Increasing Muscle Mass

Most athletes are interested in increasing muscle mass to enhance their strength and power. There is a lot of confusion as to how to go about increasing muscle size and function. Here is a step-by-step guide to muscle growth.

Step 1: Start with a good training program

Muscles need stimulation to grow. Seek expert advice from a reputable strength coach. Look for someone with at least a Level 2 coaching qualification who is recommended by other health professionals. Adjust your program as your strength develops.

Step 2: Set realistic goals

Everyone is after quick results when it comes to muscle development but the reality is that muscles take time to grow. Everyone has a different genetic potential to develop muscle mass so your goals must be realistic for you. Gains usually occur relatively quickly at the start of a training program then slow down as the body adjusts. Most athletes want to increase muscle mass while simultaneously decreasing skinfolds. This is difficult to do, as one goal is a product of a negative energy balance (intake that is less than requirements), while the other is best achieved in the face of a positive energy balance. It is important to prioritise body composition goals.

Step 3: Support your training with a high-energy diet that provides adequate protein

To gain muscle mass effectively, a positive energy balance of at least 2000-4000 kilojoules per day is required. This calls for a general increase in dietary intake. However, carbohydrate is the first nutrient to focus on, since muscle must be fuelled to do the training to stimulate muscles to grow. It is also important for athletes trying to gain muscle mass to meet their increased protein needs, but huge protein intakes are not required. In most cases, a high-energy diet that provides 1.2-2g of protein per kilogram of body mass will ensure that protein needs are met. These protein intakes are easily met by consuming a varied diet that meets your energy needs. Consuming protein above this level does not have an anabolic effect. Excess protein will be oxidised as an energy source and may contribute to gains in body fat. Very high protein diets displace other important nutrients from the diet and can be a source of saturated fat. Although it is important to achieve energy needs, a high-energy diet should not be seen as an excuse for gluttony. Excess consumption of high-fat foods can lead to excessive energy intake, and a greater risk of gaining fat mass rather than muscle.

Step 4: Get organised

Increasing your energy intake is not always easy. Many athletes with high-energy needs are surprised to find they consume less than they think. It requires considerable organisation and commitment to consistently consume a high-energy intake. This includes having a planned approach to shopping and cooking to ensure appropriate foods are available, and carrying snacks around during the day to avoid missing snacks or having to rely on less suitable options.

Step 5: Eat and drink frequently

Eating more frequently, rather than increasing the amount of food you consume at each meal, is a more effective way to ensure an increase in food and energy intake. Athletes who try to cram their nutritional needs into three meals will often suffer stomach discomfort from the sheer size of meals and end up abandoning the plan. Compact snacks and high-energy drinks offer an efficient way to top up between meals without filling up. Good choices include milk shakes and fruit smoothies, liquid meal supplements, cereal and sports bars, and flavoured dairy foods such as yoghurt.

Step 6: Time meals and snacks appropriately

Including a small serve of protein with all meals and snacks will optimise amino acid levels in the blood and may facilitate muscle development. Remember that protein comes from a wide range of sources including bread, breakfast cereal, rice and dairy products - it is not just found in meat. Eating a carbohydrate-rich, moderate-protein snack or meal immediately after training may help to optimise gains in muscle mass by increasing production of anabolic hormones, reducing protein breakdown and supplying amino acids for protein synthesis. This snack may be even more effective when consumed before the session, in the case of weight training workouts. At the AIS, we encourage athletes to consume a snack providing 1g of carbohydrate per kilogram of body weight plus 10-20 g of protein within 30 minutes of finishing training. Suitable snacks are shown below:

60kg Athlete

200g fruit yoghurt + cereal bar
200g fruit yoghurt + 250ml juice
200g fruit yoghurt + banana
50g PowerBar Protein Plus Powder in 250ml water + cereal bar
30g PowerBar Protein Plus Powder in 250ml milk + cereal bar
200ml flavoured milk + cereal bar
200ml flavoured milk + 200g fruit yoghurt
200ml flavoured milk + banana

80kg Athlete

200g fruit yoghurt + 2 cereal bars
200g fruit yoghurt + cereal bar + 400ml sports drink
200g fruit yoghurt + cereal bar + 250ml juice
200g fruit yoghurt + cereal bar + banana
75g PowerBar Protein Plus Powder in 250ml water + cereal bar
60g PowerBar Protein Plus Powder in 250ml milk + cereal bar
200ml flavoured milk + 2 cereal bars
200ml flavoured milk + cereal bar + 200g fruit yoghurt
200ml flavoured milk + cereal bar + banana

Step 7: Be patient and consistent

Increases in body mass of 2-4 kg per month are generally considered achievable but individuals can respond differently to strength training. It is important to be consistent with your training and your diet. Some athletes fall into the trap of training and eating well for a couple of days then slacking off for a couple of days. A spasmodic approach to training and nutrition will slow your progress down.

Step 8: Seek qualified advice before taking supplements

There are numerous supplements available on the market which promise to increase muscle mass and strength. The majority of these products and their claims are not based on scientific evidence and are, therefore, a waste of money. Although protein powders are all the rage in muscle building circles, these products are generally too low in carbohydrate, excessive in protein - and too expensive. However, there are a small number of supplements that may be useful for some athletes under specific circumstances. For most people wanting to increase muscle mass, the most useful supplement is one that provides carbohydrate plus moderate levels of protein and other nutrients. The AIS uses PowerBar Protein Plus powder for athletes needing to consume extra nutrients to bulk up. This has a good balance of protein and carbohydrate.

PowerBar Protein Plus Powder
Serve size	65 g
Energy per serve	1020 kJ
Carbohydrate per serve	43.7 g
Protein per serve	15 g

Before using any supplement seek expert advice regarding the efficacy, safety and legality of the product. Sports dietitians can provide qualified advice and information on the AIS Supplement Program can be found in the Supplements section of the AIS Sports Nutrition website (www.ausport.gov.au/ais/nutrition/supplements). The Australian Sports Drug Agency (www.asada.gov.au) is also a source of information.

Step 9: Monitor your progress and adjust when necessary

Individuals can respond very differently to training and nutrition programs. Some degree of trial and error is always required. The meal plan below provides an example of the type of food intake required to support a strength training program for a 70kg athlete. However, the plan will not suit everyone.

	Quantity of food required to provide high carbohydrate and high protein needs for a 70 kg athlete	Amount of carbohydrate (g)	Amount of protein (g)
Breakfast	2 cups cereal 300 ml milk 2 slices toast 2 tablespoons jam 1 cup juice	39 16 30 36 19	6 12 8 0 2
Lunch	2 bread rolls each with 50 g chicken + salad 1 banana 1 fruit bun 250 ml flavoured low fat milk	78 20 34 17	41 2 6 13
Dinner	Stir-fry with 2 cups pasta + 100 g meat + 1 cup vegetables 1 cup jelly + 1 cup custard	100 82	50 13
Snacks	750 ml sports drink 1 carton yoghurt 1 piece fruit 1 cereal bar	51 33 18 24	0 10 1 2
Analysis		594 g (8 g/kg)	166 g (2.3 g/kg)

A sports dietitian can help to develop an individualised plan to suit the specific needs and goals of each athlete. Sports dietitians can also help monitor changes in skinfolds, weight and girths using a reliable technique. These results will help the athlete to assess their real progress in changing body composition. The Sports Dietitians Australia (SDA) website provides details of qualified sports dietitians throughout Australia.

Protein - How Much?

Much controversy surrounds the protein requirements of athletes. In the scientific world, protein requirements have been well researched and, in recent years, scientists have come to a consensus. Public opinion however, tends to move in cycles with opinion being influenced by the media, the internet and pseudo-scientists. At one stage protein is unimportant, the next, protein is flavour of the month and the predominant nutrient on everybody's menu. It's no wonder many athletes are struggling to decide just how much protein is enough?

Why is protein important?

Protein is an essential nutrient in the diet, being used to manufacture body proteins that have important structural and functional roles in the human body. Structural proteins are needed to build connective tissue, cell membranes and muscle cells. Regulatory proteins act as enzymes or transport vehicles. Proteins are made up of various sequences of about 20 different amino acids. Nine of these amino acids are essential and must come from the diet or from the breakdown of other proteins in the body. Some amino acids are used as a minor fuel source during exercise.

Do athletes require more protein?

