______________________________________________________________
Nutritional Aspects of Health and Performance
at Lowland and Altitude F. Brouns Nutrition Research Center, University of Limburg, Maastricht, Netherlands
Abstract F. Brouns, Nutritional Aspects of Health and Performance at Lowland and Altitude. mt J Sports Med, Vol 13,Suppl l,ppSlOO—5106, 1992.
One of the most important nutritional goals amongst athletes is to maintain adequate energy and fluid balance, since these are subject to relatively rapid changes and are directly related to performance and health. This may especially be the case when exercise intensity is high.
Furthermore, when due to exercise and environmental stress food and fluid intake become depressed. In such conditions there may be a dramatic increase in the utilization of carbohydrate (CHO), fluid, and in some instances protein. These increased requirements may then not be covered. InInt.J.SportsMed. 13(1992)SlOO—S106 Verlag StuttgartNew York
sufficient replacement of CHO may lead to hypoglycemia, altered protein metabolism, central fatigue and exhaustion. Large sweat losses may pose a risk to health by inducing
severe dehydration, impaired blood circulation and heat transfer, leading to heat exhaustion and collapse. Inadequate CHO and protein intake leads to a negative nitrogen balance, which over the long term will lead to a loss of muscle mass. In the scope of this presentation we will refer to the most important nutritional factors which are known to affect performance over a short term, at sea level and altitude. Key words
food intake, fluid, electrolytes, carbohydrate, protein, fat, altitude Performance,
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S100 mt. J. Sports Med. 13(1992)
NutritionulAspects of Health andPerformance Lowland andAltitude NutritionalAspects and PerformanceatatLowlandandAltitude
mt. J. Sports Med. 13(1992) SIOl
Introduction
CHO sources to be used should be rapidly digested and ab-
One of the most important nutritional goals
sorbed. Most efficient are (soluble) CHO sources which can be ingested with fluid. Optimal CHO sources are processed (pre-
zation of carbohydrate (CHO) and fluid, and in some instances protein. As a result, the requirements for these
digested) and low in dietary fiber, such as: 1) monosaccharides (glucose, fructose), 2) disaccharides (sucrose, maltose), 3) polymers (maltodextrins, malt extract) and 4) starches additionally have the (soluble starch). These types types of of CHO CR0 additionally benefit of being readily dissolved in fluids. In general these
nutrients are increased. Insufficient Insufficient replacement replacement of of CHO CR0 may
types of CHO have been shown to be about equally effective in increasing blood glucose g'ucose levels and oxidation rates during ex-
lead to hypoglycemia, altered protein metabolism, central fatigue and exhaustion (9, 12, 29, 30, 44, 45, 46, 52). Large
ercise. One exception may be pure fructose, which on one hand has been shown to maintain euglycemia and not to in-
sweat losses may pose a risk to health by inducing severe dehydration, impaired blood circulation and heat transfer, leading to heat exhaustion and collapse (26, 41, 42). Inadequate protein intake induces nitrogen nitrogen (N)/protein (N)/protein loss loss and and consequently consequently leads to a negative N balance, balance, which, which, ifif itit persists, persists, may mayreduce reduce performance (20, 21, 24). It is wrong, in this respect, to state that such changes only concern top athletes, especially since at
fluence insulin secretion, resulting in a less strong inhibition of free fatty acid mobilization than glucose. On the other hand, however, fructose is slowly s'owly absorbed and may cause intestinal
equal workloads, less trained sports people sweat more profusely, use more CHO as fuel for muscle work, utilize/break down more protein and recover more slowly from exhaustion. Therefore, nutritional guidelines for top athletes and less trained people, and the use of specific foods/meals are essen-
upset when ingested above 30 g/l. Furthermore, oxidation rates of fructose in energy delivery processes have been shown to be lower, most probably because of a higher affinity of the enzyme hexokinase in muscle for glucose, compared to fructose. This makes fructose less attractive as a high energy source
during exercise (2, 12, 13). Starch hydrolysates (maltodextrin/polymers) and soluble starch may have the benefit of being less sweet than the mono and disaccharides.
tially subject to the same principles.
Carbohydrate Carbohydrate
CHO, from blood glucose, liver and muscle glycogen, is the most important fuel for high intensity perform-
ance. Liver glycogen reserves increase after meals but
diminish in between, especially during the night, as the liver constantly constantly delivers delivers glucose glucose into into the the bloodstream bloodstream to to maintain maintain aa normal blood glucose level (28). Liver glycogen depletion will, in the case of enhanced glucose utilization during exercise, in-
Carbohydrate and Recovery
After exercise, the endogenous CHO pools should be replenished. Glycogen synthesis has been shown to be most rapid during the first hours after exercise. Thereafter the synthesis rate will gradually decline (12, 13). Net glycogen synthesis rate depends on synthesis capacity and quantitative glucose supply (9, 12, 13). The latter depends largely on the type of food ingested, i.e. the rate of digestion and absorption.
