Athletics. W. H. M. Saris

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1 CHAPTER CONTENTS Energy and substrates for physical performance 472 Carbohydrate 472 Carbohydrate loading 474 Carbohydrate supplementation during exercise 474 Fat 475 Athletics W. H. M. Saris Creatine and phosphocreatine 476 Protein 476 Fluids and electrolytes 477 Vitamins and minerals 478 Ergogenic supplements 478 Key points 479 'You are what you eat' and 'eat to win' are popular statements in the athletic world and influenced the sporting population greatly during the last decade. Nutrition has become important for performance now that athletes have reached limits in training volume and intensity. This has led to renewed interest among athletes, coaches and exercise physiologists in the role of nutrition and the influence of gastrointestinal issues on physical performance and wellbeing. Clear-cut nutritional counselling, however, is often difficult for a number of reasons. First, there is no generally accepted nutritional recommendation for athletes who are involved in heavy physical training. In contrast, most countries have accepted standards for the recommended dietary allowance (RDA) of nutrients for different age groups and sexes, as well as for different levels of physical activity. The RDAs for those who are involved in heavy physical work, are normally derived from studies done among miners and lumberjacks. Information is lacking about the daily requirements of athletes who differ from miners and lumberjacks with respect to relative work intensity. Secondly, the elite athlete's food in particular, frequently has an ambience of secrecy and magic. All sorts of products, such as the reindeer milk which came onto the scene during the Montreal Olympics, or the special Chinese herbals which were presented during the Barcelona Olympics, are said to enable sportsmen to break records. The use of food in this fashion is indeed of very ancient origin. The first recorded reports date from the early days of sporting activity, around 500 BC in Greece. A very realistic conclusion that is found in the literature is that a well balanced diet is all that athletes actually require for peak performances. The question remains as to what extent this well balanced diet differs from other diets, that is, for the athlete and the rest of the population. According to results from an increasing number of scientific studies, dietary manipulation can lead to improvement of physical performance. Therefore, in the past decade, the area of sport foods and, in particular, the carbohydrate 471

2 472 DIET FOR SPECIAL GROUPS electrolyte solutions (CES) have developed rapidly. At the same time an increasing interest in functional food ingredients, which may affect body functions in a positive way, has been observed. In this respect sports nutrition has provided the first examples of scientifically developed concepts and products in the area of functional foods. Unfortunately, in such a booming market the number of 'exotic' sports drinks and supplements, which claim to improve performance with no reasonable scientific justification at all, is growing ever faster. Thus it has become necessary to discriminate between facts and fantasy. In this chapter, an overview is given of the most important aspects of the athletic diet and the problems that occur in the case of strenuous exertion. ENERGY AND SUBSTRATES FOR PHYSICAL PERFORMANCE The first and most clear difference in nutritional needs between athletes and non-athletes is energy. The energy expenditure of a sedentary adult amounts to approximately MJ per day. Physical activity by means of training or competition will increase the daily energy expenditure by 2-A MJ per hour, depending on physical fitness, and duration, type and intensity of sporting activity. For this reason, athletes must adapt their energy intake by increased food consumption to meet energy needs, according to the level of daily energy expenditure. This increased food intake should be well balanced with respect to the macronutrients and micronutrients. However, this is not always easy. Many athletic events are characterized by high exercise intensities. As a result, energy expenditure over a short period of time may be extremely high. For example, to run a marathon will expend about MJ. Depending on the time to finish, this may induce an energy expenditure of approximately 3.2 MJ/ hour in a recreational athlete and 6.3 MJ/hour in an elite athlete who finishes in approximately 2 to 2.5 hours. A professional cycling race like the Tour de France will cost an athlete about 27 MJ/day, a figure that will increase to approximately 40 MJ/day when cycling over mountain passes (Saris et al 1989a). Compensating for such high energy expenditure by ingesting normal solid meals will pose a problem to any athlete involved in such competitions, since the digestion and absorption processes will be impaired during intensive physical activity. These problems are not restricted to competition days. During intensive training days, energy expenditure is also high. In such circumstances, athletes tend to ingest a large number of 'in-between meals', contributing up to 40% of the total energy compared to about 25% in the general population (Van Erp-Baart et al 1989). These 'in-between meals' are often composed of energy-rich snacks, but unfortunately they are often low in protein and micronutrients. As such, the diet becomes imbalanced. Specially adapted nutritious foods/fluids which are easily digestible and rapidly absorbable, may solve this problem. In addition to the energy needs and the limited capacity and time to digest and metabolize, the selection of fuels in the muscle is an important issue. Metabolic capacity and power output depend on this selection. For maximal muscle performance, the muscle cell depends completely on carbohydrate (CHO) as a substrate. Therefore, diet selection is not only a matter of energy but also of substrate sources, especially carbohydrate and fat. At the other end of the energy expenditure scale, it is found that energy intake, especially for females, amongst gymnasts and ballet dancers is often extremely low (Van Erp-Baart et al 1989). Energy expenditure in the average sedentary subject ranges between 