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The Influence of Nutritional Ergogenic Aids on Exercise Heat Tolerance and Hydration Status

Lopez, Rebecca M.; Casa, Douglas J.

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Current Sports Medicine Reports: July 2009 - Volume 8 - Issue 4 - p 192-199
doi: 10.1249/JSR.0b013e3181ae4f66
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Abstract

INTRODUCTION

The use of supplements to improve athletic performance has an extensive history by a wide range of competitive athletes. Along with the proposed ergogenic effects of numerous supplements, which are often not proven or are exaggerated greatly, there could be harmful and potentially fatal side effects. Many athletes have a "win at all costs" mentality and will choose to use a supplement regardless of the possible side effects. Furthermore, many of the side effects reported with supplements are anecdotal and have not been supported by scientific evidence.

Exercise in the heat can have added risks to athletes. Recent exertional heat stroke deaths have increased media attention to some supplements and their potential connection with exertional heat stroke (7,15). Unfortunately, some of these reports and recommendations on the use of supplements are speculative and not based on factual evidence or scientific research (7,55). As a result, athletes are misinformed on which supplements may predispose them to an exertional heat illness. They are unaware what preventative measures to take when exercising in the heat. It is imperative that medical professionals educating athletes are knowledgeable about the various performance enhancers and their possible effects on heat tolerance to be able to give sound recommendations to competitive athletes and other physically active populations. Randomized-controlled trials (6,17,22,26,28,52,56,57) have resulted in evidence-based recommendations regarding the use of ergogenic supplements. Therefore, the purpose of this article is to describe the influence of popular nutritional supplements on hydration status and thermoregulation.

CREATINE

Although creatine was first identified in 1835, creatine supplementation became increasingly popular in athletic populations in the 1990s (4). Creatine's popularity stemmed from research demonstrating an increase in skeletal muscle phosphocreatine content and increased performance in some subjects (55). Creatine is a dietary element found in meat and fish; however, creatine also is synthesized by the body and can be found in skeletal muscle, heart muscle, and other organs (27). Many athletes supplement with creatine because it has been shown to increase muscle concentrations of total creatine and phosphocreatine (32). This increase in creatine and phosphocreatine potentially can act as a source of adenosine triphosphate in skeletal muscles, thereby increasing performance in short-duration, high-intensity bouts of exercise (55).

Initial reports on the efficacy and safety of creatine supplementation included warnings of possible long-term effects of creatine and anecdotal reports of increased risk of heat illness and muscle injury (Figure; Table 1). In the late '90s, several media reports and scientific papers linked creatine to several athletes' deaths (7,55). As a result, many have speculated that individuals supplementing with creatine are at greater risk of heat illness when exercising in hot and/or humid environments. In 2000, the American College of Sports Medicine's roundtable on creatine supplementation cautioned athletes on supplementing with creatine while exercising in the heat (55). However, there was no evidence at this time to support an increased risk of an exertional heat illness as a result of creatine supplementation. These anecdotal claims resulted in an increase in scientific studies investigating the relationship between creatine and hydration status and/or heat intolerance (12,14,20,26,29,30,41,48,51,56,57,59,61,62). A systematic review of the existing literature on the influence of creatine on thermoregulation and hydration status concluded that there is no evidence to support claims that creatine impedes heat tolerance or hydration status (35). As Table 2 illustrates, the systematic review failed to show significant differences in body temperature between the creatine and placebo groups while exercising in the heat.