Additonally the CHO source may be of influence. Glucose favors muscle glycogen recovery, whereas fructose, which is
duce a fall in blood glucose. Fatigue, most probably also caused by the accompanying decrease in muscle glycogen
primarily taken up by the liver, favors liver glycogen recovery. If the next activity takes place after one to two days, the athlete
levels, levels, may may then then occur occur (9, (9, 12, 12, 45, 45, 46). 46). A A negative negative energy energy balbalance may lead to a decrease in glycogen depots (60). CHO ingestion may maybe be of of benefit benefit in in this this case. case. Oral CHO intake reduces liver glycogenolysis, increases blood glucose and glucose uptake and oxidation by the muscle. Theoretically the latter will reduce the rate of muscle musc'e glycogen breakdown for energy pro-
should recover by use of normal meals having a high CHO (en % % = percentage of total daily energy content (55—65 en %) (en
duction, reduce the catabolism of protein and delay
stances be be sufficient sufficient to to restore restore glycogen glycogen depots depots at at daily daily energy energy stances expenditures up to 4000 kcal (9). However, if energy expenditures on intensive exercise days are extreme, normal meal intake, and with it CHO, may not be sufficient to cover the needs. The reason for this is the high bulk load which interferes with
fatigue/improve performance. In studies where CHO was ingested during exercise, total CHO utilization was found to be the same as that of non-CHO ingesting control groups. Since
oral CHO is shown to be oxidized, endogenous CHO, is spared. However, However, oral oral CHO CR0 has not been found to reduce reduce muscle glycogen breakdown in active muscle groups. Any sparing of endogenous CHO is thus most probably in the liver, or in non-active muscle groups.
Carbohydrate supplementation For exercise of more than 45 minutes duration it is recommended that at least 30 g, but optimally up to 80 g of dissolved CHO CR0 be be consumed consumed for for every every following following hour of exercise (9, 12). This does not delay gastric emptying and increases water absorption. During endurance exercise, low amounts of CHO may only be sufficient to maintain normal blood glucose levels, whereas higher amounts may more effectively decrease
the use of endogenous CHO stores and delay fatigue. The
intake) and being composed of low glycemic index foods such as whole grains, fruits, vegetables etc. A relatively slow digestion and absorption rate is favorable in this condition. A quan-
tity of 400—600 g of CHO/day should under these circum-
digestion and which leads ot inadequate eating. The CHO need may then be over 12 g/kg body weight/day. Such high needs can only be covered with additional ingestion of CHO dense foods/solutions (3). Additionally, if the time for for rerecovery is limited, i.e. the second training session or or competicompetition follows on the same day, then the meal in between should be composed of high glycemic index foods which are rapidly digested and absorbed. Processed, cooked and blended potato, rice, noodles or corn starches belong to this category. category. CHO solutions can be taken during exercise and may also help to enhance glycogen recovery in the first few hours after exercise (12, 13). During a stay at high altitude, total food intake, and with it CHO intake, has been found to be decreased by 10—
50%. 50 %. The
chronically increased increased catetholamine catetholamine levels may chronically
additonally lead to a constant enhanced glycogenolysis. These
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amongst athletes is to maintain adequate energy and fluid balance, since these are subject to relatively rapid changes and are directly related to performance and health. Depending on the exercise intensity there may be dramatic increases in the utili-
S102 mt. J. Sports Med. 13(1992)
Fat
Apart from CHO, fatty acids derived from adipose tissue and intramuscular triacyiglycerol, triacylglycerol, form the second main energy source for the exercising individual. The importance of fat as an energy source depends on the the degree degree of of exercise stress as well as on the availability of CHO. During
exercise aa number number of ofnervous, nervous,metabolic metabolicand andhorhor physical exercise monal stimuli will lead to an increased rate of fat utilization on
membrane, thereby enhancing FFA oxidation. However, although such an effect was found on muscle preparation in vitro, vitro, numerous studies, performed on athletes, have have failed failed to to show any effect (16, 53).
Protein Protein The body possesses possesses three three major majorprotein proteinpoo1s, pools, plasma, muscle and visceral tissue, from which amino acids (AA) may be used under stressful conditions, such as food deprivation deprivation and and energy energy deficits deficits (3, (3, 20, 20, 21, 21, 24, 24, 31, 31, 32, 32, 38, 38, 51). 51). Exercise is known to be associated with changes in plasma AA
one hand and fat mobilization on the other. This process of free fatty acid (FFA) mobilization, transport and uptake is stimulated by increased activity of the central nervous system with the action of adrenalin and noradrenalin, and addition-
composition. It has been shown that oxidation of AA, especially branched chain AA (BCAA: leucine, valine, isoleucine), will contribute to energy production during exercise. Further, that a shortage of CHO (glycogen, blood glucose) dramati-
ally by a reduction in circulating insulin (31). In particular this may become very pronounced at high altitude. The chronic hypoxia further depresses resting and exercise levels of insulin
cally increases the oxidation of AA for the production of
and increases catecholamines, growth hormone and cortisol (14, 49, 50, 60). Any shortage shortage in in energy energy production production from fromfat fat has to be compensated for by CHO (29). Energy production from CHO proceeds "faster" than from fat and needs less oxygen. During high intensity exercise CHO availability will be one of the factors limiting performance, since fat as principle energy source only allows exercise at an intensity of approximately40—60%VO2max (29, 30). Regular endurance training increases the capacity of the skeletal muscle to use fat as an energy source. This will, when working at a fixed exercise intensity, reduce the use of endogenous CHO, and may delay
This has three major consequences: A) the concentration of some AA in plasma will fall (3, 20, 21, 38, 51), B)
fatigue. Training will also increase the sensitivity of fat cells to stimuli for FFA mobilization, thereby improving the speed of
adaptation to increased needs when exercising (6). Under maximal exercise circumstances, however, endogenous CHO utilization seems to proceed at full speed and enhancement of
energy(20,21, 51).
the N, which is released, will lead to the formation of ammonia, monia, known known to to be be toxic toxic and and to to be be associated associated with with fatigue fatigue (51, (51, 52)., C) and the ratio of BCCA to other AA will change. As a result of this change, some AA will increasingly pass the bloodbrain barrier, increase their concentration in the brain, and in-
fluence neurotransmitter synthesis. As a result, neurotransmission mission and and fatigue fatigue is is thought thought to to be be influenced influenced (32). (32).