In the last 20 years, detailed research has enabled scientists to measure protein turnover during exercise and recovery, and to monitor protein balance in athletes. Endurance athletes in heavy training require extra protein to cover a small proportion of the energy costs of their training and to assist in the repair and recovery process after exercise. Strength athletes, who are interested in gaining muscle size and function, require more protein in the early stages of very intensive resistance exercise. However, strength athletes seem to adapt to the stress of exercise, so that the protein requirements to maintain protein balance in very well-trained athletes are only marginally greater than those of generally active people. Athletes who are growing, such as adolescents, have additional protein requirements. The table below summarises protein requirements for different types of athletes or exercise activities. Since athletes come in various shapes and sizes, it is easier to keep track of these requirements by relating them to the size (body mass or BM) of the athlete.

Table 1: Estimated protein requirements for athletes

Group	Protein intake (g/kg/day)
Sedentary men and women	0.8-1.0
Elite male endurance athletes	1.6
Moderate-intensity endurance athletes(a)	1.2
Recreational endurance athletes(b)	0.8-1.0
Football, power sports	1.4-1.7
Resistance athletes (early training)	1.5-1.7
Resistance athletes (steady state)	1.0-1.2
Female athletes	~15% lower than male athletes

(a)Exercising approximately four to five times per week for 45-60 min
(b)Exercising four to five times per week for 30 min at <55%>

Source: Burke and Deakin, Clinical Sports Nutrition, 3rd Edition, McGraw-Hill Australia Pty Ltd, 2006

Do athletes need to eat more protein-rich foods?

Even if an athlete has a higher requirement for protein than a sedentary individual, does this mean that they need to change their eating practices to increase their protein serves? In particular, do they need to focus only on very high protein foods, or take protein supplements? Typically, the Australian diet provides about 12-15% of energy intake in the form of protein, and the average Australian easily consumes more than 150% of the recommended dietary intake of protein from such a dietary mix. Generally, athletes enjoy the advantage of increased energy requirements that make it possible to consume even larger amounts of protein and other nutrients than the sedentary person. Numerous dietary surveys show that the dietary patterns reported by various groups of athletes provide protein intakes that are well in excess of 1.2-2.0 g/kg body mass per day - especially in the case of strength-training athletes. Therefore, there is no justification for special high protein eating strategies for sport.

It seems that some athletes swear by high-carbohydrate diets, while others have moved to high-protein diets? Who is right?

Although there appears to be a new battle looming between high-protein devotees and high-carbohydrate devotees in sport, in fact there is no need to take one extreme view or the other. Many athletes do not realise that it is possible, and beneficial, to consume a diet that achieves the muscle's carbohydrate fuel needs, as well as providing a protein intake that easily meets the increased protein needs for sport. In fact, in many situations, a meal or snack providing a combination of carbohydrate and protein is the best way to meet sports nutrition goals. These nutrients should be teamed-up, over the day, and at specific times, to achieve an optimal eating plan. The following menu plan shows food choices that achieve high carbohydrate and protein intakes simultaneously.

	Quantity of food required to provide high carbohydrate and high protein needs for a 70 kg athlete	Amount of carbohydrate (g)	Amount of protein (g)
Breakfast	2 cups cereal 300 ml milk 2 slices toast 2 tablespoons jam 1 cup juice	39 16 30 36 19	6 12 8 0 2
Lunch	2 bread rolls each with 50 g chicken + salad 1 banana 1 fruit bun 250 ml flavoured low fat milk	78 20 34 17	41 2 6 13
Dinner	Stir-fry with 2 cups pasta + 100 g meat + 1 cup vegetables 1 cup jelly + 1 cup custard	100 82	50 13
Snacks	750 ml sports drink 1 carton yoghurt 1 piece fruit 1 cereal bar	51 33 18 24	0 10 1 2
Analysis		594 g (8 g/kg)	166 g (2.3 g/kg)

How does timing and teamwork of protein and carbohydrate work?

Recovery after each workout or competition is a challenge for the athlete with a busy timetable. Recovery processes are complex and include refuelling, rehydrating and repairing. Muscle and body protein metabolism is a constant balance between protein breakdown and protein rebuilding. During exercise the balance shifts towards protein breakdown, while during recovery the balance tips in the opposite direction. Recent evidence shows that consuming protein immediately after exercise appears to enhance muscle uptake and retention of amino acids, and promote a more positive protein balance. At least this has been observed in the hours after the training session. It is still not clear whether these benefits are apparent when the 24 hour picture is looked at, or whether this change will lead to increased muscle size and strength. But, at this stage it looks like a useful strategy to promote recovery from workouts that cause muscle damage or stimulate muscle growth. The most important news is that the effect of post-exercise protein intake is best seen when the protein is combined with carbohydrate. Carbohydrate intake stimulates an increase in the hormone insulin, which in turn, stimulates the muscle to take up the amino acids. A protein-carbohydrate snack or meal after a workout makes good sense - not only for muscle repair and adaptation to training, but to provide carbohydrate fuel to restore muscle glycogen levels. Snacks or light meals that achieve this team-work include:

• Flavoured yoghurt
• Flavoured milk drinks
• Fruit smoothies
• Liquid meal supplements (e.g. Sustagen Sport)
• Sandwiches with meat, cheese, chicken or peanut butter fillings
• Breakfast cereal and milk
• Sports bars

Can athletes have too much protein?

Daily protein intakes under 2 g/kg BM in healthy people are unlikely to cause side effects. Less is known about the long-term side effects of protein intakes above 2 g/kg/BM. High protein intakes can increase the amount of calcium excreted in the urine. This may cause problems with athletes at risk of weakened bones - for example, female athletes with low energy intakes who are not menstruating. High protein intakes are also known to accelerate the progression of pre-existing kidney disease.

From a nutritional point of view, excessive intakes of protein tend to be expensive especially if protein supplements are used. High protein intakes based on the consumption of large amounts of animal foods (e.g. meat and dairy foods) can result in a greater fat intake. High protein intakes may increase fluid requirements. However, the main concern is that excessive focus on high protein foods may displace other valuable foods (e.g. fruit and vegetables) or other important nutrients such as carbohydrate and fibre from the diet.

Can athletes have too little protein?

Although the majority of athletes eat more protein than is required to meet their special needs for sport, there are some athletes who may eat diets inadequate in protein. These athletes include:

• Athletes on low energy diets trying to achieve or maintain a lower body mass/body fat level, especially those undertaking extreme restriction to lose weight rapidly
• Athletes restricting dietary variety, especially those cutting out large numbers of food groups. This includes fussy eaters and people with extensive allergies or food intolerances, athletes travelling to places with a restricted food supply, and athletes following extreme carbohydrate-rich diets or poorly constructed vegetarian diets. Athletes with eating disorders or disordered eating problems, and those following fad diets (e.g. the Cabbage soup diet, or long-term juice and vegetable fasts) are also at risk of sub-optimal protein intake among other nutrient inadequacies

Failure to consume sufficient protein on a regular basis may cause muscle to be broken down to ensure a continual supply of essential amino acids in the body. This will lead to loss of strength and power, or a failure to make optimal gains from training.

Which are the best foods to provide protein?

The following table indicates the protein content of many basic foods. Many people typically turn to meat, poultry and dairy products to obtain protein. Don't forget that plant foods such as bread, pasta, rice, breakfast cereal, legumes, lentils and nuts also contribute significant amounts of protein to the overall diet. Remember also, that many common foods or mixed dishes are made up of these basic ingredients. For example, we sometimes forget that custard or rice pudding is made with milk, or that meat or chicken are in the sandwich filling, or pasta sauce. Mixing and matching foods is a good way to team up protein, carbohydrate and other nutrients. Ideally, a mixture of protein sources should be included in the diet, and distributed at each meal and snack over the day, to ensure the full complement of amino acids is consumed on a regular basis.

Table 2: Protein rich foods for athletes. Each of the following foods provides approximately 10 g of protein. These foods have moderate to low fat contents and are rich in other nutrients.

Animal Foods	Plant Foods
2 small eggs 30 g (1.5 slices) reduced fat cheese 70 g cottage cheese 1 cup (250 ml) low-fat milk 35 g lean beef, lamb or pork (cooked weight) 40 g lean chicken (cooked weight) 50 g grilled fish 50 g canned tuna or salmon 200 g reduced fat yoghurt 150 g light fromage frais	4 slices (120 g) wholemeal bread 3 cups (90 g) wholegrain cereal 2 cups (330 g) cooked pasta 3 cups (400 g) cooked rice 3/4 cup (150 g) lentils or kidney beans 200 g baked beans 120 g tofu 400 ml soy beverage 60 g nuts or seeds 1 cup (250 ml) soy milk 100 g soy meat

Are protein supplements useful?

