Exercise at Climatic Extremes
Chapter 38 Exercise at Climatic Extremes MARK A. FEBBRAIO Introduction The relationship between nutrition and exercise has been a major scientiﬁc interest area for over 150 years. With the popularization of the muscle biopsy technique, arteriovenous (a-v) balance measurements and, more recently, the use of isotope tracers as metabolic probes during exercise, it has become possible to clearly investigate the role of nutrition in exercise physiology and biochemistry. Accordingly, growth in this area has increased exponentially. Much of the research which has examined the interaction between nutrition and exercise has been conducted in comfortable ambient conditions. It is clear, however, that environmental temperature is a major practical issue one must consider when examining nutrition and sport. In extremely low ambient temperatures, when the gradient between the skin and surrounding environment is high, the rate of endogenous heat production, even during exercise, may be insufﬁcient to offset body heat loss. In these circumstances, responses are invoked to reduce heat loss and increase heat production. In contrast, when exercise is conducted in very high ambient temperatures, the gradient for heat dissipation is signiﬁcantly reduced, which results in changes to thermoregulatory mechanisms designed to promote body heat loss. In both climatic extremes, these physiological adaptations ultimately impact upon hormonal and metabolic responses to exercise which act to alter substrate utilization. Hence, environmental temperature is an important factor to consider when determining optimal nutritional strategies for exercise performance. Exercise in a cold environment Cold stress or attenuated exercise-induced hyperthermia? Unlike heat, which can only serve to augment the exercise-induced increase in body temperature, a cold environment may invoke varied physiological responses during exercise. These responses depend on whether the interaction between the environment and the exercising organism promotes excessive heat loss or attenuates the normal rise in body core temperature associated with exercise. Most studies which have observed relative hypothermia during exercise have done so using swimming as the mode of exercise (Holmer & Bergh 1974; Galbo et al. 1979; Doubt & Hsieh 1991), since water is a much greater thermal conductant than air. In contrast, when exercise has been conducted in cold air environments ranging from 3 to 9°C, an attenuated rise, rather than a fall in body core temperature, has been observed (Jacobs et al. 1985; Febbraio et al. 1996a, 1996b). The severity of the ‘cold stress’ is an important consideration when examining nutritional requirements since a fall in body temperature will result in shivering thermogenesis (Webb 1992) and an enhanced sympathoadrenal response (Galbo et al. 1979), while an attenuated rise in body temperature blunts the exerciseinduced increase in adrenaline secretion 497 498 practical issues (Febbraio et al. 1996b). Such responses are likely to alter substrate utilization during exercise. Substrate utilization during exercise in a cold environment When the rise in body temperature is attenuated during prolonged exercise in a cold environment, the rate of glycogen utilization in contracting muscle is reduced (Kozlowski et al. 1985; Febbraio et al. 1996b; Parkin et al. 1999) and exercise performance is increased (Hessemer et al. 1984; Febbraio et al. 1996a; Parkin et al. 1999), which is not surprising, since fatigue during prolonged exercise often coincides with glycogen depletion (Coggan & Coyle 1991). In many circumstances, therefore, the cool environment may be viewed as an ‘ergogenic aid’, since it results in a conservation of ﬁnite endogenous carbohydrate stores within contracting muscles. It must be noted, however, that even in some circumstances where measures have been taken to ensure that body heat loss is eliminated, more energy is required to undertake many outdoor activities in a cold than in a temperate environment. Brotherhood (1973, 1985) has demonstrated that walking over ice or snow-covered terrain increases energy demand compared with walking at a similar speed over dry ground. In addition, wearing heavy boots and clothing as a prevention against hypothermia increases metabolic demands and substrate utilization (Campbell 1981, 1982; Romet et al. 1986). There are many athletic events, such as open water swimming and mountaineering, where extreme cold can lead to a fall in body temperature. In these circumstances, thermoregulatory mechanisms are invoked to increase body heat production and consequent substrate utilization. These include shivering and non-shivering