F1-10
FIGURE:
Results of studies investigating the influence of creatine on hydration status and exercise heat tolerance. Reprinted from Lopez RM, et al. Does creatine supplementation hinder exercise heat tolerance or hydration status: a systematic review with meta-analyses. J. Athl. Train. 2009;44:215-23. Copyright © 2009 National Athletic Trainers' Association. Used with permission. Citations pertain to references in the original article.
T1-10
TABLE 1:
Nutritional ergogenic supplements: evidence of effects on heat tolerance and hydration status.
T2-10
TABLE 2:
Results of studies investigating the influence of creatine on hydration status and exercise heat tolerance.a

The most common side effect from creatine supplementation is increased body mass as a result of water retention. This weight gain has been hypothesized to affect the body's ability to thermoregulate by altering body fluid balance. Researchers investigating the effect of creatine on body fluid balance have had varying results. One study found that 10 d of creatine supplementation did not result in altered fluid distribution or promote an osmotic fluid shift between fluid compartments (14). Several studies found that supplementing with creatine increases total body water (20,29,30,48,56,59,61,62) and intracellular water (30,59,62), yet others have found increases in extracellular water as well (20,59). This increase in total body water is evident in subjects who have an increased body mass post-supplementation. However, this influence of a possible fluid shift has not had any detrimental effects on subjects' ability to dissipate heat when exercising in the heat. Several studies have found that subjects supplementing with creatine had a lower heart rate and core body temperature (20,29,30) while exercising in the heat when compared with placebo. Also, subjects supplementing with creatine who already were dehydrated before exercising in the heat showed no signs of heat intolerance compared with placebo (57).

Researchers have concluded that both long-term and short-term creatine supplementation may have no effect or may even be advantageous for athletes exercising in the heat (12,14,20,23,29,30,35,41,57,59,61). The evidence from the recent increase in controlled studies does not support previous claims that creatine may hinder heat tolerance and/or hydration status. In fact, some studies found that creatine resulted in a lower body temperature than placebo when exercising in the heat (29,30,41). Although the rationale behind this has not been elucidated, perhaps the increased total body water seen with creatine supplementation results in improved thermoregulation. Therefore, there is a lack of evidence to support advising athletes who are supplementing with creatine to avoid exercise in the heat (35).

CAFFEINE

Caffeine, or 1,3,7-trimethylxanthine, is one of the most widely used drugs. Recently, there has been an increase in caffeine intake by athletes to improve athletic performance. Only low to moderate doses of caffeine (3-10 mg·kg−1) are needed to bring about an ergogenic effect (18). Researchers have found that these nontoxic doses of caffeine improve athletic performance, particularly endurance exercise (18). Some evidence suggests that caffeine also may serve as an ergogenic aid in high-intensity bouts of exercise (18).

Although caffeine is a popular drug among general and athletic populations alike, caffeine often has been linked with dehydration and/or heat intolerance (Table 1). The theory behind this could be attributed to caffeine's diuretic effect, the stimulation of the sympathetic nervous system, or an increase in resting metabolic rate (5). As a result, athletes sometimes are advised to abstain from caffeine while exercising, particularly in the heat. Recent studies have sought to determine whether there is evidence to support advising athletes to abstain from caffeine because of hydration and/or thermoregulatory impairments (6,17,19,21,22,42,52,54).

Studies have investigated the short- and long-term effects of various levels of caffeine ingestion on hydration status and thermoregulation in thermoneutral, warm, and hot conditions (Table 3). A study investigating the effects of acute caffeine ingestion on thermoregulation and body fluid balance during prolonged exercise in a thermoneutral environment found no adverse effects of caffeine on body fluid balance or thermoregulatory or metabolic responses (21). Following ingestion of 5 and 2.5 mg·kg−1 of caffeine at 2 and 0.5 h of exercise, respectively, there were no significant differences in final rectal temperature, total water loss, sweat rate, rise in rectal temperature, and percent change in plasma volume compared with placebo (21).

T3-10
TABLE 3:
Diuretic effect of caffeine, considering dose and volume of fluid consumed.a

Similarly, another study examined the effects of 6 mg·kg−1 of caffeine before exercise in high ambient temperatures on hemodynamic and body temperature responses (54). This study found significant increases in lactate and heart rate during exercise as a result of caffeine; however, caffeine again had no effect on body temperature during exercise. Several other researchers have found similar results with acute caffeine intake having no adverse effects on fluid-electrolyte balance (42,52), sweat rate (22,52), urine osmolality (22,52), or thermoregulatory responses (22,42,52) compared with non-caffeinated sports drinks or placebo.