Generally, athletes can obtain all the protein they require from a good mixed diet with approximately 15% of the energy coming from protein. Occasionally, an athlete may require a supplement when a practical way to consume sufficient food cannot be found. Many protein supplements are very expensive due primarily to the amount of marketing that accompanies products. They tend to provide very large amounts of protein and little other nutrients. There is no need for the amount of protein provided by many supplements and there is certainly no justification for the extra cost.

The most suitable supplement is one that provides both protein and carbohydrate. Good alternatives to protein supplements include homemade fruit smoothies, liquid meal supplements such as PowerBar Protein Plus powder and 20 g skim milk powder added to regular milk.

Do high-protein diets assist with weight loss?

The key to weight loss is to consume less kilojoules than you use over a period of time. High protein diets are currently very popular thanks to the media coverage of "shrinking" celebrities who attribute their rapid weight loss or emaciated frames to popular diets based on high-protein, low-carbohydrate eating. Around the world, it appears that people are giving up carbohydrate in favour of protein. However, in the short-term, high protein, low-carbohydrate diets result in loss of water and glycogen. This might result in a decrease on the scales but does nothing to reduce body fat. In the long-term, high-protein, low-carbohydrate diets may result in fat loss. The effect is primarily due to the fact that these diets are low in kilojoules rather than any magical effect from the protein itself. It is almost impossible to maintain a high-protein, low-carbohydrate diet for the long term because they are boring and unappetising. The lack of carbohydrate reduces energy levels, impairs performance and causes nausea and lethargy. High-protein, low-carbohydrate diets restrict the intake of many essential nutrients in the diet. In the long-term, these diets will see muscle mass decrease. High-protein, low-carbohydrate diets are not suitable for athletes.

Protein Nutrition and Endurance Exercise: What Does Science Say?

This brief review analyzes the potential for performance enhancement through protein ingestion, whether during activity or by enhancing muscle recovery.

Protein Ingestion During Exercise

A properly formulated carbohydrate-electrolyte beverage (CEB) improves performance during exercise primarily because of two key ingredients: carbohydrate (CHO), which provides fuel for working muscles, and sodium, which helps to maintain fluid balance.² Recently, two studies suggested that adding a small amount of protein (~2% whey protein) to a CEB produced improvements in endurance capacity compared to the sports drink alone.^3,4 However, the practical relevance of these studies is hampered by the way the research was conducted. First, the rate of CHO delivered in the CEB was less than what is considered optimal for performance²; and second, the method of the performance test (exercise time to fatigue) did not mimic the manner in which athletes typically compete. In a recent study⁵, we addressed these issues by having trained cyclists ingest a CEB during exercise at a rate considered optimal for CHO delivery (60 gram per hour), and perform a task that closely simulated athletic competition.

Subjects performed an 80-km cycling time trial on three occasions and drank either a 6% CHO blend, a 6% CHO + 2% whey-protein blend, or a sweetened placebo. All of the subjects consumed the solutions at a rate of 1 liter per hour. The study was “double blind” meaning neither the athletes nor the researchers knew what drink was consumed during a given trial. The study was also counterbalanced so that the order in which the subjects received the three treatments was systematically varied to prevent test-order bias. The trials determined that the average performance time was identical during the CHO and CHO+protein trials (roughly 135 min) and both were significantly faster (by approximately 4%) than the placebo trial (141 min). This study⁵ demonstrated that when athletes ingested a CEB during exercise at a rate considered optimal for CHO delivery, protein provided no additional performance benefit during an event that simulated “real life” competition.

Amino Acid Supplementation During Exercise

In addition to whole proteins, many studies have examined whether consuming specific amino acids or amino-acid mixtures improves exercise performance. These studies have generally reported no benefit, although the issue of branched-chain amino acid (BCAA) supplementation remains debateable.⁶ In this author’s opinion, the most well-controlled studies show no effect of BCAA supplementation on performance.⁷ This view is supported by two recent studies^8,9 that investigated the effect of this practice during exercise in the heat. The studies involved manipulations designed to reduce glycogen availability and induce dehydration. This technique was used in order to simulate the metabolic conditions athletes experience during the latter stages of prolonged exercise. Using a time trial and exercise to voluntary exhaustion⁹, both studies found no effect of BCAA ingestion on performance.

Protein Ingestion During Recovery and Acute Muscle Adaptations

Nutrition intake during the immediate post-exercise period may benefit the athlete in that it aids the synthesis of muscle proteins and the replenishment of muscle glycogen. Similar to the effect seen after resistance exercise¹⁰, consuming protein with CHO during recovery from endurance exercise promotes muscle repair.¹¹ This effect may be due to a direct effect of amino acids (particularly BCAA) on signaling pathways that control muscle protein synthesis.¹² A more controversial issue is whether consuming protein with CHO enhances muscle glycogen resynthesis during the first several hours of recovery from prolonged exercise. In this author’s opinion, which is consistent with leading researchers in the field¹³, most evidence suggests that ingesting a high amount of CHO at frequent intervals (e.g., ≥ 1.2 g CHO per kg body weight per hour) negates the benefits of added protein. However, if an athlete does not eat a sufficient amount of CHO during recovery, then consuming protein with CHO may augment glycogen synthesis. Thus, similar to the effect on endurance capacity, the beneficial effect of ingesting protein with CHO on glycogen storage may be due to higher energy (nutrient) intake per se rather than any unique physiological mechanism.

Protein Ingestion During Recovery and Subsequent Exercise Performance

Regardless of the potential changes in muscle, one key issue for some athletes is whether consuming protein with CHO during recovery improves subsequent endurance performance. One study¹⁴ reported that ingesting a CHO-protein drink during recovery from glycogen depleting exercise (activity lasting more than 90 minutes) markedly improved time to exhaustion during a subsequent exercise bout, as compared to a sports drink. However, the CHO-protein drink provided approximately three times as many calories as the sports drink and thus the improved endurance capacity was likely due to the higher energy intake. Another study¹⁵ that did not match energy intake compared chocolate milk, a dilute CHO sports drink and a drink matched to chocolate milk in terms of protein and CHO content. Endurance capacity was improved with chocolate milk and the sports drink compared to the third beverage, even though the latter provided CHO and protein equivalent to chocolate milk and more CHO and energy than the sports drink. The mechanisms that might explain the rather surprising findings from this study remain to be elucidated.

Studies that have compared a CHO-protein drink with a CHO drink that provided the same amount of energy, showed no difference in subsequent 5-km running time trial¹⁶ or a timed run to exhaustion¹⁷. Additional research in this area is warranted, but at present there is no compelling evidence that suggests consuming protein with CHO during recovery has a direct effect on subsequent exercise performance. Nonetheless, given that protein has been shown to promote muscle recovery after strenuous exercise¹¹, it seems prudent for athletes to consume protein with CHO as part of their recovery nutrition strategy.

Conclusions

• Some studies have suggested that consuming protein with CHO during exercise improves endurance performance while other studies have reported no benefits. Additional research will resolve this debate, but it should be remembered that there is no established mechanism by which protein intake during exercise should improve performance.
• Recent evidence indicates that when CHO is consumed in sufficient amounts during exercise, adding protein provides no performance benefit and does not enhance muscle glycogen synthesis following exercise.
• Consuming a small amount (10-20 grams) of high-quality protein after exercise promotes muscle protein synthesis compared to CHO alone and may enhance the body’s response to long-term training.

References

1. Hawley, J.A., Tipton, K.D., and Millard-Stafford, M.L. Promoting training adaptations through nutritional manipulations. J Sports Sci. 24:1-13, 2006.

2. Jeukendrup, A.E. Carbohydrate intake during exercise and performance. Nutrition 20:669-677, 2004.

3. Ivy, J.L., Res PT, Sprague, R.C., and Widzer, M.O. Effect of a carbohydrate-protein supplement on endurance performance during exercise of varying intensity. Int. J. Sports Nutr. Exerc Metab. 13:382-395, 2003.

4. Saunders, M.J., Kane, M.D., and Todd, K.M. Effects of a carbohydrate-protein beverage on cycling endurance and muscle damage. Med. Sci. Sports Exerc. 36:1233-1238, 2004.