thermogenesis. Shivering, an involuntary rhythmic contraction of skeletal muscle, is usually invoked in response to a 3–4°C fall in body temperature (Webb 1992). This increase in muscle contraction results in an approximate 2.5-fold increase in total energy expenditure. More importantly, the carbohydrate oxidation rate increases almost sixfold, while the rise in lipid oxidation is modest (Vallerand & Jacobs 1989). The rise in carbohydrate oxidation is accounted for by increases in plasma glucose turnover, glycolysis and glycogenolysis (Vallerand et al. 1995). We have recently observed that when subjects exercised at 3°C, their pulmonary respiratory exchange ratio (RER) was higher than during exercise at 20°C despite contracting muscle glycogenolysis and lactate accumulation being lower (Febbraio et al. 1996b). This suggests that involuntary activity associated with shivering in otherwise inactive muscles contributes to an increase in total body carbohydrate oxidation during exercise in a cold environment. Hence, carbohydrate availability is a critical issue during exercise in climatic conditions where a shivering response may be invoked. Apart from the increase in carbohydrate utilization as a result of shivering, cold exposure may also increase intramuscular carbohydrate utilization via an augmented sympathoadrenal response. Plasma catecholamines are elevated during exercise in response to cold stress (Galbo et al. 1979; Young et al. 1986) and exogenous increases in adrenaline often results in a concomitant increase in muscle glycogenolysis (Jannson et al. 1986; Spriet et al. 1988; Febbraio et al. 1998) and liver glucose production (Kjær et al. 1993). Shivering thermogenesis is not an absolute requirement, therefore, for increases in carbohydrate utilization during exercise in a cold environment. Dietary modiﬁcations for exercise in a cold environment In circumstances where exercise in a cold environment attenuates the exercise-induced increase in body temperature, guidelines for nutritional intake require little, if any, modiﬁcation from that which is recommended for exercise in comfortable ambient conditions. It is generally accepted that a glucose/sucrose beverage of 6–10% carbohydrate is appropriate for exercise at climatic extremes ingestion during exercise (Costill & Hargreaves 1992), since this would provide necessary glucose while allowing for optimal gastric emptying and intestinal absorption (Mitchell et al. 1989; Rehrer et al. 1989; Gisolﬁ et al. 1991). It has been suggested, however, that increasing the carbohydrate content of a ﬂuid beverage may be beneﬁcial during exercise in cooler conditions since the requirement for optimal ﬂuid delivery may be less important, due to the reduction in thermoregulatory stress, while the necessity for sufﬁcient circulating glucose levels is maintained. We have recently tested this hypothesis and found that increasing the carbohydrate content of a ﬂuid beverage which is ingested during exercise in a cool environment is not advantageous (Fig. 38.1). While such a practice does elevate blood glucose levels, it results in increased gastrointestinal discomfort, a less than efﬁcient maintenance of plasma volume and a reduction in exercise performance relative to the ingestion of a 7% carbohydrate beverage (Febbraio et al. 1996a). Therefore, when the endogenous heat produced by exercise in cool ambient conditions is sufﬁcient to offset body heat loss, feeding strategies recommended during exercise in comfortable ambient temperatures should be adhered to. During exercise in extremely cold environments which results in a fall in body core temperature, any dietary modiﬁcation which results in an increase in whole-body metabolic rate, which would generate warmer body temperatures and improve cold tolerance, would be most beneﬁcial. As a result, recent research has focused on administration of many ergogenic aids designed to increase thermotolerance during cold stress. These ergogenic aids include hormones, pharmacological agents and nutrients. Administration of the pharmacological agent dinitrophenol (Hall et al. 1948) and hormones such as thyroxin, catecholamines, cortisol and growth hormone (Sellers 1972; Le Blanc 1975) in cold exposed animals results in a delay in the onset of hypothermia. However, while these studies provide useful information regarding the mechanisms for the induction of thermogenesis, it is impractical to suggest that they be taken by humans as ergogenic aids during exercise and cold stress because of the obvious health risks. It is possible, that ingestion of b-adrenergic agonists such as caffeine, ephedrine or theophylline may improve cold tolerance, although the literature which has examined such a phenomenon has produced conﬂicting results. Ingesting the combination of ephedrine and caf- 10 * * * 6 † *† *† 4 *† 250 † 5 0 –5 –10 *† 200 † Time (min) 8 Change in PV (%) Glucose (mmol.l–1) 10 499 † 150 100 50 *† 2 (a) –15 0 50 100 150 200 250 Time (min) 0 (b) 50 100 150 200 250 Time (min) 0 (c) CON LCHO HCHO Trial Fig. 38.1 (a) Plasma glucose, (b) change in plasma volume (PV), and (c) time to exhaustion while consuming a placebo (CON, 䊏), . 