Muscle mass constitutes the the largest largest protein protein poo1 within within the the body. body. During During starvation, starvation, the breakdown of pool muscle tissue under such circumstances are the liberation of AA for the maintenance of normal plasma AA levels and the provision of substrate for gluconeogenesis. Starvation, but also physical exhaustion, due to energy deficits, are known to
blood FFA does not automatically lead to a reduction of
change the anabolic/catabolic hormone ratio towards cata-
muscle and liver glycogen utilization. There has been some de-
bolism. As a result, de novo synthesis of protein may fall to low
bate about the effect of enhanced fat utilization at high altitude. Some authors have considered this as a physiological measure to decrease the utilization of glycogen. However, it is
levels. Increased degradation and oxidation of protein to-
more likely that enhanced FFA utilization is the direct effect of the altitude induced change in the hormonal state towards cat-
abolism, reduced food and total energy intakes resulting in negative energy balance, and reduced total CHO intake, lowering glycogen.
Fat Supplementation
There is no rationale for fat supplementation during exercise. exercise. Oral Oral fat fat is is aa potent potent inhibitor inhibitor of of gastric gastric emptyemptyduring ing and fat rich pre-competition meals have been shown to be
associated with an increased incidence of gastrointestinal upset. The fat store in the body is large enough to compensate for any need. Moreover, lipolytic stimulation during exercise will enhance blood FFA levels to such an extent that a maximal rate of FFA uptake by the muscle cells and mitochondria is achieved. Oral supply of fat may additionally increase blood FFA levels but not the uptake in active muscle cells and subsequent oxidation rates (6), and may thus not be of benefit in reducing muscle and liver glycogen utilization. L-carnitine is often promoted to overcome this transport barrier. The reason being that the oral supply of L-carnitine leads to an increased
transport of long chain fatty acids across the mitochondrial
gether with decreased synthesis will then result in a net loss of functional protein.
There has been considerable debate about the question whether the AA oxidized during prolonged exercise stem from the muscle, from the gastrointestinal tract, including the liver, or from both. Measurements across particular muscle groups (performed by determination of arteriovenous AA differences) have shown that some AA are liberated from the muscle during exercise. However, this may not necessarily reflect net catabolism of muscle tissue since the muscle is able to synthesize AA, e.g. alanine from pyruvate and N derived from the oxidation of BCAA. Apart from this, microdamage to muscle muscle fibers due to mechanical stress may lead to to the the loss loss of of AA and enzymes (1).
Visceral tissues form the second largest protein
poo1 and pool and have have been been observed observed to to contribute significantly to inter-organ exchange of AA during fasting and physical stress induced induced by illness (24). Exercise may induce an increased increased concontribution of visceral protein protein to to amino amino acid acid exchange exchange between between organs (38). However, there is some speculation on the quantitative contribution of these AA to gluconeogenesis and to N losses during and after exercise. There are some recent indications that visceral tissues may have a significant contribution
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factors may reduce glycogen resynthesis and thus lower glycogendepots(8, 14, 17,18,40,43, 54, 57, 60). gendepots(8, 14,17,18,40,43,54,57,60).
F. Brouns
NutritionalAspects of Health and Performance at Lowland and Altitude
tein turnover indicated a reduced protein synthesis and increased protein degradation during exercise. A largely reduced bloodflow, inducing local ischemia may be of influence here (5, 55).
It may therefore be concluded that the nitrogen lost, as a result of endurance exercise, stems from the oxidation
of AA, derived from different pools, and that this process is known to be intensified during energetic and hormonal stress such as in a state of high energy needs, while being glycogen depleted (3, 20, 21, 51). The latter may be of special importance when climbing at high altitude, during which total food
intake and also protein intake have been shown to be depressed. These factors may further decrease glycogen stores and lead lead to to aanegativeNbalance negative N balance(8, (8,14, 14,17, 17,18, 18, 40, 40, 43, 43, 54, 57, 60).
Protein Supplementation
From the previous paragraphs it can be concluded that that endurance endurance athletes athletes have have increased increased protein protein requirerequirecluded
range of of 1.2—1.8 1.2—1.8gkg'-day gkgday —'(20, ments, in the range —'(20, 21, 21, 56). 56). These increased requirements normally should be covered by increased total energy intakes, of which protein usually represents 12— 12—15 15 en %. Protein supplementation, however, is recommended for some categories of athletes who compete in weight classes and combine intensive training with weight reduction programmes, or for athletes who, for any reason, are unable to ingest sufficient protein. Protein sources to be used for supplementation or as part of replacement meals should be low in fat, readily digestible and of appropriate quality. Milk protein, milk protein hydrolysates and combinations of proteins such as soy protein, whey protein and/or caseinates seem
to be appropriate for these purposes. In addition it must be
tinues, the interstitional space will start to draw water from the cells, leading to intracellular dehydration (41, 42). Therefore,
with continued exercise, the water content of all compart-
ments will to some extent decrease. Depending on exercise intensity, training status, climatic circumstances and body size, sweat losses may range from a few hundred milliliters to more than 2 liters per hour (6). Because plasma volume is of prime importance to maintain a normal blood flow through "exercising tissues", it may be concluded that a dramatic decrease in
plasma volume will lead to decreased blood flow of muscle tissue and of the spianchnic splanchnic circulation. This will automatically lead to a reduced transport of substrates and oxygen, needed for energy production, and of metabolic waste products, including heat, to eliminating organs. The latter will lead to a decreased heat transfer from the muscles to the skin resulting in increasing core temperatures (6, 22, 42). In particular, endurance athletes exercising in the heat may therefore — —> heat exhaustion — be prone tothe beproneto therisk riskofofdehydration dehydration — — —> heatexhaustion heatstroke/collapse (26, 42, 48). Metabolic water production during ultra-endurance exercise may be significant, but is insufficient to compensate for fluids lost through sweating.