5. van Essen MJ, and Gibala MJ. Failure of protein to improve time trial performance when added to a sports drink. Med Sci Sports Exerc. 38:1476-1483, 2006 (In press).

6. Gleeson, M. Interrelationship between physical activity and branched-chain amino acids. J Nutr. 135:1591S-1595S, 2005.

7. van Hall G, Raaymakers, J.S., Saris, W.H., Wagenmakers, A.J. Ingestion of branched-chain amino acids and tryptophan during sustained exercise in man: failure to affect performance. J Physiol. 486: 789-794, 1995.

8. Cheuvront, S.N., et. al. Branched-chain amino acid supplementation and human performance when hypohydrated in the heat. J Appl Physiol. 2004 97:1275-1282, 2004

9. Watson, P., Shirreffs, S.M., Maughan, R.J. The effect of acute branched-chain amino acid supplementation on prolonged exercise capacity in a warm environment. Eur J Appl Physiol. 93:306-314, 2004.

10. Rennie, M.J., et. al. Control of the size of the human muscle mass. Annu Rev Physiol. 66:799-828, 2004.

11. Levenhagen, D.K., et. al. Postexercise protein intake enhances whole-body and leg protein accretion in humans. Med Sci Sports Exerc. 24:828-837, 2002.

12. Blomstrand, E., Eliasson, J., Karlsson, H.K., Kohnke, R. Branched-chain amino acids activate key enzymes in protein synthesis after physical exercise. J Nutr. 136: 269S-273S, 2006.

13. Burke, L.M., Keins, B., and Ivy, J.L. Carbohydrates and fat for training and recovery. J. Sports Sci. 22:15-30, 2004.

14. Williams, M.B., Raven, P.B., Fogt, D.L., Ivy, J.L. Effects of recovery beverages on glycogen restoration and endurance exercise performance. J Strength Cond Res.17: 12-19, 2003.

15. Karp, J.R. et. al. Chocolate milk as a post-exercise recovery aid. Int J Sports Nutr Exerc Metab. 16:78-91, 2006.

16. Millard-Stafford, M., Warren, G.L., Thomas, L.M., Doyle, J.A., Snow, T., Hitchcock, K. Recovery from run training: efficacy of a carbohydrate-protein beverage? Int J Sport Nutr Exerc Metab. 15:610-624, 2005.

17. Betts, J.A., et. al. Recovery of endurance running capacity: effect of carbohydrate-protein mixtures. Int J Sport Nutr Exerc Metab. 15:590-609, 2005.

Carbohydrate Supplementation During Exercise

Does It Help? How Much is Too Much?

KEY POINTS

· Carbohydrate intake during exercise can delay the onset of fatigue and improve performance of prolonged exercise as well as exercise of shorter duration and greater intensity (e.g., continuous exercise lasting about 1 h and intermittent high-intensity exercise), but the mechanisms by which performance is improved are different.

· During prolonged exercise, the performance benefits of carbohydrate ingestion are likely achieved by maintaining or raising plasma glucose concentrations and sustaining high rates of carbohydrate oxidation, whereas during intense exercise, carbohydrate intake seems to positively affect the central nervous system.

· Carbohydrate from a single source, such as glucose, can only be oxidized at rates of approximately 60 g/h.

· When a combination of carbohydrates is ingested (e.g., glucose and fructose) oxidation rates of slightly more than 100 g/h can be achieved if large amounts of carbohydrate are ingested (e.g., > 140 g/h).

· Ingesting a carbohydrate solution that is very concentrated and/or has a high osmolality is likely to cause gastrointestinal discomfort.

· The amount of carbohydrate an individual athlete should ingest during exercise should be determined by trial and error, and a balance should be struck between increasing carbohydrate availability during exercise and minimizing gastrointestinal distress.

INTRODUCTION
As described in more detail later in this paper, carbohydrate ingestion during long-duration exercise lasting 2 h or more nearly always delays the onset of fatigue and improves performance. Carbohydrate may also be beneficial during more intense continuous exercise lasting about 1 h and during intermittent high-intensity exercise. In long-duration exercise, a greater contribution of exogenous carbohydrate (carbohydrate ingested in beverages or other foods) will spare liver glycogen, prevent a drop in blood glucose concentration, and help maintain the high rate of carbohydrate oxidation necessary to sustain exercise intensity. However, even when carbohydrate is ingested, there is almost always a negative energy balance during exercise, i.e., the energy expenditure exceeds the energy intake. For example, it has been reported that in major cycling stage races (including the Tour de France) the riders ingest on average 25 g of carbohydrate per hour (Garcia-Roves et al., 1997). This is an energy intake of only 100 kcal/h, whereas the energy expenditure could be at least ten times that value. In extreme cases of exercise that lasts 5-6 h, this could conceivably amount to a negative energy balance of 4000-5000 kcal.

The negative energy balance developed during extremely prolonged races was traditionally compensated by an exceptionally large pre-race dinner (Jeukendrup et al., 2000a); even so, it can be difficult for some athletes to maintain energy balance (Saris et al., 1989). Of course, energy intake during the race need not be restricted to carbohydrate only; fat and protein could be ingested as well in an attempt to minimize the negative energy balance. Unfortunately, fat and protein can be potent inhibitors of gastric emptying, delaying not only the delivery of energy, but also of fluids (Brouns & Beckers, 1993). For these reasons, it makes sense to increase carbohydrate intake during exercise and thereby increase carbohydrate oxidation by exercising muscles.

However, ingesting too much carbohydrate can have detrimental effects; highly concentrated carbohydrate solutions and drinks with high osmolality have been linked to the development of gastrointestinal discomfort (Rehrer et al., 1992a). Therefore, athletes must find the appropriate balance between ingesting enough carbohydrate to provide extra energy, but not so much as to increase the risk of gastrointestinal discomfort. There are other complicating factors: the development of gastrointestinal discomfort seems to be highly individualized and is dependent on the intensity and duration of exercise, hydration status, environmental conditions, and other factors.

As discussed below, the mechanism underlying the beneficial effects of carbohydrate ingestion for exercise that lasts about 1 h and perhaps for intermittent exercise (sometimes lasting longer than 1 h) appears to be different than that for more prolonged continuous exercise and is associated with effects on the central nervous system. For exercise of shorter duration, lesser amounts of ingested carbohydrate are required compared to more prolonged exercise. As with prolonged exercise, there is a potential for gastrointestinal distress if an athlete ingests too much carbohydrate during high-intensity exercise.

The major purpose of this article is to provide a brief review of the scientific literature related to the effects of carbohydrate intake on performance and the optimal dose and type of carbohydrate ingested during exercise. Attention is also given to the metabolism of ingested carbohydrate, gastrointestinal disturbances during prolonged exercise, the relationship between carbohydrate intake and fluid delivery, and the possibility that carbohydrate ingestion during exercise might adversely affect genetic adaptations to physical training.

RESEARCH REVIEW
Effects of Carbohydrate Intake on Performance
The beneficial effects of carbohydrate feeding on exercise performance have been well described. In older studies, the ergogenic effects of carbohydrate feeding were typically seen during exercise lasting at least 2 h (Bjorkman et al., 1984; Coyle et al., 1983; Hargreaves et al., 1984; Ivy et al., 1983; Murray et al., 1989; Neufer et al., 1987). More recent studies have found positive effects of carbohydrate feeding during exercise of relatively high intensity (>75% VO2max) lasting approximately 1 h (Anantaraman et al., 1995; Below et al., 1995; Carter et al., 2003; el-Sayed et al., 1997). As an example, Jeukendrup et al. (1997) investigated the effects of carbohydrate ingestion during the equivalent of a 40-km time trial (~ 1 h) in well-trained cyclists and found that performance was improved by 2.3%. However, it should also be noted that some investigators were unable to detect an ergogenic effect of carbohydrate feedings for high-intensity exercise (Clark et al., 2000; McConell et al., 2000; Powers et al., 1990). Carter and colleagues (2004b) concluded that any beneficial effect was unrelated to substrate availability because glucose infusion at high rates did not affect performance; rather, this group suggested that the effects might operate via the central nervous system (Jeukendrup et al., 1997).