7% carbohydrate (LCHO, 䊊) or 14% carbohydrate (HCHO, 䊉), beverage during fatiguing exercise at 70% Vo2max. in 5°C conditions. *, difference (P < 0.05) compared with CON; †, difference (P < 0.05) compared with HCHO. Data expressed as means ± SE (n = 6). From Febbraio et al. (1996a). 500 practical issues feine (Vallerand et al. 1989) or ephedrine, caffeine and theophylline (Vallerand et al. 1993) results in a signiﬁcant increase in heat production in coldexposed humans, but the ingestion of caffeine alone produces no such effect (Graham et al. 1991). Likewise, some researchers (Wang et al. 1987) but not others (Vallerand et al. 1993) have demonstrated that the ingestion of theophylline during cold exposure attenuates the fall in body temperature. It appears, therefore, that ingestion of b-adrenergic agonists may provide some means of enhancing thermoregulatory thermogenesis, although further work in this area is required to conﬁrm this theory. In addition, since b-adrenergic agonists such as ephedrine and caffeine are substances banned by the International Olympic Committee, they may be impractical as a mechanism for overcoming cold stress during athletic competition. Since carbohydrate is the major substrate utilized in shivering thermogenesis, it has been suggested that low endogenous glycogen stores may reduce cold tolerance. This is true of very lean individuals (Martineau & Jacobs 1989) but not of moderately lean and fatter individuals (Young et al. 1989). Therefore, adequate carbohydrate stores are not only important to fuel muscle contraction during exercise, they possibly allow for a better maintenance of body core temperature, especially in leaner athletes. In summary, during exercise in a cold environment, effort should be made to ensure that pre-exercise carbohydrate stores are adequate in order to offset the potential increase in carbohydrate oxidation associated with shivering and non-shivering thermogenesis. This is especially important for those individuals who live and repeatedly exercise in a cold environment. The concentration of carbohydrate within a ﬂuid beverage should not be increased to more than 12%, despite the fact that ﬂuid loss via sweating is minimized or abolished, because of potential gastrointestinal distress. Finally, the ingestion of b-adrenergic agonists such as caffeine and theophylline may provide some beneﬁt against acute cold exposure, but further work examining this phenomenon is required. Exercise in a hot environment Substrate utilization during exercise in the heat Although there is some conﬂict in the literature, it is generally accepted that exercise in a hot environment results in a substrate shift towards increased carbohydrate utilization. Muscle glycogenolysis (Fink et al. 1975; Febbraio et al. 1994a, 1994b), liver glucose production (Hargreaves et al. 1996a) and respiratory exchange ratio (Febbraio et al. 1994a, 1994b; Hargreaves et al. 1996a) are higher during exercise in a hot environment. Furthermore, both muscle (Young et al. 1985; Febbraio et al. 1994a, 1994b) and plasma (Rowell et al. 1968; Fink et al. 1975; Powers et al. 1985; Young et al. 1985; Yaspelkis et al. 1993; Febbraio et al. 1994a) lactate accumulation are increased in humans during exercise in the heat compared with during similar exercise in a cool environment. The increase in plasma lactate accumulation is likely to reﬂect an increase in muscle lactate production, since hepatic lactate removal, although decreased during exercise in the heat, does not account for the increase in plasma lactate accumulation (Rowell et al. 1968) while muscle lactate efﬂux is unaffected during exercise and heat stress (Nielsen et al. 1990). It must be noted, however, that not all studies have observed an increase in intramuscular glycogen utilization during exercise in the heat (Nielsen et al. 1990; Yaspelkis et al. 1993; Young et al. 1996). It is likely that the discrepancy in the literature is related to methodological differences such as the use of acclimatized subjects (Yaspelkis et al. 1993) or differences in pre-exercise glycogen concentrations (Nielsen et al. 1990; Young et al. 1996) when comparing exercise in the heat with that in a cooler environment. These factors will inﬂuence