1.4 and 1.6 times basal metabolic rate (BMR), while reported energy intake in top gymnasts is usually below this level, despite the fact that they train for 3-i hours each day. This can probably be explained by the two factors of underreporting and the urge to limit energy intake, the former factor being a result of the latter. Whether the intake data are realistic or not, some sports athletes often limit their energy intake to reduce body mass and fat mass, with consequences for energy turnover and probably performance. On one hand, these athletes are training frequently and intensively, but on the other they have to maintain a low bodyweight. The low energy intakes in these circumstances may lead to a low intake of essential nutrients such as protein, iron, calcium, zinc, magnesium and vitamins. The required CHO intake to balance the CHO used in training, may also be marginal. This aspect should receive special attention since many of these athletes are young and still in a period of growth and development. CARBOHYDRATE Carbohydrate is the most important fuel for high-intensity exercise and performance. Since the human body has only limited stores of CHO as glycogen in muscle and liver, oral supplementation will definitely affect performance. This is especially true when the endogenous CHO stores are emptied, which normally occurs after min of moderately intense exercise. During the past decade, sports nutrition research has focused attention on this problem and on how nutritional support can be of benefit. The body CHO reserve is limited to about 400 g of glycogen, in liver (100 g) and muscle (300 g). With the introduction of the muscle biopsy technique, by Bergstrom & Hultman (1966), it became clear that muscle/liver glycogen concentration plays a pivotal role in the onset of fatigue. The muscle glycogen concentration is influenced by

3 ATHLETICS 473 exercise, dietary carbohydrate intake and training status. The principal role of the liver glycogen reserve is to maintain a constant plasma glucose level between meals or during exercise, thereby ensuring an adequate substrate supply for those organs that depend upon glucose for their energy needs, such as the brain. In rest, the utilization of muscle glycogen is negligible. However, with increasing exercise intensity the contribution increases to up to % at intensity levels higher than 80-90% of the maximal aerobic power. Muscle fibres recruited during anaerobic exercise, such as sprinting and weight lifting, rely heavily on the cellular content of high-energy phosphates (ATP, phosphocreatine) and glycogen-derived glucose. Under such conditions, fatty acids are less suitable substrates because the chemical energy stored in the lipid molecule can only be utilized for ATP regeneration when a sufficient amount of molecular oxygen is available. During highintensity exercise this is an important limiting factor. The maximum rate of high-energy phosphate formation from lipid oxidation is too low to match the rate of utilization of high-energy phosphates (HEP) during highintensity exercise. In Table 30.1 the maximum rate of HEP formation in the muscle from the different fuel sources is presented as well as the rate of utilization during different types of running exercise. This illustrates the limitation in the power output of the skeletal muscle fibre. The endogenous muscle energy stores of glycogen and triglycerides are especially important when energy demands are higher with the increasing power output and, at the same time, oxygen and substrate supply by the circulation is limited. Using the stable isotope techniques, Romijn et al (1993) investigated the energetic contribution of the different substrates during exercise at three different levels of intensity (25%, 65% and 85% of maximal aerobic power (V02max) respectively). Figure 30.1 shows that, with increasing intensity, the skeletal muscle depends more and more on endogenous fuel stores. On the other hand, at 65% V02max, about 50% of the fuel mix is delivered by the circulation. Since the rate of ATP production from fat is considerably slower, it is clear that in the case of glycogen depletion E o> "O <D -o c LU >. O) 0 1 I i % Maximal oxygen consumption I j Muscle glycogen I Muscle triglyceride Plasma FFA Plasma glucose Fig Contribution of substrates to the muscle energy expenditure at different levels of exercise intensity; FFA, free fatty acids (Romijn et al 1993). in the muscle, exercise intensity is limited since the ATP production will slow down due to the reliance on fat as a substrate. This phenomenon is often referred to as 'hitting the wall' and usually occurs after about 90 minutes of intense exercise. Based on these physiological observations, it became clear that the level of pre-exercise muscle glycogen concentration as well as the CHO ingestion during exercise are of importance since these affect the time of onset of fatigue. That symptoms of fatigue are related to glycogen depletion in the exercising muscle was shown in the early muscle biopsy studies by Bergstrom & Hultman (1966). Thus, loss of CHO ranks directly behind dehydration and electrolyte imbalance as a major cause of fatigue. CHO status is directly related to intake rather than to other variables. Thus, nutritional modulation of CHO status has been explored more in detail to enhance or prolong performance. The liver seems to be the central coordinator for an optimal circulatory glucose homeostasis when the CHO demand in the muscle is increased due to the level of exercise. The hepatic glucose production (HGP) will then be increased up to about 1 g/min. This HGP is derived from liver glycogen or from CHO absorption in the gastrointestinal tract and transport through the portal vein to the liver. Ingestion of CHO during exercise will increase the contribution of so-called exogenous CHO to muscle CHO oxidation and decrease Table 30.1 Maximal rate of high-energy phosphate (HEP) (ATP and phosphocreatine) production and utilization in skeletal muscle Substrate utilized Maximum rate of formation mol HEP/min Type of exercise Rate of utilization mol HEP/min Endogenous glycogen* Endogenous glycogen 1 " Bloodborne glucose* Bloodborne fatty acids f Rest 100 m sprint 1500 m sprint Marathon Anaerobic. taerobic.