Consistent with this idea, our laboratory showed that rinsing the mouth with a carbohydrate solution improved cycling performance during a 1-h time trial by 2-3% even when the subjects did not actually swallow the carbohydrate (Carter et al., 2004a). This performance improvement was of the same magnitude as that seen with carbohydrate ingestion during a similar exercise task (Jeukendrup et al., 1997). These results suggest the existence of receptors in the mouth that communicate with the brain to affect exercise performance. Although direct evidence for such receptors is lacking, it is clear that the brain can sense changes in the composition of the contents of the mouth and stomach. Oropharyngeal receptors, including those situated in the oral cavity, are known to have important roles in perceptual responses during rehydration and exercise in the heat (Maresh et al., 2001; Riebe et al., 1997). In these studies, ratings of perceived exertion (RPE) and thirst sensations were lower when fluid intake was by mouth compared to infusing fluids intravenously. These findings are supported by reports of temporary reductions in thirst due to the gargling of tap water (Seckl et al., 1986). Although speculative, it is possible that triggering of stimuli within the oral cavity by the carbohydrate solution could initiate a chain of neural messages in the central nervous system, resulting in the stimulation of the reward and/or pleasure centers in the brain.

It must be noted that maximal continuous exercise lasting less than 45 min may not benefit from carbohydrate feeding (Palmer et al., 1998). At such high exercise intensities, other factors may override a possible beneficial central effect of carbohydrate. Relatively few studies have been conducted using exercise durations less than 1 h, so additional work in this area is needed. However, some laboratories have observed positive effects of carbohydrate drinks on high-intensity intermittent exercise using a shuttle run as a model for team sports such as basketball and soccer (Davis et al., 1999; Nicholas et al., 1995; Welsh et al., 2002).

Although central mechanisms might play a role in enhancing performance during exercise lasting approximately 1 h, the long-established mechanism during more prolonged exercise remains the maintenance of blood glucose concentrations and relatively high rates of carbohydrate oxidation. Once the effect of carbohydrate on endurance performance had been established in the 1980s, the next obvious objective was to determine the optimal dose.

The Optimal Dose
There are only a few published reports of the effects of different doses of carbohydrate on exercise performance. Mitchell and co-workers (1989) compared ingestion of 37 g, 74 g, or 111 g of carbohydrate per hour (6%, 12%, and 18% carbohydrate solutions, respectively) or flavored water. Compared to water, only the trial using 74 g of carbohydrate per hour significantly enhanced the performance of a 12-min isokinetic cycling time-trial following 105 min of continuous exercise. However, all of the performance results for the three carbohydrate trials were statistically similar. In an earlier investigation using a similar isokinetic performance ride, but following 105 min of intermittent exercise, the same authors found improved performance compared to a water trial for 5%, 6% and 7.5% carbohydrate solutions (33, 40, and 50 g/h, respectively), with no significant differences among the carbohydrate trials (Mitchell et al., 1988). However, in this study both the amount and type of carbohydrate ingested were varied.

A study by Fielding and colleagues (1985) is often used to claim that a minimum of 22 g of carbohydrate per hour is required to achieve a performance benefit. In that study, subjects performed a cycling sprint after having exercised for 4 h. Performance improvements were observed when 22 g of carbohydrate were ingested every hour, whereas no effects were observed when half this dose was consumed (11 g/h). But in an experiment reported by Maughan’s group (1996), the intake of 16 g of glucose per hour improved endurance capacity by 14% compared with water. (However, no placebo was given in this study). To add to the uncertainty, Flynn and colleagues (1987) did not find any differences in performance with the ingestion of placebo, 5%, or 10% carbohydrate solutions that provided 0, 15, and 30 g of carbohydrate per hour, respectively, during 2 h of cycling.

The majority of studies provided 40-75 g of carbohydrate per hour and observed performance benefits. Ingesting carbohydrate from a single source, e.g., glucose or maltodextrins, at a rate greater than 60-70 g/h does not appear to be any more effective at improving performance than ingesting carbohydrate at 60-70 g/h, perhaps, as discussed later, because of limitations in the rate of absorption of a single type of carbohydrate from the intestine. It is also possible that the current performance measurements are not sensitive enough to pick up the small differences in performance that may exist when comparing different carbohydrate solutions.

One might conclude that performance benefits can sometimes be observed with the ingestion of relatively small amounts of carbohydrate, e.g., 16 g/h, but more reliably with greater amounts. If carbohydrate ingestion is to improve endurance performance, it is likely that the beneficial effect is primarily dependent on the oxidation of that carbohydrate.

Oxidation of Ingested Carbohydrate Several factors can influence the oxidation of exogenous carbohydrate supplied in liquid and solid foods, including feeding schedule, type and amount of carbohydrate ingested, and the exercise intensity. These factors independently affect the rate of carbohydrate oxidation.

Type of single-source carbohydrate. Some types of carbohydrate from a single source are oxidized more readily than others (Jeukendrup et al., 2000b). They can be divided into two arbitrary categories: carbohydrates that can be oxidized at rates up to approximately 30 g/h and up to 60 g/h (Table 1).

TABLE 1. Oxidation of different carbohydrates

Amount of carbohydrate. The optimal amount of ingested carbohydrate should ideally be the amount that results in the maximal rate of exogenous carbohydrate oxidation without causing gastrointestinal discomfort. Rehrer et al. (1992b) studied the oxidation of different amounts of carbohydrate ingested during 80 min of cycling exercise at 70%VO2max. Subjects received either a 4.5% glucose solution (a total of 58 g glucose during 80 min of exercise) or a 17% glucose solution (220 g during 80 min of exercise). Total exogenous carbohydrate oxidation was only slightly higher with the larger dose of carbohydrate (42 g versus 32 g in 80 min). Even though the amount of carbohydrate ingested was increased almost four-fold, the oxidation rate was hardly affected. Jeukendrup et al. (1999) investigated even larger carbohydrate intakes (up to 180 g/h) and found that oxidation rates peaked at 56 g/h at the end of 120 min of cycling exercise. These results suggest some sort of limitation in the maximal rate of oxidation of ingested carbohydrates.

Based on the scientific literature in this area, it must be concluded that the maximal rate at which a single source of ingested carbohydrate can be oxidized is about 60-70 g/h (Figure 1). Although the vast majority of studies was performed with men, the same conclusion seems to hold true for endurance-trained women, i.e., the highest rates of exogenous glucose oxidation and the greatest endogenous carbohydrate sparing were observed when carbohydrate was ingested at moderate rates (60 g/h) during exercise (Wallis et al., 2007). This knowledge implies that athletes who ingest a single type of carbohydrate should ingest about 60-70 g/h for optimal carbohydrate delivery. Ingesting more than this will not increase carbohydrate oxidation rates any further and is likely to be associated with gastrointestinal discomfort.

FIGURE 1. Oxidation of ingested carbohydrate. This figure is modified from Jeukendrup ( 2004) and is compiled from studies investigating the oxidation of exogenous (ingested) carbohydrate during exercise. The oxidation rate is plotted as a function of the ingestion rate. In green are the values from studies in which a single type of carbohydrate was studied. In black are the oxidation rates from combinations of multiple carbohydrates. The green line is an estimation of the average from all studies of single carbohydrates and the black line for multiple transportable carbohydrates. As the amount ingested increases, the oxidation rate also increases, but only up to a certain point. Ingesting more than 60-70 g/h of a single carbohydrate source does not further increase the rate of oxidation of that carbohydrate, and the excess carbohydrate is likely to accumulate in the intestine. However, if multiple carbohydrates are ingested at high rates, greater maximal rates of exogenous carbohydrate oxidation can be achieved, perhaps because multiple carbohydrates stimulate multiple transport mechanisms to transfer carbohydrate from the intestine into the blood and thereby increase carbohydrate delivery to muscles.

Multiple transportable carbohydrates. As reviewed by Jeukendrup (2004), it is likely that oxidation of a single exogenous carbohydrate is limited to approximately 60 g/h because there is a limitation in the rate of intestinal absorption of that carbohydrate. It is suggested that by feeding a single carbohydrate source (e.g., glucose or fructose or maltodextrins) at high rates, the specific transporter proteins that aid in absorbing that carbohydrate from the intestine become saturated. Once these transporters are saturated, feeding more of that carbohydrate will not result in greater intestinal absorption and increased oxidation rates.