The electrolyte (E) concentration of sweat is lower than that of blood. This means that relatively more water
than E is lost from the blood. Therefore dehydration due to sweat loss will lead to an increase in the concentration of blood E (22). However, this is only the case when no or little water is ingested to compensate for the fluids lost. Large sweat losses
compensated by plain water intake may result in low blood sodium levels. Hyponatremia and consequently signs of water intoxication have been observed in marathon runners and tnathletes athletes (27, (27, 33). 33). Regular Regular endurance endurance training, training, resulting resulting in in large large
sweat responses will lead to to adaptations adaptations in in favor favor of of aa better better maintenance of fluid and electrolyte balance. Sweat glands adapt to reabsorb sodium, and thus plasma volume tends to
become increased. increased. Also Also the the sensitivity sensitivity for for fluid fluid regulatory regulatory horhorstated that that adequate adequate CHO CHO intake intake is is of of equal equal importance. importance. CHO CHO become stated deficits induce increased protein utilization, thus N losses. Al- mones will be enhanced (22, 26). Sweating will become more though research is currently being conducted relative to the "economical and effective". Less sweat will drip off the body. role of amino acid supplementation during exercise, there is Nevertheless, trained people exercising at their maximal levels no solid evidence that supplementation of (single) AA, such as of endurance performance capacity will be prone to dehydraarginine, ornithine, tryptophan and BCAA will be of direct tion during competition or training. The thermogenic stress, caused by by the the extremely extremely high high metabolic metabolic rates, rates, will will initiate initiate maxmaxbenefit to performance (7, 19, 58, 59). However, BCAA and caused imal sweat rates. At high altitude, a substantial water loss may glutamine have characteristics which may make them interesting for supplementation. BCAA are known to pass the liver al- also occur due to decreased water intake, increased altimost exclusively, and may thus be an optimal N supplier for tude/cold induced diuresis and low H20 saturation. As a remuscle tissue in periods of recovery when protein synthesis is sult, plasma volume is known to be decreased at high altitude increased (24, 56). The plasma glutamine concentration has (8, 34, 39, 47). Appropriate fluid and electrolyte intake rerecently been observed to be depressed in exhausted/over- mains necessary to maintain normal physiological functiontrained endurance athletes. Appropriate plasma glutamine ing under these conditions. levels are suggested to be essential for optimal immune-comExercise Rehydratkrn Rehydration Solutions petence as well as for maintenance of muscle mass/protein
synthesis (32, 51). Research is needed to support these possible applications. Performance improvement claims have been made with respect to supplementation of single AA, however, without solid evidence.
Fluid and Elecfrolytes If sweat losses and, especially at high altitude, respiratory fluid losses are large, causing plasma volume to decrease and blood electrolyte levels to increase, then water will be shifted from the interstitial space to the plasma. As a re-
sult, interstitial water content will fall. If this situation con-
Rehydration solutions for athletes are generally designed to replace fluid and minerals lost by sweating
and also limited amounts of energy in the form of CHO. Higher exercise intensities require a higher degree of energy production for which CHO as the energy source is most suitable. Accordingly, with higher exercise intensities, more metabolic heat will be produced. Consequently sweat production/loss will be increased, as will the excretion of electrolytes. The longer the exercise, the larger the amount of fluid, E and CHO needed to replace the losses.
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(38). a recent study on the effects of exercise on intestinal pro-
mt. J. Sports Med. 13 (1992) S103
S104 mt. J. Sports (1992) Sports Med. Med. 13 13(1992)
F. Brouns
mulate a rehydration solution which precisely compensates for the losses of all individuals, in all situations. Therefore, rehydration solutions for athletes are generally designed to broadly broadly cover the needs of a representative exercising exercising populapopulation under different circumstances. General guidelines for the composition of rehydration solutions have recently been obtained from a large number of studies in the fields of gastric emptying, intestinal absorption, fluid balance regulatory factors and fatigue/performance. These have been summarized in a number number of of reviews reviews (4, (4,6, 10, 11, II, 15, 25, 27, 35, 37). 6, 10, 15, 22, 25, 37), The general outcome from these studies is that the addition of small to moderate amounts of CHO and sodium to a drink does not delay gastric emptying and improves water absorption, relative to plain water. The scientific reason behind these indings relates findings relates to to the the fact fact that that coupled glucose-sodium glucose-sodium transtrans-
more concentrated CHO — electrolyte solutions (up to 100 g CHO/l), which are known to only slightly reduce net fluid absorption, do not differ in effects on fluid homeostasis, compared to less concentrated CHO solutions, but enhance CHO availability (6, 10, 22, 25). A general guideline should be that rehydration solutions should not be strongly hypertonic. Hypertonic solutions have been shown to reduce net fluid absorption by inducing fluid secretion into the gastrointestinal tract
and may also reduce gastric emptying. The latter may influence/limit quantitative fluid intake (4, 6, 22, 23, 37). The source of CHO will influence fluid osmolality. A general recommendation for the composition of oral rehydration beverages for sport is given in Table 1.
References Armstrong R. B.: Muscle Muscie damage and endurance events. Sports 2
dition of other E, in the small quantities lost by whole body sweat, does not influence gastric emptying, or absorption (35, 36). The CHO fraction will additionally contribute to the maintenance of a normal normal blood blood glucose glucose level level and and will willlead leadto toaa sparing of the endogenous CHO reserves. The latter may influence protein breakdown, delay fatigue and thus influence A large body of scientific scientific evieviperformance (3, 12, 13, 51, 51, 52). 52). A
' 6
carbohydrate sodium* osmolality
optional
30lOU g/l** 30_lOOg/l**
Nutritional Aspects of Health and Performance
at Lowland and Altitude F. Brouns Nutrition Research Center, University of Limburg, Maastricht, Netherlands
Abstract F. Brouns, Nutritional Aspects of Health and Performance at Lowland and Altitude. mt J Sports Med, Vol 13,Suppl l,ppSlOO—5106, 1992.