Although most studies have investigated the effects of acute or short-term caffeine ingestion, one study looking at the effects of long-term caffeine intake on hydration found little evidence to suggest that chronic caffeine intake may lead to dehydration or heat intolerance (6). Armstrong et al. examined the effects of three levels of caffeine consumption on fluid-electrolyte balance and renal function. Although this study supported previous findings that caffeine may act as a diuretic soon after consumption, urine volumes, serum osmolality, and all other hydration measures were similar among all treatment groups on all days (6). Until this study, little was known about the effects of chronic caffeine consumption greater than 24 h on hydration status. Across 11 d, changes in caffeine dosage had no significant effect on body mass or urinary indices (6). This study found no evidence to suggest that athletes consuming moderate amounts of caffeine (3-6 mg·kg−1·d−1) are at greater risk of hypohydration and/or heat intolerance while exercising (6 mg·kg−1·d−1 for a 70-kg individual is equivalent to the following intake per day: 4 cups of coffee (8 oz.), 10 cans of Coca-Cola (12 oz.), or 5 cans of Red Bull (8 oz.), for example). Furthermore, caffeine's influence as a mild diuretic likely is negated when fluid status is compromised during exercise in the heat. Results of the various studies examining the effects of caffeine on hydration status suggest that there is little evidence to support the recommendations for exercising individuals to avoid caffeine, particularly when exercising in the heat.

GLYCEROL

Compared with the other nutritional supplements mentioned in this review, glycerol is the only ergogenic supplement with purported beneficial effects on hydration status and/or heat tolerance (Table 1). Researchers have investigated various means of hyperhydrating athletes before exercise, particularly for endurance events where hydrating during exercise can be difficult. Hyperhydration with water alone has resulted in increased urine output soon after ingestion, thus leading to ineffective hyperhydration. As a result, several studies have investigated the effects of glycerol ingestion with water to promote hyperhydration to decrease cardiovascular and thermoregulatory strain and improve exercise performance (3,24,25,28,31,36,38,40,44,46,49,60). A metaanalysis on the effects of glycerol-induced hyperhydration on fluid retention and exercise performance revealed that hyperhydration with glycerol improved fluid retention by 50% compared to hyperhydration with water alone (24). Glycerol is a naturally occurring three-carbon molecule that increases the concentration of fluid in the blood and tissues (49). Increases in blood osmolality together with increased water intake would result in an osmotic drive resulting in water retention (49).

The available literature on the influence of glycerol on increasing athletic performance and/or improving various physiological responses is inconclusive. Various researchers have found that hyperhydration with glycerol has improved performance significantly versus hyperhydration with water alone (25,28,44). However, others have found no significant differences in exercise performance when subjects were hyperhydrated with glycerol (28,36,38,60). Data regarding the influence of glycerol on cardiovascular, thermoregulatory, and other physiologic responses also have varied. While some studies have found decreases in cardiovascular and/or thermoregulatory strain with glycerol ingestion (25,28,38,44,46,60), others have found no significant differences between glycerol and placebo (31,38).

Studies investigating the effects of glycerol ingestion before exercise in the heat also varied in the results; however, overall there were no definite advantages to glycerol over water in three out of these four studies (3,28,38,60). Anderson et al. found that glycerol ingestion prior to 90 min of steady-state cycling resulted in fluid retention that was capable of reducing cardiovascular strain and enhancing thermoregulation (3). Similarly, Kavouras et al.'s findings in a rehydration study suggest that glycerol ingestion prior to cycling in the heat improved exercise capacity (28). Subjects exercised 19% and 72% longer in the glycerol trial versus both the water only and the non-fluid trials, respectively. However, despite increased cutaneous vascular conductance and improved exercise capacity attributed to plasma volume expansion, there were no significant differences in urine output or other cardiovascular measures with glycerol compared with water or no fluid conditions (28).

