Maximising Performance
The specific nutritional requirements of athletes depend on many factors, including the type of sport, the intensity, duration and frequency. Moreover individual preferences and aversions have to be taken into account.
It is well-recognized that an excellent nutritional status is a prerequisite for athletic performance at the highest levels. It is the aim of this article to give an overview of the generally accepted views about optimal nutrition for sports people and to address particularly the measures that are indicated to meet the specific needs of athletes. The specific nutritional requirements of athletes depend on many factors, including the type of sport, the intensity, duration and frequency. Moreover individual preferences and aversions have to be taken into account.
Creating Balance
Maintenance of energy balance at the desirable body weight is a prerequisite for physical performance. Sports related physical activity increase the energy requirements and may lead to a negative energy balance. A continuing negative energy balance may result in loss of energy stores and even muscle mass and decreased performance. Whereas sedentary adults with light to moderate physical activity display energy expenditures in the order of 8.5 to 12.0 MJ per day, athletes may spend an additional 2-4 MJ per hour of exercise depending on the duration and intensity of the exercise in training and competition. It is essential to restore energy stores of glycogen in muscles and liver and of fat in the adipose tissue. Restoration of energy stores is also essential to neutralise the decrease of the activity of the immune system, which results from the exercise stress (3).
Optimal Weight
Body weight is a critical factor in physical performance in several types of sports. Athletes with excess body weight will adapt their energy intake until the individual optimal body weight has been reached. Generally, there is no need for the use of specific slimming products, because at high energy expenditure levels, it is not difficult to reach safely and gradually the optimal weight, simply by reducing energy intake from a balanced diet.
In weight related sports, like judo and boxing, it may occur that rapid loss of weight is needed to enter in a certain weight category. If this is not done properly, it may result in loss of performance. Decreased energy intake could result in severe reduction of glycogen stores. The water lost in addition with glycogen may in total result in the required body weight loss. However, dehydration as a consequence of this fluid loss is known to affect performance negatively. Therefore, the acute strategies are detrimental for physical performance.
Specific Requirements
In addition to a balanced basic diet providing all essential nutrients in adequate amounts, athletes, and especially endurance athletes, have specific requirements that cannot always be met easily by traditional foods or drinks, like soft drinks, dairy products, bread and other basic foods and normal meals. It occurs frequently in the life of the athlete that the time between exercise events is short, whereas in these short periods specific nutritional requirements need to be met. Under these conditions the use of specific foods or drinks may be desirable. Moreover, athletes may prefer to use nutrient supplements and ergogenic aids to increase performance, although it is recognized within the scientific community that nutrient supplements are without any beneficial effect when the habitual diet is well balanced and that only for a small number of bioactive substances (caffeine, creatine and some alkalising agents) the scientific evidence for a performance enhancing effect is sufficient (2).
High in Carbs
It is well-recognized that a high-carbohydrate diet increases the time to exhaustion in the endurance athlete performing at energy expenditure levels corresponding to more than 50% of the maximal oxygen consumption (VO2-max). Under these circumstances the metabolic fuel consists of muscle glycogen in addition to fatty acids. Increasing the level of exercise requires a further shift to the use of glycogen instead of fatty acids as fuel. This is so because per unit of time oxidation of carbohydrates results in a higher energy production than oxidation of fatty acids. Exhaustion of stores of glycogen in muscles and liver causes loss of power and is associated with heavy fatigue, called by athletes ‘meeting the man with the hammer’. It has been shown that ingestion of a high carbohydrate diet (approximately 9-10 g/kg body weight per day) for 3 days before the competition will maximise glycogen stores in muscles and liver and will delay the time to exhaustion. Glycogen stores in the body are limited to an amount of 400-600g and this is only sufficient for high-intensity endurance performance for not more than about 1.5-2 hours. By training, lipolytic capacity in adipose tissue and oxygen supply to muscles is improved and this results in a better utilisation of fatty acids as metabolic fuel, saving the glycogen stores and resulting in prolongation of time to exhaustion.