One of the most important nutritional goals amongst athletes is to maintain adequate energy and fluid balance, since these are subject to relatively rapid changes and are directly related to performance and health. This may especially be the case when exercise intensity is high.
Furthermore, when due to exercise and environmental stress food and fluid intake become depressed. In such conditions there may be a dramatic increase in the utilization of carbohydrate (CHO), fluid, and in some instances protein. These increased requirements may then not be covered. InInt.J.SportsMed. 13(1992)SlOO—S106 Verlag StuttgartNew York
sufficient replacement of CHO may lead to hypoglycemia, altered protein metabolism, central fatigue and exhaustion. Large sweat losses may pose a risk to health by inducing
severe dehydration, impaired blood circulation and heat transfer, leading to heat exhaustion and collapse. Inadequate CHO and protein intake leads to a negative nitrogen balance, which over the long term will lead to a loss of muscle mass. In the scope of this presentation we will refer to the most important nutritional factors which are known to affect performance over a short term, at sea level and altitude. Key words
food intake, fluid, electrolytes, carbohydrate, protein, fat, altitude Performance,
Downloaded by: Universite Laval. Copyrighted material.
S100 mt. J. Sports Med. 13(1992)
NutritionulAspects of Health andPerformance Lowland andAltitude NutritionalAspects and PerformanceatatLowlandandAltitude
mt. J. Sports Med. 13(1992) SIOl
Introduction
CHO sources to be used should be rapidly digested and ab-
One of the most important nutritional goals
sorbed. Most efficient are (soluble) CHO sources which can be ingested with fluid. Optimal CHO sources are processed (pre-
zation of carbohydrate (CHO) and fluid, and in some instances protein. As a result, the requirements for these
digested) and low in dietary fiber, such as: 1) monosaccharides (glucose, fructose), 2) disaccharides (sucrose, maltose), 3) polymers (maltodextrins, malt extract) and 4) starches additionally have the (soluble starch). These types types of of CHO CR0 additionally benefit of being readily dissolved in fluids. In general these
nutrients are increased. Insufficient Insufficient replacement replacement of of CHO CR0 may
types of CHO have been shown to be about equally effective in increasing blood glucose g'ucose levels and oxidation rates during ex-
lead to hypoglycemia, altered protein metabolism, central fatigue and exhaustion (9, 12, 29, 30, 44, 45, 46, 52). Large
ercise. One exception may be pure fructose, which on one hand has been shown to maintain euglycemia and not to in-
sweat losses may pose a risk to health by inducing severe dehydration, impaired blood circulation and heat transfer, leading to heat exhaustion and collapse (26, 41, 42). Inadequate protein intake induces nitrogen nitrogen (N)/protein (N)/protein loss loss and and consequently consequently leads to a negative N balance, balance, which, which, ifif itit persists, persists, may mayreduce reduce performance (20, 21, 24). It is wrong, in this respect, to state that such changes only concern top athletes, especially since at
fluence insulin secretion, resulting in a less strong inhibition of free fatty acid mobilization than glucose. On the other hand, however, fructose is slowly s'owly absorbed and may cause intestinal
equal workloads, less trained sports people sweat more profusely, use more CHO as fuel for muscle work, utilize/break down more protein and recover more slowly from exhaustion. Therefore, nutritional guidelines for top athletes and less trained people, and the use of specific foods/meals are essen-
upset when ingested above 30 g/l. Furthermore, oxidation rates of fructose in energy delivery processes have been shown to be lower, most probably because of a higher affinity of the enzyme hexokinase in muscle for glucose, compared to fructose. This makes fructose less attractive as a high energy source
during exercise (2, 12, 13). Starch hydrolysates (maltodextrin/polymers) and soluble starch may have the benefit of being less sweet than the mono and disaccharides.
tially subject to the same principles.
Carbohydrate Carbohydrate
CHO, from blood glucose, liver and muscle glycogen, is the most important fuel for high intensity perform-
ance. Liver glycogen reserves increase after meals but
diminish in between, especially during the night, as the liver constantly constantly delivers delivers glucose glucose into into the the bloodstream bloodstream to to maintain maintain aa normal blood glucose level (28). Liver glycogen depletion will, in the case of enhanced glucose utilization during exercise, in-
Carbohydrate and Recovery
After exercise, the endogenous CHO pools should be replenished. Glycogen synthesis has been shown to be most rapid during the first hours after exercise. Thereafter the synthesis rate will gradually decline (12, 13). Net glycogen synthesis rate depends on synthesis capacity and quantitative glucose supply (9, 12, 13). The latter depends largely on the type of food ingested, i.e. the rate of digestion and absorption.