4 474 DIET FOR SPECIAL GROUPS the contribution of CHO originating from the liver glycogen pool. If there is no CHO intake, the HGP depends entirely on liver glycogen and hepatic gluconeogenesis to produce a constant outflow of glucose and to prevent hypoglycaemia, which is in itself a life-threatening situation. Studies have shown that, although the oxidative capacity of the muscle for CHO coming from the circulation is up to 2 g/min, the liver will not allow an increase in glucose output and thus muscle uptake greater than about 1 g/min (Rehrer et al 1992). This regulatory control mechanism also limits the contribution of exogenous CHO, taken in the form of CHO drinks during the exercise, to the oxidative process in the exercising muscle, to a maximum of about 1 g/min. Therefore, the important determinants of endurance performance that can be manipulated by dietary habits are the pre-exercise levels of liver/muscle glycogen, and CHO intake during endurance exercise in order to ensure the maximal exogenous CHO contribution to the CHO oxidation rate in the muscle. Carbohydrate loading To ensure maximal glycogen stores at the start of an important event, several dietary regimens have been proposed. The currently preferred method, suggested by Sherman & Lamb (1988), is to keep the body saturated with CHO in order to maximize tissue glycogen levels. To accomplish this goal, an athlete actually has to consume 60-70% of dietary energy as CHO, up to g CHO daily. This amount of CHO-rich food is equivalent to approximately three loaves of bread; 15 cooked potatoes or 15 cups of rice daily. To train for a particular event, exercise is tapered starting a week prior to the event. Every second day, the amount of exercise is halved until no exercise has to be performed on the last day before the event will take place. The high CHO diet is consumed throughout this week. Research showed that this method of glycogen loading, also called supercompensation, yields tissue glycogen levels that improve performance. As mentioned earlier, this CHO loading is indicated for events that last for 1.5 up to 2.0 hours or longer. Carbohydrate supplementation during exercise Despite the increase of blood glucose levels, there is little evidence that CHO ingestion improves performance for events shorter than 1 hour. Recently, Jeukendrup et al (1997) showed that performance improves when CHO is ingested during intense exercise (80-90% V02max) of about 1 hour. After 1 to 3 hours of continuous exercise at 70-80% V02max athletes fatigue, due to CHO depletion. Particularly when no CHO is consumed during exercise, athletes may experience a premature lowering of blood c I O) CO D X o 0) ra to O 60 CD M -D 50 >> n o Q to O 40 E o 30 E? <5 c CD 20 ra,o 1-10 "o 0 s Fatigue at 70% V02 max due to inadequate CHO energy when fasted fl^^llkt. *? / Blood Blood glucose glucose use when fasted use when fed CHO Muscle glycogen use when fasted or fed CHO i 1 I 1 : Exercise time (h) Fig The percentage of energy and the absolute rate of CHO oxidation derived from muscle glycogen and blood glucose during prolonged exercise at 70-75% V02max (Coyle 1991). glucose. Both depletion of muscle glycogen stores and a less than optimal circulatory supply of glucose will accelerate the process of fatigue. CHO feeding during exercise appears to delay fatigue by as much as min, allowing the exercising muscles to rely mostly upon bloodborne glucose, as energy substrate late in exercise (Coggan & Mendenhall 1992). Interestingly enough, carbohydrate feeding during exercise does not lead to a sparing of muscle glycogen, which is illustrated in Figure 30.2, based on the work of Coyle et al (1986). During the first phase of exercise, most carbohydrate energy is derived from muscle glycogen. When exercise progresses, muscle glycogen is reduced, especially when the glycogen levels are already at a suboptimal level at the start of the exercise. This reduction in contribution to the muscle energy supply is balanced by an increased reliance upon blood glucose, originating from liver glycogen stores or orally ingested CHO. Without CHO feeding, blood glucose concentration normally declines later on in the exercise period. The liver reduces its output of glucose, due to the depletion of liver glycogen stores, when the influx of glucose into the muscle is increased. Without additional CHO feeding fatigue may occur, for there is not enough blood glucose available. It is generally recommended that exogenous CHO intake should at least be 0.5 g/min, and preferably up to g/min, during the exercise period. As shown by Wagenmakers et al (1993), larger intakes do not increase CHO oxidation. Glucose, sucrose and maltodextrines appear to be equally effective in maintaining blood glucose concentrations, CHO oxidation and in improving performance. Fructose, galactose and amylose result in lower CHO oxidation rates (Saris et al 1993). Most efficient are the soluble CHO sources that can be ingested with fluids. Since water is the other most important nutrient needed by athletes, in the past decade