The high-carbohydrate diet preceding the competition (or training) is one of the important nutritional issues that enhance athletic endurance performance. This is illustrated in table 1, derived from the classical experiment of Bergstrom et al. (4) with subjects on a cycle -ergometer performing to exhaustion at approximately 75% of their VO2-max.
Table 1. Glycogen content in muscles (g/100 g wet tissue) and time to exhaustion (min)
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Diet Muscle glycogen content Time to exhaustion
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High fat and protein 0.63 56.9
Mixed 1.75 113.6
High carbohydrate 3.31 166.5
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Reloading Glycogen
To top off the glycogen stores it is recommended (5) to use a carbohydrate rich meal 3-4 hours before competition.
To prolong further the time to exhaustion, the endurance athlete should ingest rapidly available carbohydrates (high glycaemic index) during exercise at the opportunities that are available. Carbohydrate-electrolyte solutions are convenient and effective also to prevent dehydration. Such drinks should not be hypertonic, because this will delay gastric emptying and may cause gastro-intestinal side effects (osmotic diarrhoea). It has been established that an upper limit exists of 1 g per minute for the oxidation of carbohydrate administered during exercise (30-60g/hour) (6). Glucose absorption is enhanced by sodium. All together it is recommended for drinks taken during exercise to contain 20-50 mmol Na per liter at an osmolality between 200 and 300 mOsm/l (2), which is slightly lower than the osmolality of the blood.
Post-Exercise
In the post-exercise period it is important for the athlete to reload the glycogen stores as soon as possible. This is particularly important when a next competition or training is close, for instance the game or next day. Replenishment of glycogen stores requires approx. 16-20 hours and in this period a high carbohydrate intake is needed. It has been shown that carbohydrates taken in the first minutes after exercise result in a more rapid repletion than carbohydrates taken after 2 hours.
Muscle glycogen resynthesis is near optimal (5-7% per hour) when carbohydrate intake rate is about 50 g every two hours. The preferred type of carbohydrate to ingest during the first 4 hours should have a high glycaemic index. This type of carbohydrate strongly stimulates insulin secretion facilitating rapid uptake of glucose by liver and muscles. Co-ingestion of proteins or protein hydrolysates, because of insulinotropic effects, may enhance the rate of glycogen replenishment by 4-8 hours (2). In the later stages of glycogen replenishment, carbohydrates with a lower glycaemic index are recommended.
Loss of Water
Dehydration of as little as 2% of the body weight is known to result in decreased performance (6). Heavy endurance exercise in hot climates will cause loss of water (and electrolytes) via sweat in order to maintain the normal body temperature. This loss of water may reach levels of more than 2 liters per hour. Sweat contains proportionally more sodium and less potassium than blood plasma, so that exercise and heat exposure can cause an increased concentration of electrolytes in the plasma. A sodium deficit in the body of ultra endurance athletes may occur when electrolyte losses are not compensated, while water losses are. This results in hyponatraemia, also called water intoxication. This situation is prevented by the ingestion of carbohydrate drink containing sodium as indicated above.
Disturbance of thermoregulation and dehydration causes loss of cognitive functions, skills and an increased risk of gastro-intestinal problems. It is recommended that the athlete compensates the fluid lost by sweating and uses the opportunities within their activities to drink as much as is practical (6). It is further of importance to train fluid intake during training to get accustomed to the possible gastro-intestinal side effects and adapt to the amount and type of fluid.
Protein Intake
A lot of attention has been given in the past to protein nutrition of athletes, having in mind particularly the seeming relationship between dietary protein and muscle mass. Opinions about protein requirements of athletes have varied widely in the past. Nowadays, the generally accepted view is that a normal balanced diet will provide adequate amounts of protein for athletes in all kind of sports. In fact, protein intake levels in the range of 1.2 g/kg of body weight (endurance athletes) – 1.4 g/kg body weight (weight lifters) – a little higher than those recommended for the general population –, will largely cover the protein needs of the athlete (2). When the energy expenditure of the athlete is taken into account, a diet that provides 10-12% of the energy as protein, will largely meet the protein requirements and a normal balanced diet is adequate to supply this.