In 1995, Shi et al. suggested that the ingestion of carbohydrates that use different transporters might increase total carbohydrate absorption. Subsequently, we began a series of studies using different combinations of carbohydrates to determine their effects on exogenous carbohydrate oxidation. In the first study, subjects ingested a drink containing glucose and fructose (Jentjens et al., 2004a). Glucose was ingested at a rate of 72 g/h and fructose at a rate of 36 g/h. In the control trials, the subjects ingested glucose at a rate of 72 g/h and 108 g/h (matching glucose intake or energy intake). We found that the ingestion of glucose at a rate of 72 g/h resulted in oxidation rates of about 48 g/h. Ingesting glucose at 108 g/h did not increase the oxidation rate. However, after ingesting glucose plus fructose, the rate of total exogenous carbohydrate oxidation increased to 76 g/h, an increase in oxidation of 45% compared with a similar amount of glucose. In the following years, we tried different combinations and amounts of carbohydrates in an attempt to determine the maximal rate of oxidation of mixtures of exogenous carbohydrates (Jentjens et al., 2004abc, 2005ab, 2006; Wallis et al., 2007). We observed very high oxidation rates with combinations of glucose plus fructose, with maltodextrins plus fructose, and with glucose plus sucrose plus fructose. The highest rates were observed with a mixture of glucose and fructose ingested at a rate of 144 g/h. With this feeding regimen, exogenous carbohydrate oxidation peaked at 105 g/h. This is 75% greater than what was previously thought to be the absolute maximum.

The increased oxidation resulting from the ingestion of multiple types of carbohydrate is theoretically beneficial, although considerably more research needs to be accomplished in this area. From a study in which subjects cycled for 5 h at 50% of their maximal work rates (~58% VO2max) with water, glucose, or glucose plus fructose, we saw some indication that ingesting multiple carbohydrates may result in greater improvements in performance (Jeukendrup et al., 2006). In this study, carbohydrate was ingested at a rate of 90 g/h. The first indication of improved performance was that the subjects’ ratings of perceived exertion (RPE) tended to be lower with the mixture of glucose and fructose compared with glucose alone; the water placebo treatment produced the greatest RPE values. In fact, not all participants were able to complete the 5-h ride when they drank the water placebo. In addition, the self-selected cadence dropped significantly with water, which is generally acknowledged as an indication of developing fatigue. With glucose, cycling cadence was somewhat greater than with water, but with glucose plus fructose, cadence was highest and remained almost unchanged from the beginning of exercise. We have since confirmed the beneficial effects on prolonged exercise performance of drinking solutions of glucose plus fructose compared with glucose only (K. Currell et al., unpublished findings).

We introduced the term oxidation efficiency to describe the percentage of the ingested carbohydrate that is oxidized (Jeukendrup et al., 2000b). High oxidation efficiency means that smaller amounts of carbohydrate remain in the gastrointestinal tract, reducing the risk of causing gastrointestinal discomfort that is frequently reported during prolonged exercise (Brouns & Beckers, 1993; Rehrer et al., 1992a). Importantly, in our studies, the oxidation efficiency of drinks containing carbohydrates that use different transporters for intestinal absorption was higher than for drinks with a single carbohydrate source. Therefore, compared to a single source of carbohydrate, ingesting multiple carbohydrate sources results in a smaller amount of carbohydrate remaining in the intestine, and osmotic shifts and malabsorption may be reduced. This probably means that drinks with multiple transportable carbohydrates are less likely to cause gastrointestinal discomfort. Interestingly, this is a consistent finding in studies that have attempted to evaluate gastrointestinal discomfort during exercise (Jentjens et al., 2004abc, 2005b, 2006; Wallis et al., 2007). Subjects tended to feel less bloated with the glucose plus fructose drinks compared to drinking glucose solutions. A larger-scale study of the effects of drinks with different types of carbohydrates on gastrointestinal discomfort has not yet been published.

Exercise intensity. With increasing exercise intensity, the active muscle mass becomes progressively more dependent on carbohydrate as a source of energy. However, the oxidation of exogenous carbohydrate seems to remain constant at intensities of 50-60% VO2max or greater (Pirnay et al., 1982).

Gastrointestinal Discomfort During Exercise
Gastrointestinal discomfort is very common during exercise, especially in endurance and ultra-endurance sports. Peters et al. (1999) mailed a questionnaire to 606 athletes (runners, cyclists and triathletes) to assess the prevalence of gastrointestinal problems as well as their training background and nutrition habits. Symptoms presumably originating in the upper gastrointestinal tract (nausea, vomiting, belching, heartburn, chest pain) and in the lower gastrointestinal tract (bloating, abdominal cramps, side ache, urge to defecate, and diarrhea) were evaluated in all participants. For all subjects, 45-79% reported symptoms of lower gastrointestinal distress, and 36-67% had upper gastrointestinal symptoms. Symptoms generally seem to be more severe during running than cycling, are more prevalent in women than in men, and seem to be more frequent in prolonged exercise. For example, in an extreme long-distance triathlon event, 93% of the participants reported some type of gastrointestinal disturbance, and 45% of these problems were classified as serious (Jeukendrup et al., 2000c).

The occurrence of gastrointestinal disturbances has been related to carbohydrate intake during exercise (Brouns & Beckers, 1993). A relatively high intake of carbohydrate during exercise may increase the incidence of gastrointestinal symptoms such as diarrhea and abdominal cramps, either by the osmotic attraction of fluid from the blood into the intestine (Brouns & Beckers, 1993) or by malabsorption. The fact that mesenteric blood flow to the intestines is reduced during high-intensity exercise and even more with dehydration (Brouns & Beckers, 1993) might explain the fact that symptoms seem to be more prevalent if exercise is more prolonged and performed in hot conditions. Although the occurrence of gastrointestinal distress has been related to carbohydrate intake during exercise, this may be more related to the hyperosmolality of solutions than the actual carbohydrate content (Rehrer et al., 1992). In fact, in a laboratory study, 7% hypotonic carbohydrate drinks did not result in significantly greater discomfort during 2.5 h of running and cycling compared with water (Peters et al., 2000). Although direct evidence is lacking, it is likely that carbohydrate ingested at very high rates (>60 g/h), which almost certainly results in hyperosmolality of the stomach contents, will cause an increased incidence of gastrointestinal problems. It is also likely, however, that the gastrointestinal discomfort associated with a particular source or sources of carbohydrate is mainly dictated by the oxidation efficiency of the carbohydrate. It is therefore tempting to speculate that multiple transportable carbohydrates ingested at high rates will be associated with reduced gastrointestinal discomfort while providing carbohydrate at high rates. The tolerance of athletes for large doses of various carbohydrate drinks and the likelihood that an athlete will develop gastrointestinal discomfort seem highly individualized. Therefore, strategies for carbohydrate intake should always be developed on an individual basis, largely by trial and error.

Carbohydrate and Fluid Delivery
Another reason to avoid the intake of highly concentrated carbohydrate solutions is that such solutions have been shown to delay gastric emptying and fluid absorption. But impairment of fluid delivery is minimized when combinations of multiple transportable carbohydrates are ingested. We found that fluid delivery with a solution of glucose plus fructose is greater than with a glucose solution (Jentjens et al., 2006). Both of these carbohydrate solutions contained about 15 g of carbohydrate per 100 ml (i.e., a 15% carbohydrate solution), and such highly concentrated carbohydrate solutions would normally result in severely impaired fluid delivery. Interestingly, the rate of fluid delivery to the blood with the glucose-plus-fructose drink was closer to that of plain water than it was to glucose. Nevertheless, in hot, humid environments, especially at relatively low exercise intensities, fluid delivery is more important than carbohydrate delivery, and athletes should consume less-concentrated carbohydrate solutions.

Carbohydrate needs in endurance athletes are fairly constant in different environmental conditions, although carbohydrate oxidation rates are somewhat increased in the heat. This increased carbohydrate oxidation is mainly from muscle glycogenolysis, and the contribution from exogenous carbohydrate may actually be decreased (Jentjens et al., 2002). The most logical explanation for this decrease is a redistribution of blood flow to the skin and muscle, with a reduction in blood flow to the intestines. This redistribution of blood would presumably adversely affect carbohydrate absorption. However, combinations of multiple carbohydrates can at least partly overcome this problem, and high rates of exogenous carbohydrate oxidation can be achieved even in these hot conditions (Jentjens et al., 2002).