5 ATHLETICS 475 research has been focused on the so-called carbohydrate electrolyte solutions (CES) (see also the section below on Fluids and electrolytes). Depending on the needs, either CHO or fluid availability may become the limiting factor for performance. The exercise duration and intensity, climatological conditions and metabolic status (such as glycogen-depleted versus glycogen-loaded, dehydrated versus euhydrated) will primarily determine whether CHO or water becomes rate-limiting. In general, under hot (and humid) conditions fluid deficit may hamper performance or threaten health before a CHO deficit will occur. In long-lasting, fairly intense sports in relatively cool environments, the possibility is far greater that lack of sufficient CHO will reduce performance before fluid losses are substantial enough to be detrimental. Thus, both climatological conditions and quantitative sweat loss determine the priorities for water and carbohydrate ingestion. In addition to the environmental conditions, gastric emptying and intestinal absorption also play an important role in the selection of the type and amount of carbohydrate. The energy content of the solution, mostly in the form of CHO, is an important regulator for the rate of gastric emptying. Addition of CHO, especially by means of glucose, is important to stimulate the water absorption in the small intestine. An increase in CHO content, however, increases osmolarity and reduces gastric emptying and also decreases the delivery rate of fluid. Within a broad CHO concentration range of 3-8%, this effect is offset by the stimulation of intestinal water uptake in the presence of glucose. Based on these principles, water and CHO uptake can be maximized within a certain period of time. Optimal CHO sources for high-intensity endurance exercise are processed CHO low in dietary fibre, such as glucose, sucrose, maltose and maltodextrine, or soluble starch, such as corn starch. With respect to palatability and gastrointestinal comfort, starch hydrolysates and soluble starch may have the benefit of being less sweet than mono- and disaccharides. These complex CHOs also have less influence on fluid osmolarity at higher concentrations ( g/1). Drinks would be strongly hypertonic with dissolved mono- and/or disaccharides, but not with maltodextrins/polymers or starch. The rate of muscle glycogen resynthesis following exercise is obviously an important factor in recovery. In man, muscle glycogen is restored at a rate of only 3-7% under normal conditions. Therefore, approximately 20 hours are required for the muscle glycogen stores to recover. It appears that the rate of muscle glycogen resynthesis is higher during the first 2 hours after exercise has taken place. Therefore, one should start to eat CHO as soon as possible when the exercise is done. Moderate to high glycaemic index foods (Ch. 5) are more efficient in muscle glycogen resynthesis than low glycaemic index foods (Coyle 1991). Studies on the optimal amount needed for a fast recovery of the glycogen stores showed that ingestion of more than 50 g every 2 hours does not seem beneficial. Recent studies indicate the beneficial effects of additional small amounts of protein or specific amino acids ( g/kg/h) to increase plasma insulin levels which gives an extra activation of the glycogen synthase enzyme. FAT The importance of muscle and liver glycogen and CHO feeding in the reduction of fatigue and the improvement of athletic performance has led to a quite complete understanding of carbohydrate metabolism under exercise conditions. Far less information is available on the role of fat as an endogenous and exogenous substrate during exercise. Large amounts of fat are stored as triglycerides (TG) within the muscle fibres (intramuscular triglycerides; IMTG). Nevertheless, compared to glycogen stores, the rate of fat mobilization from these stores is limited. The reason for this limitation is still not fully elucidated, but the lipophilic characteristic of the muscle slows down transport and needs binding to proteins. In the circulation, free fatty acids are bound to albumin while TG are transported within the low-density lipoprotein types. Different fatty acid-binding proteins (FABP) are active for the transportation of fatty acids in the muscle across membranes and cytosol (Van der Vusse & Reneman 1996). From Figure 30.1 it can be calculated that the rate of total fat oxidation is higher at 65% V02max compared to 25% V02max (i.e. 110 kcal/kg/min versus 70 kcal/ kg/min). At low-intensity exercise, almost all energy is derived from fat. Yet, since the total energy expenditure is so much higher at 65% V02max (2.6-fold higher), the absolute fat oxidation is higher and therefore evaluating the substrate utilization without consideration of the overall energy expenditure is misleading. Another important finding regarding fat as a fuel substrate is the adaptation to increased reliance on fat as a substrate during endurance training. The increase in fat oxidation is accompanied by a reduction in glycogen use at a given submaximal intensity. There is much interest in the origin of this fat. In Figure 30.3, the results of the fuel utilization in the muscle during exercise before and after a 12-week endurance training programme are displayed. The reduction in muscle glycogen oxidation can be explained by the increase in IMTG oxidation. Additional research has shown that a major adaptation, which allows trained subjects to oxidize more fat compared with untrained subjects, seems to be an increase in muscle oxidative capacity rather than in the ability to mobilize more fat from the adipose tissue. Several studies have shown that fat oxidation during exercise is very sensitive to dietary carbohydrates in the hours before the exercise