Additonally the CHO source may be of influence. Glucose favors muscle glycogen recovery, whereas fructose, which is
duce a fall in blood glucose. Fatigue, most probably also caused by the accompanying decrease in muscle glycogen
primarily taken up by the liver, favors liver glycogen recovery. If the next activity takes place after one to two days, the athlete
levels, levels, may may then then occur occur (9, (9, 12, 12, 45, 45, 46). 46). A A negative negative energy energy balbalance may lead to a decrease in glycogen depots (60). CHO ingestion may maybe be of of benefit benefit in in this this case. case. Oral CHO intake reduces liver glycogenolysis, increases blood glucose and glucose uptake and oxidation by the muscle. Theoretically the latter will reduce the rate of muscle musc'e glycogen breakdown for energy pro-
should recover by use of normal meals having a high CHO (en % % = percentage of total daily energy content (55—65 en %) (en
duction, reduce the catabolism of protein and delay
stances be be sufficient sufficient to to restore restore glycogen glycogen depots depots at at daily daily energy energy stances expenditures up to 4000 kcal (9). However, if energy expenditures on intensive exercise days are extreme, normal meal intake, and with it CHO, may not be sufficient to cover the needs. The reason for this is the high bulk load which interferes with
fatigue/improve performance. In studies where CHO was ingested during exercise, total CHO utilization was found to be the same as that of non-CHO ingesting control groups. Since
oral CHO is shown to be oxidized, endogenous CHO, is spared. However, However, oral oral CHO CR0 has not been found to reduce reduce muscle glycogen breakdown in active muscle groups. Any sparing of endogenous CHO is thus most probably in the liver, or in non-active muscle groups.
Carbohydrate supplementation For exercise of more than 45 minutes duration it is recommended that at least 30 g, but optimally up to 80 g of dissolved CHO CR0 be be consumed consumed for for every every following following hour of exercise (9, 12). This does not delay gastric emptying and increases water absorption. During endurance exercise, low amounts of CHO may only be sufficient to maintain normal blood glucose levels, whereas higher amounts may more effectively decrease
the use of endogenous CHO stores and delay fatigue. The
intake) and being composed of low glycemic index foods such as whole grains, fruits, vegetables etc. A relatively slow digestion and absorption rate is favorable in this condition. A quan-
tity of 400—600 g of CHO/day should under these circum-
digestion and which leads ot inadequate eating. The CHO need may then be over 12 g/kg body weight/day. Such high needs can only be covered with additional ingestion of CHO dense foods/solutions (3). Additionally, if the time for for rerecovery is limited, i.e. the second training session or or competicompetition follows on the same day, then the meal in between should be composed of high glycemic index foods which are rapidly digested and absorbed. Processed, cooked and blended potato, rice, noodles or corn starches belong to this category. category. CHO solutions can be taken during exercise and may also help to enhance glycogen recovery in the first few hours after exercise (12, 13). During a stay at high altitude, total food intake, and with it CHO intake, has been found to be decreased by 10—
50%. 50 %. The
chronically increased increased catetholamine catetholamine levels may chronically
additonally lead to a constant enhanced glycogenolysis. These
Downloaded by: Universite Laval. Copyrighted material.
amongst athletes is to maintain adequate energy and fluid balance, since these are subject to relatively rapid changes and are directly related to performance and health. Depending on the exercise intensity there may be dramatic increases in the utili-
S102 mt. J. Sports Med. 13(1992)
Fat
Apart from CHO, fatty acids derived from adipose tissue and intramuscular triacyiglycerol, triacylglycerol, form the second main energy source for the exercising individual. The importance of fat as an energy source depends on the the degree degree of of exercise stress as well as on the availability of CHO. During
exercise aa number number of ofnervous, nervous,metabolic metabolicand andhorhor physical exercise monal stimuli will lead to an increased rate of fat utilization on
membrane, thereby enhancing FFA oxidation. However, although such an effect was found on muscle preparation in vitro, vitro, numerous studies, performed on athletes, have have failed failed to to show any effect (16, 53).
Protein Protein The body possesses possesses three three major majorprotein proteinpoo1s, pools, plasma, muscle and visceral tissue, from which amino acids (AA) may be used under stressful conditions, such as food deprivation deprivation and and energy energy deficits deficits (3, (3, 20, 20, 21, 21, 24, 24, 31, 31, 32, 32, 38, 38, 51). 51). Exercise is known to be associated with changes in plasma AA
one hand and fat mobilization on the other. This process of free fatty acid (FFA) mobilization, transport and uptake is stimulated by increased activity of the central nervous system with the action of adrenalin and noradrenalin, and addition-
composition. It has been shown that oxidation of AA, especially branched chain AA (BCAA: leucine, valine, isoleucine), will contribute to energy production during exercise. Further, that a shortage of CHO (glycogen, blood glucose) dramati-
ally by a reduction in circulating insulin (31). In particular this may become very pronounced at high altitude. The chronic hypoxia further depresses resting and exercise levels of insulin
cally increases the oxidation of AA for the production of
and increases catecholamines, growth hormone and cortisol (14, 49, 50, 60). Any shortage shortage in in energy energy production production from fromfat fat has to be compensated for by CHO (29). Energy production from CHO proceeds "faster" than from fat and needs less oxygen. During high intensity exercise CHO availability will be one of the factors limiting performance, since fat as principle energy source only allows exercise at an intensity of approximately40—60%VO2max (29, 30). Regular endurance training increases the capacity of the skeletal muscle to use fat as an energy source. This will, when working at a fixed exercise intensity, reduce the use of endogenous CHO, and may delay
This has three major consequences: A) the concentration of some AA in plasma will fall (3, 20, 21, 38, 51), B)
fatigue. Training will also increase the sensitivity of fat cells to stimuli for FFA mobilization, thereby improving the speed of
adaptation to increased needs when exercising (6). Under maximal exercise circumstances, however, endogenous CHO utilization seems to proceed at full speed and enhancement of
energy(20,21, 51).
the N, which is released, will lead to the formation of ammonia, monia, known known to to be be toxic toxic and and to to be be associated associated with with fatigue fatigue (51, (51, 52)., C) and the ratio of BCCA to other AA will change. As a result of this change, some AA will increasingly pass the bloodbrain barrier, increase their concentration in the brain, and in-
fluence neurotransmitter synthesis. As a result, neurotransmission mission and and fatigue fatigue is is thought thought to to be be influenced influenced (32). (32).