rates of glycogen utilization, since heat acclimation attenuated glycogenolysis during exercise in the heat (King et al. 1985) while pre-exercise glycogen concentration is directly related to rates of utilization during submaximal exercise (Chesley et al. 1995; Hargreaves et al. 1995). In general, the exercise at climatic extremes literature suggests that exercise and heat stress results in a shift towards increased carbohydrate catabolism. The increase in carbohydrate oxidation indicates that lipid utilization is decreased during exercise in the heat. Few studies, however, have examined the effect of exercise and heat stress on lipid catabolism. Plasma free-fatty acid concentration (Fink et al. 1975; Nielsen et al. 1990) and uptake (Nielsen et al. 1990) are similar when comparing exercise in the heat with that in a cooler environment. These ﬁndings, however, do not demonstrate unequivocally that lipid utilization is unaffected by heat stress during exercise, since Fink et al. (1975) also observed a decreased intramuscular triglyceride utilization. These data, along with the consistent observation of an increased RER during exercise and heat stress, suggest a substrate shift away from lipid. Recently, Mittleman et al. (1998) have demonstrated that branched-chain amino acid (BCAA) supplementation increased endurance performance during exercise in the heat. This ﬁnding is in contrast with studies conducted during exercise in cooler environments (van Hall et al. 1995; Madsen et al. 1996). This discrepancy could arise because protein catabolism may be augmented during exercise in the heat. We have observed an increase in ammonia (NH3) accumulation during exercise and heat stress (Snow et al. 1993; Febbraio et al. 1994b). Although a major pathway for NH3 production during exercise is via the deamination of adenosine 5¢-monophosphate to form NH3 and inosine 5¢-monophosphate (IMP), NH3 can also be formed in skeletal muscle via the oxidation of BCAA. Accordingly, BCAA supplementation augments muscle NH3 production during exercise (MacLean et al. 1996). During our study (Febbraio et al. 1994b), the augmented muscle NH3 accumulation when comparing exercise in the heat with that in a cooler environment was observed in the absence of any difference in IMP accumulation, suggesting that enhanced BCAA oxidation may have accounted for the increase. It should be noted, however, that others (Dolny & Lemon 1988) have estimated protein degradation, as measured by urea excre- 501 tion, to be reduced during exercise in the heat. Further work examining the effect of exercise and heat stress on protein catabolism is warranted. Factors inﬂuencing fatigue during exercise in the heat: substrate depletion vs. hyperthermia During submaximal exercise in comfortable ambient temperatures, the rate of energy utilization is closely matched by rates of energy provision. It is well established that in these circumstances fatigue is often associated with glycogen depletion and/or hypoglycaemia (Coyle et al. 1986; Sahlin et al. 1990) and endurance can be increased by providing exogenous carbohydrate during exercise (Coyle et al. 1986; Coggan & Coyle 1987). At fatigue the muscle is characterized by low glycogen levels and a concomitant elevation in IMP accumulation (Sahlin et al. 1990; Spencer et al. 1991), since glycogen depletion may impair the tricarboxylic acid cycle and adenosine triphosphate must be generated from alternative pathways such as the adenylate kinase reaction. Since carbohydrate utilization is augmented during exercise in the heat and fatigue often coincides with depletion of this substrate, it is somewhat paradoxical that fatigue during exercise in the heat is often related to factors other than substrate depletion. We (Parkin et al. 1999) and others (Nielsen et al. 1990) have demonstrated that intramuscular glycogen content is approximately 300 mmol · kg–1 dry weight at fatigue when, during exercise in cooler environments, this ﬁgure is usually less than 150 mmol · kg–1 dry weight (Fig. 38.2). This may be because hyperthermia may lead to fatigue prior to carbohydrate stores being compromised. This hypothesis is supported by the observations that, when exercising in the heat to exhaustion, subjects will fatigue at the same body core temperature even if interventions such as acclimatization (Nielsen et al. 1993) or ﬂuid/carbohydrate ingestion (Febbraio et al. 1996a) alter the duration of exercise. There may be circumstances, however, where carbohydrate may be limiting during exercise in the heat. If the 502 practical issues 500 1.25 400 1.0 * IMP (mmol.kg–1) Glycogen (mmol.kg–1) * 300 200 100 0.75 0.5 0.25 0 Rest 0 Fatigue (a) Rest Fatigue (b) Fig. 38.2 (a) Glycogen