Does Carbohydrate Ingestion Impair Metabolic Adaptations to Training?
Civitarese et al. (2005) suggested that carbohydrate ingestion during exercise may suppress the gene expression of oxidative enzymes involved in fat metabolism and might therefore interfere with the process of training adaptations that involve a greater reliance on fat metabolism for energy. They showed that the transcription of several genes related to fat metabolism is transiently induced after exercise when no feeding is provided during exercise and that glucose feeding interfered with these adaptations. Moreover, Cluberton et al. (2005) demonstrated that glucose ingestion attenuated the exercise-induced increase in other enzymes involved in energy metabolism and of certain types of messenger RNA. However, there may be a flaw in extrapolating these results to practical implications for the athlete; carbohydrate ingestion may allow the athlete to train harder, which would then most likely result in enhanced transcription of metabolic genes. So it may be too early to provide practical advice based on the small number of laboratory studies currently published (Hawley et al., 2006).

SUMMARY
Although carbohydrate ingestion can improve exercise performance, consuming large amounts of carbohydrate is not necessarily a good strategy. Carbohydrate from a drink or food will spare liver glycogen, increase carbohydrate oxidation by muscle, and positively impact certain central nervous system responses, but ingesting too much carbohydrate can have detrimental effects. Highly concentrated carbohydrate solutions and drinks with high osmolality have been linked to the development of gastrointestinal discomfort. It therefore appears that there is a fine balance between ingesting optimal amounts of carbohydrate that can be oxidized for energy while at the same time avoiding gastrointestinal discomfort that can lead to poor performance.

REFERENCES
Anantaraman, R., A. A. Carmines, G. A. Gaesser, and A. Weltman (1995). Effects of carbohydrate supplementation on performance during 1 h of high intensity exercise. Int. J. Sports Med. 16(7):461-465.

Below, P. R., R. Mora-Rodriguez, J. Gonzales Alonso, and E. F. Coyle (1995). Fluid and carbohydrate ingestion independently improve performance during 1 h of intense exercise. Med. Sci. Sports Exerc. 27(2):200-210.

Bjorkman, O., K. Sahlin, L. Hagenfeldt, and J. Wahren (1984). Influence of glucose and fructose ingestion on the capacity for long term exercise in well trained men. Clin. Physiol. 4:483-494.

Brouns, F., and E. Beckers (1993). Is the gut an athletic organ? Digestion, absorption and exercise. Sports Med. 15(4): 242-257.

Carter, J., A. E. Jeukendrup, T. Mundel, and D. A. Jones (2003). Carbohydrate supplementation improves moderate and high-intensity exercise in the heat. Pflugers Arch. 446(2):211-219.

Carter, J. M., A. E. Jeukendrup, and D. A. Jones (2004a). The effect of carbohydrate mouth rinse on 1-h cycle time trial performance. Med. Sci. Sports Exerc. 36(12):2107-2111.

Carter, J. M., A. E. Jeukendrup, C. H. Mann, and D. A. Jones (2004b). The effect of glucose infusion on glucose kinetics during a 1-h time trial. Med. Sci. Sports Exerc. 36(9):1543-1550.

Clark, V. R., W. G. Hopkins, J. A. Hawley, and L. M. Burke (2000). Placebo effect of carbohydrate feedings during a 40-km cycling time trial. Med. Sci. Sports Exerc. 32(9):1642-1647.

Cluberton, L. J., S. L. McGee, R. M. Murphy, and M. Hargreaves (2005). Effect of carbohydrate ingestion on exercise-induced alterations in metabolic gene expression. J. Appl. Physiol. 99:1359-1363.

Coyle, E. F., J. M. Hagberg, B. F. Hurley, W. H. Martin, A. A. Ehsani, and J. O. Holloszy (1983). Carbohydrate feeding during prolonged strenuous exercise. J. Appl. Physiol. 55(1):230-235.

Davis, J.M., R.S. Welsh, K.L. De Volve, and N.A. Alderson (1999). Effects of branched-chain amino acids and carbohydrate on fatigue during intermittent, high-intensity running. Int. J. Sports Med. 20(5):309-314.

el-Sayed, M. S., J. Balmer, and A. J. Rattu (1997). Carbohydrate ingestion improves endurance performance during a 1 h simulated time trial. J. Sports Sci. 15(2):223-230.

Fielding, R. A., D. L. Costill, W. J. Fink, D. S. King, M. Hargreaves, and J. E. Kovaleski (1985). Effect of carbohydrate feeding frequencies and dosage on muscle glycogen use during exercise. Med. Sci. Sports Exerc. 17(4):472-476.

Flynn, M. G., D. L. Costill, J. A. Hawley, W. J. Fink, P. D. Neufer, R. A. Fielding, and M. D. Sleeper (1987). Influence of selected carbohydrate drinks on cycling performance and glycogen use. Med. Sci. Sports Exerc. 19(1):37-40.

Garcia-Roves, P., N. Terrados, S. Fernandez, and A. Patterson (1997). Macronutrients intake of top level cyclists during continuous competition-change in the feeding pattern. Int. J. Sports Med. 19:61-67.

Hargreaves, M., D. L. Costill, A. Coggan, W. J. Fink, and I. Nishibata (1984). Effect of carbohydrate feedings on muscle glycogen utilisation and exercise performance. Med. Sci. Sports Exerc. 16(3):219-222.

Hawley, J. A., K. D. Tipton, and M. L. Millard-Stafford (2006). Promoting training adaptations through nutritional interventions. J. Sports Sci. 24(7):709-721.

Ivy, J. L., W. Miller, V. Dover, L. G. Goodyear, W. M. Sherman, S. Farrell, and H. Williams (1983). Endurance improved by ingestion of a glucose polymer supplement. Med. Sci. Sports Exerc. 15(6):466-471.

Jentjens, R. L., A. J. Wagenmakers, and A. E. Jeukendrup (2002). Heat stress increases muscle glycogen use but reduces the oxidation of ingested carbohydrates during exercise. J. Appl. Physiol. 92(4):1562-1572.

Jentjens, R. L., L. Moseley, R. H. Waring, L. K. Harding, and A. E. Jeukendrup (2004a). Oxidation of combined ingestion of glucose and fructose during exercise. J. Appl. Physiol. 96(4):1277-1284.

Jentjens, R. L., M. C. Venables, and A. E. Jeukendrup (2004b). Oxidation of exogenous glucose, sucrose, and maltose during prolonged cycling exercise. J. Appl. Physiol. 96(4):1285-1291.

Jentjens, R. L., and A. E. Jeukendrup (2005a). High rates of exogenous carbohydrate oxidation from a mixture of glucose and fructose ingested during prolonged cycling exercise. Br. J. Nutr. 93(4):485-492.

Jentjens, R. L., C. Shaw, T. Birtles, R. H. Waring, L. K. Harding, and A. E. Jeukendrup (2005b). Oxidation of combined ingestion of glucose and sucrose during exercise. Metabolism 54(5):610-618.

Jentjens, R. L., K. Underwood, J. Achten, K. Currell, C. H. Mann, and A. E. Jeukendrup (2006). Exogenous carbohydrate oxidation rates are elevated after combined ingestion of glucose and fructose during exercise in the heat. J. Appl. Physiol. 100(3):807-816.

Jentjens, R. L. P. G., J. Achten, and A. E. Jeukendrup (2004c). High oxidation rates from combined carbohydrates ingested during exercise. Med. Sci. Sports Exerc. 36(9):1551-1558.

Jeukendrup, A., F. Brouns, A. J. Wagenmakers, and W. H. Saris (1997). Carbohydrate-electrolyte feedings improve 1 h time trial cycling performance. Int. J. Sports Med. 18(2):125-129.

Jeukendrup, A. E., A. J. Wagenmakers, J. H. Stegen, A. P. Gijsen, F. Brouns, and W. H. Saris (1999). Carbohydrate ingestion can completely suppress endogenous glucose production during exercise. Am. J. Physiol. 276(4 Pt 1):E672-E683.

Jeukendrup, A. E., N. P. Craig, and J. A. Hawley (2000a). The bioenergetics of World Class Cycling. J. Sci. Med. Sport 3(4):414-433.

Jeukendrup, A. E., and R. Jentjens (2000b). Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research. Sports Med. 29(6):407-24.

Jeukendrup, A. E., K. Vet-Joop, A. Sturk, J. H. Stegen, J. Senden, W. H. Saris, and A. J. Wagenmakers (2000c). Relationship between gastro-intestinal complaints and endotoxaemia, cytokine release and the acute-phase reaction during and after a long- distance triathlon in highly trained men. Clin. Sci. (Colch) 98(1):47-55.

Jeukendrup, A. E. (2004). Carbohydrate intake during exercise and performance. Nutrition 20(7-8):669-677.