Muscle mass constitutes the the largest largest protein protein poo1 within within the the body. body. During During starvation, starvation, the breakdown of pool muscle tissue under such circumstances are the liberation of AA for the maintenance of normal plasma AA levels and the provision of substrate for gluconeogenesis. Starvation, but also physical exhaustion, due to energy deficits, are known to
blood FFA does not automatically lead to a reduction of
change the anabolic/catabolic hormone ratio towards cata-
muscle and liver glycogen utilization. There has been some de-
bolism. As a result, de novo synthesis of protein may fall to low
bate about the effect of enhanced fat utilization at high altitude. Some authors have considered this as a physiological measure to decrease the utilization of glycogen. However, it is
levels. Increased degradation and oxidation of protein to-
more likely that enhanced FFA utilization is the direct effect of the altitude induced change in the hormonal state towards cat-
abolism, reduced food and total energy intakes resulting in negative energy balance, and reduced total CHO intake, lowering glycogen.
Fat Supplementation
There is no rationale for fat supplementation during exercise. exercise. Oral Oral fat fat is is aa potent potent inhibitor inhibitor of of gastric gastric emptyemptyduring ing and fat rich pre-competition meals have been shown to be
associated with an increased incidence of gastrointestinal upset. The fat store in the body is large enough to compensate for any need. Moreover, lipolytic stimulation during exercise will enhance blood FFA levels to such an extent that a maximal rate of FFA uptake by the muscle cells and mitochondria is achieved. Oral supply of fat may additionally increase blood FFA levels but not the uptake in active muscle cells and subsequent oxidation rates (6), and may thus not be of benefit in reducing muscle and liver glycogen utilization. L-carnitine is often promoted to overcome this transport barrier. The reason being that the oral supply of L-carnitine leads to an increased
transport of long chain fatty acids across the mitochondrial
gether with decreased synthesis will then result in a net loss of functional protein.
There has been considerable debate about the question whether the AA oxidized during prolonged exercise stem from the muscle, from the gastrointestinal tract, including the liver, or from both. Measurements across particular muscle groups (performed by determination of arteriovenous AA differences) have shown that some AA are liberated from the muscle during exercise. However, this may not necessarily reflect net catabolism of muscle tissue since the muscle is able to synthesize AA, e.g. alanine from pyruvate and N derived from the oxidation of BCAA. Apart from this, microdamage to muscle muscle fibers due to mechanical stress may lead to to the the loss loss of of AA and enzymes (1).
Visceral tissues form the second largest protein
poo1 and pool and have have been been observed observed to to contribute significantly to inter-organ exchange of AA during fasting and physical stress induced induced by illness (24). Exercise may induce an increased increased concontribution of visceral protein protein to to amino amino acid acid exchange exchange between between organs (38). However, there is some speculation on the quantitative contribution of these AA to gluconeogenesis and to N losses during and after exercise. There are some recent indications that visceral tissues may have a significant contribution
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factors may reduce glycogen resynthesis and thus lower glycogendepots(8, 14, 17,18,40,43, 54, 57, 60). gendepots(8, 14,17,18,40,43,54,57,60).
F. Brouns
NutritionalAspects of Health and Performance at Lowland and Altitude
tein turnover indicated a reduced protein synthesis and increased protein degradation during exercise. A largely reduced bloodflow, inducing local ischemia may be of influence here (5, 55).
It may therefore be concluded that the nitrogen lost, as a result of endurance exercise, stems from the oxidation
of AA, derived from different pools, and that this process is known to be intensified during energetic and hormonal stress such as in a state of high energy needs, while being glycogen depleted (3, 20, 21, 51). The latter may be of special importance when climbing at high altitude, during which total food
intake and also protein intake have been shown to be depressed. These factors may further decrease glycogen stores and lead lead to to aanegativeNbalance negative N balance(8, (8,14, 14,17, 17,18, 18, 40, 40, 43, 43, 54, 57, 60).
Protein Supplementation
From the previous paragraphs it can be concluded that that endurance endurance athletes athletes have have increased increased protein protein requirerequirecluded
range of of 1.2—1.8 1.2—1.8gkg'-day gkgday —'(20, ments, in the range —'(20, 21, 21, 56). 56). These increased requirements normally should be covered by increased total energy intakes, of which protein usually represents 12— 12—15 15 en %. Protein supplementation, however, is recommended for some categories of athletes who compete in weight classes and combine intensive training with weight reduction programmes, or for athletes who, for any reason, are unable to ingest sufficient protein. Protein sources to be used for supplementation or as part of replacement meals should be low in fat, readily digestible and of appropriate quality. Milk protein, milk protein hydrolysates and combinations of proteins such as soy protein, whey protein and/or caseinates seem
to be appropriate for these purposes. In addition it must be
tinues, the interstitional space will start to draw water from the cells, leading to intracellular dehydration (41, 42). Therefore,
with continued exercise, the water content of all compart-
ments will to some extent decrease. Depending on exercise intensity, training status, climatic circumstances and body size, sweat losses may range from a few hundred milliliters to more than 2 liters per hour (6). Because plasma volume is of prime importance to maintain a normal blood flow through "exercising tissues", it may be concluded that a dramatic decrease in
plasma volume will lead to decreased blood flow of muscle tissue and of the spianchnic splanchnic circulation. This will automatically lead to a reduced transport of substrates and oxygen, needed for energy production, and of metabolic waste products, including heat, to eliminating organs. The latter will lead to a decreased heat transfer from the muscles to the skin resulting in increasing core temperatures (6, 22, 42). In particular, endurance athletes exercising in the heat may therefore — —> heat exhaustion — be prone tothe beproneto therisk riskofofdehydration dehydration — — —> heatexhaustion heatstroke/collapse (26, 42, 48). Metabolic water production during ultra-endurance exercise may be significant, but is insufficient to compensate for fluids lost through sweating.