content and (b) inosine 5¢-monophosphate (IMP) concentration before (rest) and after (fatigue) submaximal exercise to exhaustion in different ambient temperatures: 䊏, 40 °C; , 20 °C; 䊐, 3 °C. Data expressed as mean ± SE (n = 8). From Parkin et al. (1999), with permission. intensity of exercise is moderate, resulting in a relatively low rate of endogenous heat production, or the exercise is intermittent in nature allowing for effective heat dissipation, carbohydrate may be limiting. Accordingly, carbohydrate ingestion may (Murray et al. 1987; Davis et al. 1988b; Millard-Stafford et al. 1992) or may not (Davis et al. 1988a; Millard-Stafford et al. 1990; Febbraio et al. 1996a) increase exercise performance in the heat. The beneﬁt of carbohydrate ingestion during and following exercise in the heat may, however, be related to factors other than exercise performance. Immune function has been demonstrated to be depressed by increases in stress hormones such as catecholamines, corticosteroids and growth hormone (Keast et al. 1988). These hormones are elevated when comparing exercise in the heat with that in a cooler environment (Febbraio et al. 1994a; Hargreaves et al. 1996a). There may be, therefore, a possible relationship between exercise in a hot environment and immune suppression. Indeed, it has been demonstrated that exercise and heat stress results in a decrease in lymphocyte production (Cross et al. 1996). Carbohydrate feeding during exercise in comfortable ambient conditions results in a decrease in circulating adrenaline (McConell et al. 1994), cortisol (Mitchell et al. 1990) and growth hormone (Smith et al. 1996). In addition, plasma elastase, a marker of in vivo neutrophil activation, is reduced during exercise with carbohydrate feedings (Smith et al. 1996). It is possible, therefore, that carbohydrate ingestion during and following exercise in the heat may attenuate the rise in the counterregulatory hormones which depress immune function, and we are currently undertaking experiments to examine this hypothesis. As mentioned previously, glycogen content within human skeletal muscle at the point of fatigue during exercise in the heat is often adequate to maintain energy turnover via oxidative phosphorylation. It is somewhat surprising, therefore, that a marked increase in IMP accumulation at fatigue during exercise and heat stress is observed despite glycogen concentration being adequate to maintain the oxidative potential of the contracting skeletal muscle (Fig. 38.2) (Parkin et al. 1999). These data suggest a disruption to mitochondrial function during exercise and heat stress and support recent ﬁndings by Mills et al. (1996), who observed an increase in plasma concentrations of lipid hydroperoxides, a marker of oxidative exercise at climatic extremes stress, in horses exercising in the heat. In addition, when examining the ratio between adenosine diphosphate (ADP) production and mitochondrial oxygen consumption (ADP/O ratio) in isolated rat skeletal muscle mitochondria, Brooks et al. (1971) observed a constant ADP/O ratio at temperatures ranging from 25 to 40°C. Above 40°C, however, the ADP/O ratio declined linearly with an increase in temperature, suggesting that for a given oxygen consumption the increase in ADP rephosphorylation was lower than the rate of ATP degradation. Interestingly, in our previous studies in which we observed increased phosphocreatine degradation and IMP formation (Febbraio et al. 1994b; Parkin et al. 1999), intramuscular temperature was greater than 40°C following exercise in the hot environment but not the control trial. The data indicate, therefore, that the combination of exercise and heat stress may affect mitochondrial function resulting in oxyradical formation. Although speculative, antioxidant supplementation may be of beneﬁt during exercise in the heat and we are currently examining such a phenomenon. Candas et al. 1986; Hamilton et al. 1991; Montain & Coyle 1992), which also improves exercise performance (Maughan et al. 1989; Walsh et al. 1994; McConell et al. 1997). In addition to the physiological alterations caused by dehydration, we have also observed that ﬂuid ingestion reduces muscle glycogen use during prolonged exercise (Fig. 38.3), since it also results in a reduced intramuscular temperature and a blunted sympathoadrenal response (Hargreaves et al. 1996b). It is clear from these data that ﬂuid ingestion not only attenuates the rise in body core temperature, thereby preventing hyperthermia, it also reduces the likelihood of carbohydrate depletion. Since sweat rate is exacerbated during exercise in the heat, dehydration progresses more rapidly and therefore the importance of ﬂuid ingestion is