6 476 DIET FOR SPECIAL GROUPS Before training Intramuscular triglyceride Plasma FFA After training Fig Substrate utilization in the muscle as percentage of total energy expenditure at a given absolute intensity (64% of pretraining V02max); * significant differences before and after training; FFA, free fatty acids. Redrawn from Martin et al (1993). period, even in relatively small amounts. This is partly due to the deviation in plasma insulin in response to the carbohydrate meal and the inhibition of lipolysis of endogenous triglycerides. This suppression of fat oxidation is reversed after about 60 min of exercise. Further stable isotope studies have confirmed these observations and strongly indicate that carbohydrate availability regulates fat oxidation directly during exercise. It is likely that insulin plays an important role in this process. Even diets with a low content of carbohydrate, or those containing carbohydrate which causes less insulin secretion, most probably elicit enough insulin to reduce fat oxidation. Therefore, research has been initiated to study the effects of fat supplements or high-fat diets on performance. The medium-chain triglycerides (MCT) are of particular interest. These C8 to C10 carbons are more soluble in water and gastric emptying is not inhibited. Furthermore, MCT oxidation is not as limited by transport into the mitochondria as that of long-chain triglycerides (LCT) is. Exercise studies with MCT supplementation have revealed that MCT is oxidized almost as rapidly as glucose and even better in combination with glucose (Jeukendrup et al 1996). However, since MCT intake is limited, because of gastrointestinal distress with larger amounts, the contribution to total energy expenditure is limited to about 7%. Also performance studies have not yet shown an effect additional to carbohydrate feeding alone. Besides investigating the type of fat, research has focused on the effect on performance of long-term use of a high-fat diet (Van der Vusse & Reneman 1996). These types of diet may provoke adaptive responses, leading to an increased capacity to oxidize fat and to preserve muscle glycogen. However, at present, the results do not show an improvement of performance and, from a general health perspective, are therefore not recommended to athletes. CREATINE AND PHOSPHOCREATINE When the different energy substrates for performance are discussed creatine (Cr; not to be confused with chromium, for which Cr is the normal abbreviation) must be added to the list of food ingredients which can be of importance, especially in high-intensity short-term exercise, like sprinting (Greenhaff 1995). In healthy individuals, the total Cr pool is approximately 120 g, 95% of which is located in skeletal muscle. Each day, 2 g is replenished by endogenous synthesis and dietary intake, especially through meat and fish. A reversible equilibrium exists between Cr and phosphocreatine (PCr), and the development of fatigue during maximal short-duration exercise has been associated with the depletion of muscle PCr stores. Chronic high levels of Cr ingestion (20 g/day) can lead to an average increase in muscle total Cr of about 20%, of which approximately 30% is in the form of PCr. This increase can reduce the muscle ATP degradation by 25% during maximal isokinetic cycling exercise, leading to a significant increase in work output of about 5%. It has been questioned whether the use of such abnormally high amounts of a regular dietary component should be banned in athletic competition. The regulations of the International Olympic Committee (IOC) prohibit the use of dietary components that are taken in abnormal amounts. Caffeine is one example of such a substance. The doping limit of the IOC could be reached by ingesting eight cups of coffee (100 mg/cup), indicating that athletes have to be careful in coffee consumption prior to the competition. For Cr the normal daily intake is less than 10% of the effective dose and therefore it may come into the category of banned supplements. This also demonstrates that there is a grey area between normal dietary components and what are generally accepted as doping substances, such as anabolic drugs. It is expected that in the near future more normal dietary components will be revealed to be effective in enhancing performance at abnormal intake levels. PROTEIN For many years the effects of exercise on dietary protein requirements has been a controversial topic. Most RDA committees have not provided an additional allowance for protein for active individuals. However, a considerable amount of experimental evidence indicates that regular exercise does in fact increase protein needs. The weight of current evidence suggests that athletes should consume approximately 1.2 to 1.6 g/kg body weight per