The electrolyte (E) concentration of sweat is lower than that of blood. This means that relatively more water
than E is lost from the blood. Therefore dehydration due to sweat loss will lead to an increase in the concentration of blood E (22). However, this is only the case when no or little water is ingested to compensate for the fluids lost. Large sweat losses
compensated by plain water intake may result in low blood sodium levels. Hyponatremia and consequently signs of water intoxication have been observed in marathon runners and tnathletes athletes (27, (27, 33). 33). Regular Regular endurance endurance training, training, resulting resulting in in large large
sweat responses will lead to to adaptations adaptations in in favor favor of of aa better better maintenance of fluid and electrolyte balance. Sweat glands adapt to reabsorb sodium, and thus plasma volume tends to
become increased. increased. Also Also the the sensitivity sensitivity for for fluid fluid regulatory regulatory horhorstated that that adequate adequate CHO CHO intake intake is is of of equal equal importance. importance. CHO CHO become stated deficits induce increased protein utilization, thus N losses. Al- mones will be enhanced (22, 26). Sweating will become more though research is currently being conducted relative to the "economical and effective". Less sweat will drip off the body. role of amino acid supplementation during exercise, there is Nevertheless, trained people exercising at their maximal levels no solid evidence that supplementation of (single) AA, such as of endurance performance capacity will be prone to dehydraarginine, ornithine, tryptophan and BCAA will be of direct tion during competition or training. The thermogenic stress, caused by by the the extremely extremely high high metabolic metabolic rates, rates, will will initiate initiate maxmaxbenefit to performance (7, 19, 58, 59). However, BCAA and caused imal sweat rates. At high altitude, a substantial water loss may glutamine have characteristics which may make them interesting for supplementation. BCAA are known to pass the liver al- also occur due to decreased water intake, increased altimost exclusively, and may thus be an optimal N supplier for tude/cold induced diuresis and low H20 saturation. As a remuscle tissue in periods of recovery when protein synthesis is sult, plasma volume is known to be decreased at high altitude increased (24, 56). The plasma glutamine concentration has (8, 34, 39, 47). Appropriate fluid and electrolyte intake rerecently been observed to be depressed in exhausted/over- mains necessary to maintain normal physiological functiontrained endurance athletes. Appropriate plasma glutamine ing under these conditions. levels are suggested to be essential for optimal immune-comExercise Rehydratkrn Rehydration Solutions petence as well as for maintenance of muscle mass/protein
synthesis (32, 51). Research is needed to support these possible applications. Performance improvement claims have been made with respect to supplementation of single AA, however, without solid evidence.
Fluid and Elecfrolytes If sweat losses and, especially at high altitude, respiratory fluid losses are large, causing plasma volume to decrease and blood electrolyte levels to increase, then water will be shifted from the interstitial space to the plasma. As a re-
sult, interstitial water content will fall. If this situation con-
Rehydration solutions for athletes are generally designed to replace fluid and minerals lost by sweating
and also limited amounts of energy in the form of CHO. Higher exercise intensities require a higher degree of energy production for which CHO as the energy source is most suitable. Accordingly, with higher exercise intensities, more metabolic heat will be produced. Consequently sweat production/loss will be increased, as will the excretion of electrolytes. The longer the exercise, the larger the amount of fluid, E and CHO needed to replace the losses.
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(38). a recent study on the effects of exercise on intestinal pro-
mt. J. Sports Med. 13 (1992) S103
S104 mt. J. Sports (1992) Sports Med. Med. 13 13(1992)
F. Brouns
mulate a rehydration solution which precisely compensates for the losses of all individuals, in all situations. Therefore, rehydration solutions for athletes are generally designed to broadly broadly cover the needs of a representative exercising exercising populapopulation under different circumstances. General guidelines for the composition of rehydration solutions have recently been obtained from a large number of studies in the fields of gastric emptying, intestinal absorption, fluid balance regulatory factors and fatigue/performance. These have been summarized in a number number of of reviews reviews (4, (4,6, 10, 11, II, 15, 25, 27, 35, 37). 6, 10, 15, 22, 25, 37), The general outcome from these studies is that the addition of small to moderate amounts of CHO and sodium to a drink does not delay gastric emptying and improves water absorption, relative to plain water. The scientific reason behind these indings relates findings relates to to the the fact fact that that coupled glucose-sodium glucose-sodium transtrans-
more concentrated CHO — electrolyte solutions (up to 100 g CHO/l), which are known to only slightly reduce net fluid absorption, do not differ in effects on fluid homeostasis, compared to less concentrated CHO solutions, but enhance CHO availability (6, 10, 22, 25). A general guideline should be that rehydration solutions should not be strongly hypertonic. Hypertonic solutions have been shown to reduce net fluid absorption by inducing fluid secretion into the gastrointestinal tract
and may also reduce gastric emptying. The latter may influence/limit quantitative fluid intake (4, 6, 22, 23, 37). The source of CHO will influence fluid osmolality. A general recommendation for the composition of oral rehydration beverages for sport is given in Table 1.
References Armstrong R. B.: Muscle Muscie damage and endurance events. Sports 2
dition of other E, in the small quantities lost by whole body sweat, does not influence gastric emptying, or absorption (35, 36). The CHO fraction will additionally contribute to the maintenance of a normal normal blood blood glucose glucose level level and and will willlead leadto toaa sparing of the endogenous CHO reserves. The latter may influence protein breakdown, delay fatigue and thus influence A large body of scientific scientific evieviperformance (3, 12, 13, 51, 51, 52). 52). A
' 6
carbohydrate sodium* osmolality
optional
30lOU g/l** 30_lOOg/l**