increased during exercise in extreme heat. Indeed, Below et al. (1995) have demonstrated that ﬂuid ingestion improves exercise performance in a hot environment. Since the negative effects of dehydration are well documented, it would be desirable to hyperhydrate prior to exercise in a hot environment. Accordingly, glycerol added to a bolus of water Beneﬁt of ﬂuid ingestion 500 400 ∆ GLY (mmol.kg–1 dry wt) Although a more comprehensive review of ﬂuid ingestion is covered in previous chapters of this book (see Chapters 15–17), it is necessary to reiterate the importance of ﬂuid when discussing nutrition for exercise in climatic extremes. In circumstances where the endogenous heat production and high environmental temperature result in fatigue prior to carbohydrate stores being compromised, ﬂuid ingestion, irrespective of whether it contains carbohydrate, is of major importance in delaying the rise in body core temperature. Exercise-induced dehydration is associated with an increase in core temperature (Hamilton et al. 1991; Montain & Coyle 1992), reduced cardiovascular function (Hamilton et al. 1991; Montain & Coyle 1992) and impaired exercise performance (Walsh et al. 1994). These deleterious physiological effects are attenuated, if not prevented, by ﬂuid ingestion (Costill et al. 1970; 503 * 300 200 100 0 No fluid Fluid ingestion Fig. 38.3 Net muscle glycogen utilization (GLY; postexercise minus pre-exercise) during 120 min of exercise in the absence or presence of ﬂuid ingestion. *, difference (P < 0.05) compared with no ﬂuid. Data expressed as mean ± SE (n = 5). From Hargreaves et al. (1996b), with permission. 504 practical issues and ingested has been demonstrated by some (Lyons et al. 1990; Koenigsberg et al. 1991; Freund et al. 1995) but not others (Murray et al. 1991) to increase ﬂuid retention, reduce sweat rate and consequently result in an enhanced thermoregulatory capacity, especially during exercise in a hot environment (Lyons et al. 1990). Although not clearly understood, it appears that the effectiveness of glycerol may be related to an attenuated rate of free water clearance, and/or an increase in the kidney’s medullary concentration gradient resulting in increased glomerular reabsorption (Freund et al. 1995). On balance, the literature suggests that glycerol hyperhydration may be effective prior to exercise in a hot environment. As sweat rate increases during exercise in the heat, the potential for electrolyte loss, in particular sodium, is increased. It has been suggested that sodium be included in rehydration beverages to replace sweat sodium losses, prevent hyponatraemia, promote the maintenance of plasma volume and enhance intestinal absorption of glucose and ﬂuid (for detailed review, see Chapter 17). Although the addition of sodium to a ﬂuid beverage will maintain the drive for drinking and minimize urinary ﬂuid loss in recovery from exercise (Nose et al. 1988; Maughan & Leiper 1995), we have observed little effect of alterations in beverage sodium content on glucose or ﬂuid bioavailability during exercise (Hargreaves et al. 1994). Guidelines for dietary intake when exercising in the heat In examining the literature, it is clear that both carbohydrate and ﬂuid availability are very important when making dietary recommendations for those exercising in the heat. The intake of carbohydrate should be increased with repeated exercise bouts in the heat because even though acclimation reduces glycogenolytic rate (King et al. 1985), glycogen use is still higher in an acclimated individual exercising in the heat than in an unacclimated individual exercising in cooler conditions (Febbraio et al. 1994a). In addition, those individuals who undergo daily exercise in hot conditions must pay careful attention to ﬂuid intake, since heat acclimatization increases sweat rate (Armstrong & Maresh 1991) and, hence, body ﬂuid loss. It is important to note that while a high carbohydrate diet may exaggerate the core temperature response in rats (Francesconi & Hubbard 1986), such a diet does not cause any deleterious thermoregulatory responses during exercise in humans (Schwellnus et al. 1990). When exercising, one should ingest a carbohydrate/ﬂuid/electrolyte beverage frequently. Since the relative importance of ﬂuid delivery is increased during exercise in the heat, one may be tempted to ingest water in these circumstances. This practice should be avoided, since the ingestion of a carbohydrate/electrolyte/ﬂuid beverage empties from the gut at the same rate as water (Francis 1979; Owen et al. 1986; Ryan et al. 1989), while it can spare muscle glycogen (Yaspelkis & Ivy 1991), during exercise in the heat. In addition, the relative importance of