7 ATHLETICS 477 day (Lemon 1991). The reason for this increase is an enhanced utilization of amino acids in oxidative energy production during exercise, a process which is known to be intensified at higher work levels and in a state of glycogen depletion. Studies appear to show an accelerated rate of protein turnover in athletes in training who demonstrate increased protein catabolism. However, in general, the increased protein needs are covered by an increased food intake. For example, in a nationwide study in elite athletes, a close relationship was found between protein and energy intake (Van Erp-Baart et al 1989). In the extreme case of cyclists in the Tour de France, this close association between protein and energy intake resulted in an absolute intake of over 3 g protein/kg/day. At the other end of the energy intake scale, it is suggested that restricted food intake could result in an inadequate protein intake. These categories include gymnasts, female long-distance runners, and weight class athletes. The use of single amino acids, such as arginine and ornithine to influence metabolic pathways involved in hormone production, needs further research before definite statements can be made (Lemon 1991). Although amino acids, when supplied intravenously in high dosages, have been observed to enhance hormone release, there is no evidence that this is also the case after oral ingestion of amounts, which are normally present in supplements. Branched-chain amino acids (BCAA; leucine, isoleucine, valine) and tryptophan have been suggested to reduce central fatigue by serotonergic mechanisms. However, well controlled studies failed to show an effect on the postponement of fatigue (Van Hall et al 1995). Based on the experience in clinical nutrition, the ingestion of extra glutamine has recently been proposed as a means to support the immune competence of athletes involved in intense daily training. So far, the available scientific data are too limited for any conclusions to be drawn. FLUIDS AND ELECTROLYTES Most of the energy required for intensive endurance exercise is produced by oxidative metabolism. As such, each litre of oxygen consumed will contribute to an energy production of approximately 20 kj. However, 75% of this energy is released as heat and only 25% is used for mechanical work. In order to avoid hyperthermia, the heat produced must be transferred from the working muscles to the periphery, primarily by the circulating blood. This heat is eliminated at the surface of the body by radiation, convection and evaporation. Radiation and convection are the means by which dry heat is transferred to the immediate surroundings. Heat loss by convection can be increased substantially by streaming air (wind) or water. Evaporation of sweat is the most important way of eliminating heat when working in a warm environment because convection and radiation will be minimal in these circumstances. With intense physical activity, sweat rates will be maximized. At maximal sweat rates, a 70-kg male athlete may lose >30 ml/min or >1800 ml/hour of sweat. Body size, training status, exercise intensity, and weather conditions all influence the sweating rate. One of the highest sweat rates ever reported amounted to 3.71/hour in a 67-kg marathon runner. Without appropriate rehydration, blood flow through the extremities and the skin will decrease, and in addition, sweat response and heat flux will be diminished. This may cause hyperthermia with associated health risks when exercising in the heat. In any situation, dehydration may also impair performance. With sweat loss, electrolytes are also excreted. Because sweat contains lower concentrations of minerals than plasma, and water intake is insufficient to compensate for the losses, the electrolyte concentration in blood normally increases as a result of intensive endurance exercise. When water intake is equal to water loss by sweating, theoretically the plasma electrolyte levels, especially the major electrolytes sodium and chloride, should fall. However, because of the high plasma sodium content and the large extracellular space, this is not apparent until late in exercise, after significant amounts of water have been ingested. For example, if a 70-kg athlete (141 extracellular fluid) lost 61 of sweat (Na + concentration 30 meq/1 or 600 mg/1), then 180 meq Na + would be lost. If the water loss occurs equally from extracellular fluid (containing meq Na + ) and intracellular fluid, then the extracellular Na + concentration would still be above 130 meq/1. This may explain why drinking large amounts of plain water when competing in the heat has only been shown to result in significant hyponatraemia in about 10-20% of ultra-endurance runners and triathletes. Therefore, in most exercise events lasting less than 3 or 4 hours, the major concern is that fluid and glucose should be available to prevent exhaustion and heatstroke. In the case of maximal performance, the availability of carbohydrate together with fluid may limit performance. In this respect both carbohydrate and sodium have beneficial characteristics for the stimulation of water absorption. On the other hand, from a number of observational studies as well as experimental studies in laboratory settings, it became clear that a higher frequency of gastrointestinal distress symptoms occur when athletes are dehydrated, especially when fluid loss exceeds 4% of body weight. Restoration of fluid balance after exercise is an important part of the recovery process. From several studies it became clear that rehydration after exercise can only be achieved if sweat electrolyte losses as well as water are replaced. The sodium content of most CESs is in the range