Glycogen Resynthesis
In the early post-exercise period, protein or protein hydrolysate intake may accelerate glycogen resynthesis via stimulation of insulin secretion. This means that particularly proteins that are rich in amino acids that stimulate insulin secretion can have a place in this regard. It has been shown that the amino acid pattern of dietary proteins may have different effects on insulin secretion. Insulin secretion appeared to be correlated with plasma levels of leucine, phenylalanine and arginine (9). Taking into account the amino acid composition of whey proteins as compared to casein or soy protein, there may be an advantage for whey only for leucine, but the rapid digestibility and uptake of amino acids from whey proteins could have a boosting effect on insulin secretion and glucose uptake by muscles in the post exercise period. Further research would be needed to substantiate a specific benefit for whey proteins in the post exercise period.
Restricting Fats
Sport people are advised to ingest a high carbohydrate diet with approximately 60-70 % of the energy originating from carbohydrates. Taking into consideration that at least 10% of the energy should originate from proteins, athletes should restrict their fat intake to about 20-30 % of energy intake. This amount is sufficient for adequate intake of indispensable fatty acids contained in the fat, and which are required by the body for specific physiological functions. Depending on their structure, these essential fatty acids are of n-3 or n-6 type. The ratio between n-6 and n-3 essential fatty acids should preferably be below 5 (2).
There is evidence of weak anti-inflammatory effects of long chain n-3 fatty acids as occur in fatty fish as well as of gamma linolenic acid (from evening primrose oil). Such fatty acids could be of benefit for the athlete to reduce the inflammatory response to physical stress. The use by athletes of medium chain triglycerides as a source of rapidly available energy should no longer be recommended, because of their gastro-intestinal side effects and lack of efficacy to enhance performance (2).
Micronutrients
The general expert view is (2) that micronutrient requirements (vitamins, minerals, trace elements) of athletes are not higher than those of the general population and this means that the levels recommended for the general population also apply to athletes and that there is no need for micronutrient supplements when the diet of the athlete is balanced.
However, during periods of competition or heavy training, for a variety of reasons, the diet may not be optimal regarding micronutrient intake and in those cases micronutrient supplements could be useful. It should however be pointed out that intake levels of some vitamins (A and D), minerals (Mg) and trace elements (Fe, Zn, Cu, Se) in excess of the recommended daily allowances can do more harm than good. A lot of attention has been given to the requirements of athletes of antioxidant vitamins, e.g. vitamin C, vitamin E and beta-carotene. There is no evidence that intake levels by athletes of these vitamins in excess of those recommended for the general population do have any beneficial effect, despite the higher oxidant stress caused by exercise. It is well-known that the antioxidant capacity of the athlete is enhanced by training. Training results in higher activities of the endogenous antioxidant systems of super oxide dismutase and catalase. Therefore the need of athletes for exogenous antioxidants is reduced to levels similar to those of the general population.
Ergogenic Aids
In spite of the fact that numerous preparations are offered on the market to enhance performance, scientific evidence for their efficacy is mostly lacking. The majority of these supplements are not effective and some do more harm than good and buying them is really spilling money. Accepted ergogenic aids with a proven efficacy are caffeine, creatine (2) and alkalising agents as bicarbonate and sodium citrate (6,7). Of other substances, well-known in the sports world (e.g. L-carnitine, medium chain triglycerides, branched chain amino acids), the scientific basis for efficacy as ergogenic aid is still insufficient and their use should not be recommended.
Enhancing with Caffeine
Caffeine is a normal constituent of coffee, tea and some soft drinks. One cup of coffee may contain between 50 and 150 mg of caffeine. Caffeine ingestion, shortly before exercise, at levels between 3-8 mg/kg body weight, enhances performance in short term intense exercise and prolonged endurance exercise (2). Three explanations have been given for the ergogenic effect of caffeine: (1) a stimulatory effect on the neural signals between brain and neuromuscular junction, (2) a direct effect on muscles by increasing the second messenger cAMP in muscle cells and (3) enhancing fat oxidation, thereby sparing glycogen stores.