In contrast to Anderson et al.'s findings, Marino et al. found no significant improvements in exercise performance with glycerol ingestion (38). Despite increases in sweat rate and urine osmolality in the glycerol condition compared with placebo, a 60-min self-paced cycling bout in 34.5°C resulted in no decrease in thermoregulatory strain or increased performance. An investigation on the effects of pre-exercise glycerol on performance and physiological function during a mountain bike race in the heat found that glycerol with water did not affect cardiovascular or thermoregulatory responses compared with hydration without glycerol, despite decreased urine output and water retention found with glycerol (60). Researchers also examined the effects of glycerol ingestion for both hyperhydration and as a rehydrating agent following 120 min of exercise-induced dehydration. Results indicated the glycerol trial had significantly greater fluid retention (approximately 700 mL) compared with placebo (36).

The varying results in the literature could be attributed to differences in methodology, such as the time between glycerol hyperhydration and exercise. Studies with a greater gap between hyperhydration and exercise, such as 3 h post-glycerol, found approximately 600 mL greater total body water (34). Moreover, when subjects were hyperhydrated pre-exercise and rehydrated during exercise, the hydration advantages of glycerol over water alone were lost. Similarly, when exercise start time was within 1 h of hyperhydration, there were no differences in hydration status between glycerol and water (34). Differences in exercise and hydration protocols also have been inconclusive. Therefore, the variability in the literature makes it difficult to elucidate whether hyperhydration with glycerol improves thermoregulation or decreases cardiovascular strain during exercise, particularly while exercising in the heat. However, in certain situations (such as the inability to consume large amounts of fluid during intense exercise in the heat or not having access to fluids), the use of preexercise glycerol may assist with maintaining hydration status during exercise.

EPHEDRA

The use of the supplement ephedra, from the Chinese plant species ma huang, has been controversial in the last decade. Although the use of ephedra dates back more than 5000 yr (1), only recently has the supplement been exposed as a potential performance enhancer and weight loss aid. Recent high-profile athlete deaths have been linked to the use of supplements such as ephedrine (7,15). Ephedrine is the major isomer making up 30% to 90% of the total alkaloids found in ephedra (1); the other ephedra alkaloids include pseudoephedrine, norpseudoephedrine, and methylephedrine (2). Ephedra, often in combination with caffeine, has been associated with possibly predisposing athletes to exertional heat stroke, particularly with individuals exercising in hot, humid environments (15,33).

As a result of speculations that ephedra is to blame for some of these high-profile deaths, several sports organizations have banned the use of ephedra for athletes. Several consumer health groups requested that the U.S. Food and Drug Administration (FDA) ban the sale of ephedra-containing products, despite the lack of scientific evidence linking ephedrine to these reported deaths or serious side effects. Dietary supplements are regulated by the Dietary Supplement Health and Education Act of 1994 (DSHEA); the FDA can only ban a supplement if there is sufficient convincing evidence that the product is harmful (53). A review of 52 randomized controlled trials investigating the effects of ephedra-containing dietary supplements on weight loss and/or athletic performance found there were no serious adverse events, such as death or cardiovascular events; however, some of the side effects reported included anxiety, autonomic hyperactivity, palpitations, and headache (53). It could be theorized that increases in heart rate and metabolic rate as a result of ephedra may lead to an increased heat load or some impairment in the body's ability to dissipate heat at the same rate that it is being generated. However, there has been no evidence of elevated core body temperature or heat intolerance in these randomized controlled trials (Table 1). The subjects used in these clinical trials, however, were healthy and had been screened for predisposing factors to illness.

There is a paucity of data regarding the potential influence of ephedra-containing supplements on thermoregulation. Several studies have investigated the effects of ephedrine with caffeine (8-11,37), since many products on the market contain both of these ingredients. However, many of these studies were examining the influence of ephedrine on exercise performance and did not report thermoregulatory measures (8,10,11,45). One study by Bell and associates examined the effects of caffeine and ephedrine (C+E) ingestion on thermal regulation during heat stress trials in 40°C (9). Initial pre-exercise rectal temperatures (Tre) were not different, and changes in Tre throughout the exercise bouts were not significantly different between C+E and placebo conditions (9).