8 478 DIET FOR SPECIAL GROUPS of mmol/1. These values are already at the lower level for an effective rehydration solution. The available soft drinks contain virtually no sodium and are therefore unsuitable for use as rapid rehydration solutions. VITAMINS AND MINERALS Vitamins have attracted the most widespread attention in the world of sport because of their supposed role in enhancing performance. Therefore the use of vitamin supplements is very popular, sometimes reaching extreme levels of intake, exceeding the RDAs by 10 to 100 times (Saris et al 1989b). Of course, a marginal or low vitamin status will result in decreased performance or even illness. However, considering the RDAs, there are no indications that long-term vitamin intake among athletes is insufficient (Van der Beek 1991). As with other nutrients, athletes involved in intensive training, but consuming low-energy diets, are the most prone to marginal vitamin intakes. Supplementation at moderate levels can contribute under these circumstances to adequate daily intake. Vitamin supplementation with quantities exceeding those needed for optimal blood levels has never been shown to improve performance. In addition, vitamins E, C and beta-carotene are important antioxidants. The growing interest in the health-related aspects of antioxidants has also aroused the interest of sports nutritionists. Exercise has been associated with increases in free radical production in skeletal muscle. This increased oxidative stress could lead to tissue damage. It is suggested that, especially in the condition of overtraining, the balance between oxidative stress and repair can be interrupted, leading to dysfunction of membranes and increased lipid peroxidation. However, it has been shown that athletes have high levels of cellular antioxidant enzymes, such as superoxide dismutase (SOD), catalase and glutathione peroxidase. Most probably this is the result of a physiological adaptation to the higher oxidative stress and therefore it is questionable whether additional supplementation of antioxidants above the normally adequate intake levels is of benefit. So far, in this respect, the available literature does not allow any claims to be made. Of the minerals, iron has attracted most attention with regard to additional supplementation for athletes. Although other minerals, such as calcium, magnesium, zinc, copper and selenium, are as important as iron, the status of these minerals is adequate in most athletes. Many athletes, in particular females, are iron-depleted, but true iron deficiencies are rarely seen. Iron supplements have not been shown to enhance performance, except where iron-deficiency anaemia exists. Iron loss is increased in athletes, partly due to some iron loss in sweat or increased gastrointestinal or urinary blood loss, as a consequence of the intensity and duration of the exercise. However, as mentioned before with regard to other nutrients, this is usually well compensated by the increased dietary intake that is associated with the high energy intake. Iron supplements are therefore not usually considered to be necessary and can cause gastrointestinal disturbances or even haemosiderosis. Iron deficiency may be suspected in athletes who have a slightly lower than average haemoglobin (hb) from the so-called athletes' anaemia (<14% in men and <12% in women endurance athletes). This is usually a training-induced physiological condition and is caused by an increase in plasma volume. Despite a slightly reduced haemoglobin, the total red-cell mass is normal or greater than usual. Although athletes' anaemia may be associated with low serum ferritin, this physiological condition does not respond to iron supplementation. Based on the literature, Watts (1989) recommended that if the mean cell volume (MCV) is <75 fl and the mean cell hb <25 mg, or the ferritin is >5 mg lower than the lower limit of the reference value of the laboratory, a trial of iron supplementation for 1 month is justified. If iron deficiency is really present the hb value should rise by 1 g. ERGOGENIC SUPPLEMENTS Substances such as caffeine, L-carnitine, aspartate and sodium bicarbonate have received scientific attention because of their possible ergogenic effects on performance. In the case of caffeine ingestion (3-9 mg/kg body weight) prior to exercise, the body of evidence for improvement in performance is overwhelming (Spriet 1995). In the case of L-carnitine it has been suggested that oral supplementation leads to an improved fat oxidation which may favour athletic performance as in the case of caffeine. However, recent well controlled studies showed no change in muscle carnitine levels after supplementation, leaving no room for further speculations on the ergogenic properties of L-carnitine. However, for a considerable number of suggested ergogenic substances, such as bee-pollen, royal jelly and other exotic preparations, no scientific data are available at all. Therefore one should be aware of the nutritional quackery which offers athletes a wide variety of products and misleading information.