electrolyte intake may be increased during exercise in the heat and thus rehydration beverages should include electrolytes. The amount of the carbohydrate within a ﬂuid beverage ingested during exercise in the heat appears to have little effect on ﬂuid availability or exercise performance, provided the carbohydrate is not too concentrated. The change in plasma volume and exercise performance in the heat is not different when ingesting beverages containing 0%, 4.2% and 7% carbohydrate, respectively. Of note, however, when a 14% carbohydrate solution is ingested during exercise in the heat, the maintenance of plasma volume is reduced while the rise in rectal temperature tends to be augmented. Accordingly, exercise performance tends to fall (Fig. 38.4) (Febbraio et al. 1996a). It is important, therefore, to keep the concentration of carbohydrate within a ﬂuid beverage to approximately 10% during exercise in the heat, even though carbohydrate utilization is augmented in these circumstances. In terms of volume and frequency, a practical recommendation might be 400 ml every 15 min since the rate of ﬂuid loss during exercise in the heat is approximately 1.6 l · h–1 (M. exercise at climatic extremes –5 –10 –15 * –20 0 (a) 120 39 100 38.5 38 37.5 37 36.5 20 40 60 80 100 120 Time (min) 39.5 Exercise time (min) 0 Rectal temperature ( C) Change in PV (%) 5 0 20 40 60 (b) 80 100 120 Time (min) 505 80 60 40 20 0 (c) CON 4.2% 7% CHO CHO 14% CHO Fig. 38.4 The change in (a) plasma volume (PV), (b) rectal temperature, and (c) time to exhaustion, while consuming a placebo (CON, 䊏) or carbohydrate (CHO) beverage of differing concentrations: 4.2% CHO (䊉), 7% . CHO (䉭) or 14% CHO (䉮) during fatiguing exercise at 70% Vo2max. in 33°C conditions. *, difference (P < 0.05) from other trials. Data expressed as mean ± SE. Data from Febbraio et al. (1996a). Febbraio, unpublished observations). It is also recommended that the carbohydrate beverage be ingested into recovery to replenish intramuscular glycogen stores and promote rehydration, especially important for those individuals repeatedly exercising in a hot environment. As previously discussed, there is some evidence to suggest that protein catabolism is increased during exercise in the heat. One may be tempted to recommend that protein intake be increased prior to and during such exercise. However, it must be noted that there is a relative paucity of research examining protein requirements during exercise in the heat and more is required before deﬁnitive recommendations can be made. Likewise, there is some evidence to suggest that oxyradical generation may be increased via the combination of exercise and heat stress and it may be of some beneﬁt to supplement those undertaking repeated exercise in a hot environment with antioxidants such as a-tocopherol (vitamin E) and ascorbic acid (vitamin C). This recommendation is speculative, however, since the hypothesis that such supplementation is advantageous during exercise in the heat is yet to be experimentally investigated. The deleterious effects of dehydration during exercise, especially that which is conducted in a hot environment, have been well documented. It would be desirable to hyperhydrate before exercise and, as has been demonstrated, glycerol ingestion may provide some beneﬁt in achieving hyperhydration by attenuating urine output. It appears that a regime consisting of 1 g glycerol · kg–1 body weight in approximately 2 l of ﬂuid ingested in the 120 min prior to exercise provides some beneﬁt during subsequent exercise (Lyons et al. 1990). Competitive athletes should, however, experiment with this regime during training, since not all individuals may respond favourably to glycerol hyperhydration. In summary, during exercise in the heat, a balance between preventing hyperthermia and maintaining adequate fuel supply to fuel muscle contraction must be maintained. In order to achieve this, athletes need to closely monitor hydration levels and carbohydrate intake leading up to exercise. Daily monitoring of body weight and ensuring that urine is pallid will provide a guide to hydration status. During competition, a 4–8% carbohydrate/ﬂuid/electrolyte solution should be ingested at approximately 400 ml every 15 min and such ingestion should be maintained during recovery to ensure ﬂuid and energy replacement. Other dietary modiﬁcations such as increased protein intake, antioxidant supplementation and glycerol hyperhydration may provide some beneﬁt but further research in these areas is required before deﬁnitive 506 practical issues recommendations can be made regarding their efﬁcacy. References Armstrong, L.E. & Maresh, C.M. 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