Jeukendrup, A. E., L. Moseley, G. I. Mainwaring, S. Samuels, S. Perry, and C. H. Mann (2006). Exogenous carbohydrate oxidation during ultraendurance exercise. J. Appl. Physiol. 100(4):1134-1141.

Maresh, C. M., J. A. Herrera-Soto, L. E. Armstrong, D. J. Casa, S. A. Kavouras, F. T. Hacker, Jr., T. A. Elliott, J. Stoppani, and T. P. Scheett (2001). Perceptual responses in the heat after brief intravenous versus oral rehydration. Med. Sci. Sports Exerc. 33(6):1039-1045.

Maughan, R. J., L. R. Bethell, and J. B. Leiper (1996). Effects of ingested fluids on exercise capacity and on cardiovascular and metabolic responses to prolonged exercise in man. Exp. Physiol. 81(5):847-859.

McConell, G. K., B. J. Canny, M. C. Daddo, M. J. Nance, and R. J. Snow (2000). Effect of carbohydrate ingestion on glucose kinetics and muscle metabolism during intense endurance exercise. J. Appl. Physiol. 89(5):1690-1698.

Mitchell, J. B., D. L. Costill, J. A. Houmard, W. J. Fink, D. D. Pascoe, and D. R. Pearson (1989). Influence of carbohydrate dosage on exercise performance and glycogen use. J. Appl. Physiol. 67(5):1843-1849.

Murray, R., J. G. Seifert, D. E. Eddy, G. L. Paul, and G. A. Halaby (1989). Carbohydrate feeding and exercise: effect of beverage carbohydrate content. Eur J. Appl. Physiol. 59:152-158.

Neufer, P. D., D. L. Costill, M. G. Flynn, J. P. Kirwan, J. B. Mitchell, and J. Houmard (1987). Improvements in exercise performance: effects of carbohydrate feedings and diet. J. Appl. Physiol. 62(3): 983-988.

Nicholas, C.W., C. Williams, H.K. Lakomy, G. Phillips, and A. Nowitz (1995). Influence of ingesting a carbohydrate-electrolyte solution on endurance capacity during intermittent, high-intensity shuttle running. J. Sports Sci. 13(4):283-290.

Palmer, G. S., M. C. Clancy, J. A. Hawley, I. M. Rodger, L. M. Burke, and T. D. Noakes (1998). Carbohydrate ingestion immediately before exercise does not improve 20km time trial performance in well trained cyclists. Int. J. Sports Med. 19:415-418.

Peters, H. P., M. Bos, L. Seebregts, L. M. Akkermans, G. P. van Berge Henegouwen, E. Bol, W. L. Mosterd, and W. R. de Vries (1999). Gastrointestinal symptoms in long-distance runners, cyclists, and triathletes: prevalence, medication, and etiology. Am J. Gastroenterol. 94(6):1570-1581.

Peters, H. P., J. W. Wiersma, J. Koerselman, L. M. Akkermans, E. Bol, W. L. Mosterd, and W. R. de Vries (2000). The effect of a sports drink on gastroesophageal reflux during a run-bike-run test. Int. J. Sports Med. 21(1):65-70.

Pirnay, F., J. M. Crielaard, N. Pallikarakis, M. Lacroix, F. Mosora, G. Krzentowski, A. S. Luyckx, and P. J. Lefebvre (1982). Fate of exogenous glucose during exercise of different intensities in humans. J. Appl. Physiol. 53:1620-1624.

Powers, S. K., J. Lawler, S. Dodd, R. Tulley, G. Landry, and K. Wheeler (1990). Fluid replacement drinks during high intensity exercise: effects on minimizing exercise-induced disturbances in homeostasis. Eur. J. Appl. Physiol. 60:54-60.

Rehrer, N. J., M. van Kemenade, W. Meester, F. Brouns, and W. H. M. Saris (1992a). Gastrointestinal complaints in relation to dietary intake in triathletes. Int. J. Sport Nutr. 2:48-59.

Rehrer, N. J., A. J. M. Wagenmakers, E. J. Beckers, D. Halliday, J. B. Leiper, F. Brouns, R. J. Maughan, K. Westerterp, and W. H. M. Saris (1992b). Gastric emptying, absorption and carbohydrate oxidation during prolonged exercise. J. Appl. Physiol. 72(2):468-475.

Riebe, D., C. M. Maresh, L. E. Armstrong, R. W. Kenefick, J. W. Castellani, M. E. Echegaray, B. A. Clark, and D. N. Camaione (1997). Effects of oral and intravenous rehydration on ratings of perceived exertion and thirst. Med. Sci. Sports Exerc. 29(1):117-24.

Saris, W. H. M., M. A. van Erp-Baart, F. Brouns, K. R. Westerterp, and F. ten Hoor (1989). Study on food intake and energy expenditure during extreme sustained exercise: the Tour de France. Int. J. Sports Med. 10(suppl.1):S26-S31.

Seckl, J. R., T. D. Williams, and S. L. Lightman (1986). Oral hypertonic saline causes transient fall of vasopressin in humans. Am J. Physiol. 251(2 Pt 2):R214-R217.

Wallis, G. A., S. E. Yeo, A. K. Blannin, and A. E. Jeukendrup (2007). Dose-response effects of ingested carbohydrate on exercise metabolism in women. Med. Sci. Sports Exerc. 39(1):131-138.

Welsh, R. S., J. M. Davis, J. R. Burke, and H. G. Williams (2002). Carbohydrates and physical/mental performance during intermittent exercise to fatigue. Med. Sci. Sports Exerc. 34(4):723-731.

S U P P L E M E N T

OPTIMIZING CARBOHYDRATES DURING EXERCISE

THE BENEFITS of consuming carbohydrates during endurance exercise are well known, but what is the optimal type and amount of carbohydrate intake? Too much or the wrong type of carbohydrate can cause bloating, nausea, and other symptoms of digestive disturbances; too little will have no real benefit to your performance. Table 1 provides recommendations for the optimal types and amounts of carbohydrate to be consumed for various types of exercise. Note that extreme endurance events like the Tour de France require fairly large amounts of carbohydrate to maintain energy balance. If the athlete is not careful, consuming these large amounts of carbohydrate could cause digestive problems. Combinations of carbohydrates (glucose and fructose for example) ingested at high rates seem to minimize the negative side effects and optimize carbohydrate delivery in these situations.

Strategies for Ingesting Carbohydrate
When? Carbohydrate ingestion often can enhance performance during exercise of 45 min or longer. So to maintain or improve the quality of a training session or to enhance your performance in competition, consuming some form of carbohydrate will probably help. If it is logistically possible in your event, you should consume a carbohydrate-containing sports drink every 15-20 minutes. Otherwise, you should drink during recovery periods or breaks in the training session or competition.

What Type of Carbohydrate? Some types of carbohydrates deliver energy at higher rates than others. The greatest rates of energy delivery occur when you ingest a combination of two or more types of carbohydrates. Examples of suitable combinations include maltodextrins and fructose, glucose and fructose, or glucose, sucrose and fructose.

How Much Carbohydrate? How much you ingest depends on a number of factors, including:

· The intensity and duration of exercise (See Table 1)

· The type of carbohydrate (or combination of carbohydrates)

· Your individual tolerance for various volumes and concentrations of carbohydrate solutions. Only trial and error with different drinking schedules during training sessions and in competitions will enable you to discover the best carbohydrate/fluid feeding schedule for you.

How? Although carbohydrates in solid foods can deliver carbohydrate, they cannot deliver fluid, which is especially critical in hot environments. Highly concentrated carbohydrate solutions can slow fluid delivery, so you should use a well-formulated sports drink containing not more than 7% carbohydrate (7 g/100 ml or 16.3 g/8 oz). Drink 240-600 ml (8-20 oz) of water or a sports drink about 10-15 minutes before exercise to stimulate fluid delivery from the stomach and then keep the stomach volume high by drinking smaller amounts of a sports drink every 15-20 minutes during exercise. Drink enough to minimize the body weight you typically lose during a similar type of training session or competition, but do not drink so much that you gain weight. (Drinking too little or too much can be dangerous to your health.)

SUGGESTED ADDITIONAL RESOURCES
Jeukendrup, A.E. (2004). Carbohydrate intake and exercise performance. Nutrition 20:669-677.

Jeukendrup, A.E., and R. Jentjens (2000). Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research. Sports Med.. 29:407-424.

Jeukendrup, A.E., R.L. Jentjens, and L. Moseley (2005). Nutritional considerations in triathlon. Sports Med.. 35:163-181.