9 ATHLETICS 479 KEYPOINTS The main nutritional needs of an athlete are to consume enough food to provide sufficient energy. Daily energy needs may double for a marathon, and increase three- to fourfold for a cycle race like the Tour de France. Extra food for athletes must be consumed as meals and snacks which are high in energy but sufficient in protein and micronutrients, and easily digested and absorbed. Gymnasts and ballet dancers may not eat enough because they want to remain slim. Carbohydrate (CHO) is the best fuel for exercise of high intensity and high performance. Body stores of CHO are limited, so CHO loading by eating g CHO daily for a week can ensure maximum glycogen stores for events of more than 1 hour in duration. During exercise, CHO intake of g/min in the form of drinks is recommended. Recovery is aided by taking low protein CHO-rich drinks and food soon after exercise. Fat is favoured as an energy substrate in lowintensity exercise and during endurance training. The protein requirements of athletes are g/kg bodyweight/day. This is about twice the normal RDA and is generally covered by the increased food intake. Fluids and electrolytes are needed to replace sweat loss of 1-2 l/hour. CHO and sodium stimulate water absorption and the sodium content of rehydration solutions should be at least mmol/l. Vitamin and mineral supplements above normal RDAs are probably not necessary and sufficient amounts should come from the increased dietary intake. Caution is needed in assessing the dubious nutritional merits of a wide range of products available to athletes. REFERENCES Bergstrom J, Hultman E 1966 Muscle glycogen synthesis after exercise: an enhancing factor localized in the muscle cells in man. Nature 210: Coggan A R, Mendenhall L A 1992 Effect of diet on substrate metabolism during exercise. In: Lamb D R, Gisolfi C V (eds) Perspectives in exercise, sciences and sport medicine. Energy metabolism in exercise and sport. Benchmark Press, Indianapolis, p 435 Coyle E F 1991 Timing and method of increased carbohydrate intake to cope with heavy training competition and recovery. Journal of Sport Sciences 9:29-52 Coyle E F, Coggan A R, Hemmert M K, Ivy J L 1986 Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrates. Journal of Applied Physiology 6: Greenhaff P L 1995 Creatine and its application as an ergogenic aid. International Journal of Sports Nutrition 5:S100-S110 Jeukendrup A E, Wagenmakers A J M, Brouns F, Halliday D, Saris W H M 1996 Effects of carbohydrate (CHO) and fat supplementation on CHO metabolism during prolonged exercise. Metabolism 45: Jeukendrup A E, Brouns F, Wagenmakers A J M, Saris W H M 1997 Carbohydrate-electrolyte feedings improve 1 h time trial cycling performance. International Journal of Sports Medicine 18: Lemon P W R 1991 Does exercise alter dietary protein requirements? In: Brouns F, Saris W H M, Newsholm E A (eds) Advances in nutrition and top sport. In: Medicine and Sports Science, Karger, Basel, vol 32, p Martin W H, Dalsky G P, Hurley B F et al 1993 Effect of endurance training on plasma FFA turnover and oxidation during exercise. American Journal of Physiology 256:E708-E714 Rehrer N J, Wagenmakers A J M et al 1992 Gastric emptying, absorption and carbohydrate oxidation during prolonged exercise. Journal of Applied Physiology 72: Romijn J A, Coyle E F, Sidossis L S et al 1993 Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. American Journal of Physiology 265:E380-E391 Saris W H M, Schrijver J, Van Erp-Baart A M J, Brouns F 1989a Adequacy of vitamin supply under maximal sustained workloads: the Tour de France. International Journal on Vitamin and Nutrition Research 59: Saris W H M, Van Erp-Baart A M J, Brouns F, Westerterp K R, Ten Hoor F 1989b Study on food intake and energy expenditure during extreme sustained exercise: the Tour de France. International Journal of Sports Medicine 10(1):S26-S31 Saris W H M, Goodpaster B H, Jeukendrup A E, Brains F, Halliday D, Wagenaters A J M 1993 Exogenous carbohydrate oxidation from different carbohydrate sources during exercise. Journal of Applied Physiology 75: Sherman W H, Lamb D R 1988 Nutrition and prolonged exercise. In: Lamb D R, Murray R (eds) Perspectives in exercise sciences and sport medicine: prolonged exercise. Benchmark Press, Indianapolis, vol 1, p 213 Spriet L L 1995 Caffeine and performance. International Journal of Sport Nutrition 5:S84-S99 Van der Beek E J 1991 Vitamin supplementation and physical exercise performance. Journal of Sport Sciences 9:77-89 Van der Vusse G J, Reneman R S 1996 Lipid metabolism in muscle. In: Rowell L B, Shephard J T (eds) Handbook of physiology. Oxford Press, New York, section 12, p 952

10 480 DIET FOR SPECIAL GROUPS Van Erp-Baart A M J, Saris W H M, Binkhorst R A, Vos J A, Elvers JWH 1989 Nationwide survey on nutritional habits in elite athletes, part I: energy carbohydrates, protein and fat intake. International Journal of Sports Medicine 10(1):S3-S10 Van Hall G, Raaymakers J S H, Saris W H M, Wagenmakers A J M 1995 Ingestion of branched-chain amino acids and tryptophan during sustained exercise - failure to affect performance. Journal of Physiology 486: Wagenmakers A J M, Brouns F, Saris W H M, Halliday D 1993 Oxidation rates of orally ingested carbohydrates during prolonged exercise in men. Journal of Applied Physiology 77:2774^2780 Watts E 1989 Athletes' anaemia. British Journal of Sports Medicine 23:81-83