Dietary Peptide
Creatine is a non-essential dietary peptide-like compound. It is synthesized within the body from the amino acids arginine and glycine and is found in substantial amounts in meat and dairy products. Creatine-phosphate in the muscles serves as a source of energy and contributes substantially to ATP resynthesis from ADP after short-duration-high intensity exercise. Short-term creatine supplementation (loading dose for 5-7 days of 20 g/day) augments creatine concentration of muscles and can lead to an improvement of performance, particularly in repetitive high power output exercise bouts (2). After the loading period it is recommended by manufacturers to ingest a maintenance dose of 2-5 g/day.
The use of alkalising agents at least one hour before exercise such as sodium citrate (at ingestion levels of 500 mg/kg body weight) and sodium bicarbonate (at ingestion levels of 300 mg/kg body weight) to neutralize the acid formed in the body during exercise has been reported to improve performance in exercise of short - and moderate duration (between 1 and 7 minutes), but no significant effects on performance have been established during periods less than 1 minute or greater than 7 minutes (12).
Future Challenges
As stressed by Wilmore and Freund (8), athletes are very complex models to study, being unique and not representing a ‘normal’ population. Their performance is often substantially influenced by psychological factors. Since nutrition is much more than just the delivery of the good nutrients at the right moments and may have a large impact on the mood status of the athlete, individual preferences should be taken into account in advising athletes about nutrition. Challenges for TNO Quality of Life will be focused on innovations in food, which, in the end, should be beneficial for athletes.
Further individualisation of nutrition advice is certainly a future issue; therefore it is important that the recent knowledge about sports nutrition is up-to-date on the level of coaches, trainers and athletes. Knowledge transfer using courses to communicate recent available developments in nutrition to target groups with an emphasis on practical applications is recommended, e.g. International Advanced Course Nutrition & Sports, First edition November 2004, Zeist & Arnhem, The Netherlands organized by the Graduate School VLAG, in cooperation with the Graduate School WIAS, NOC*NSF, TNO Quality of Life and TNO Defense, Security and Safety.
Convenience Foods
Nearby is the challenge for the food industry to design new tasteful and healthy convenience foods, especially designed for the athlete, who is frequently in situations that wholesome meals are not available at the right moments. This may result too often in fast food consumption and snacking, resulting in a higher fat intake than recommended. Optimal composition adjusted to relevant conditions and application of innovative technology for preparation, preservation and packaging of high-quality convenience foods can be applied in the development of these foods.
Further fundamental research is needed in the search for a biomarker for physical load. It is of utmost importance to establish on an individual level if athletes need to rest or is well enough recovered for an extra training. Strain due to exercise has also adverse effects on the condition and resistance to infections. Because there is not just one biomarker to monitor this condition of the human body, research is needed to identify a biomarker pattern that could provide direct information about the catabolic or anabolic state of sportspeople. Such a pattern could eventually serve as a biomarker pattern to establish effects of functional foods aimed at accelerating the recovery of athletes.
Mood and Cognitive
Up to now little attention has been given to the effects of diet on mood status and cognitive functions of the athlete and on the impact these effects may have on physical performance. In this regard the close relation between the brain and the functioning of the gut deserves more attention in sports nutrition. Foods or drinks containing tryptophan-enriched proteins or protein hydrolysates are a promising approach to influence the mood status of athletes.
In conclusion, up-to-date information on food and exercise performance, innovative food products as well as fundamental research on physical stress and mood, will be the main challenges for TNO Quality of Life in the field of sports nutrition.
By Robin van den Berg, Gertjan Schaafsma and Wilrike Pasman of TNO Quality of Life in Zeist, The Netherlands. For more information please contact food-office@voeding.tno.nl
References
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http://www.gnc.com/wellness/natpharm/Concern/Athletic_Performance.htm.
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