There have, however, been several case studies linking ephedra to exertional heat stroke (15,47). The death of the Baltimore Orioles' pitcher Steve Bechler was linked initially to ephedrine supplements that he was taking when he collapsed from exertional heat stroke (15). However, it has been suggested that there were several other factors in Bechler's case that may have either predisposed him to heat stroke (e.g., history of heat illness, lack of acclimatization, and several layers of clothing) or contributed to his death (improper treatment). Therefore, ephedrine could not be solely responsible for his death (33).

Another case, of a highly trained infantry soldier, may be the only case, to our knowledge, to report a concrete connection between ephedra and exertional heat stroke (47). Although the mechanism through which ephedra may cause exertional heat stroke is not immediately clear, it is believed that ephedra may activate dopamine receptors and impair heat dissipation through vasoconstriction, thus producing a thermogenic effect (45). To date, there is insufficient evidence that ephedra can impair thermoregulation; however, the inability to conduct randomized controlled trials because of its potential negative effects may leave the effects of ephedra while exercising in the heat unknown. Furthermore, there have been sufficient case reports of deaths linked to ephedra (whether heat-related or not) to make one wary of suggesting ephedra as a safe supplement for athletes or other exercising individuals.

OTHER SUPPLEMENTS

With the abundance of supplements in the sports market and the lack of regulation by the FDA, it is not surprising that there are many unknowns regarding the effects of other supplements on hydration and thermoregulation. There are a few other supplements that can be used as ergogenic aids and warrant mention in this review with regard to their effect on hydration and/or thermoregulation. Although many studies have investigated the effects of branched-chain amino acids (BCAA) on exercise performance, few have examined the effect that BCAA supplementation may have on hydration status and/or thermoregulation. Cheuvront et al. examined the impact of BCAA supplementation and performance in the heat when hypohydrated (16). This study found that exercise in the heat (40°C) after ingestion of either an isocaloric BCAA and carbohydrate drink or a carbohydrate-only drink resulted in no differences in core body temperature. Similarly, Mittleman et al. compared the effects of a BCAA drink to a placebo during exercise in the heat (34.4°C) and found no significant differences in core body temperature or sweat losses (43). In a study comparing the effects of varying levels of protein intake on hydration indexes, Martin et al. found that although plasma osmolality and urine specific gravity were elevated as protein intake increased, these hydration indexes were still within normal limits (39). Therefore, the effect of increased protein intake on measures of hydration status was minimal.

The combination of several dietary supplements was also believed to have caused a malignant hyperthermia reaction in an otherwise healthy trauma patient (13). A 24-yr-old male trauma patient developed malignant hyperthermia after receiving the anesthetic sevoflurane in the operating room. The family denied a history of malignant hyperthermia and heat/exercise intolerance; however, upon recovery the patient admitted to 6 months of daily intense exercise, ingestion of creatine monophosphate and Ripped Fuel® (containing caffeine, ephedrine, and aspirin), daily injections of deca-durabolin (an anabolic steroid), and weekly testosterone (13). Although the authors of this case study had several hypotheses for the occurrence of malignant hyperthermia, the most prominent conclusion was that the patient may have increased intracellular calcium pharmacologically and altered skeletal muscle metabolism, which combined with the anesthetic may have contributed to the rapid malignant hyperthermia. Although the use of these supplements could not be confirmed as the cause of the hyperthermia, this case study epitomizes the idea that the effects of combining uncontrolled supplements may be deleterious.

The effects of some prescribed medications on hydration status and/or thermoregulation also warrant more randomized controlled trials, particularly when combined with exercise in the heat. Recent studies on the effects of acute dopamine reuptake inhibition have demonstrated elevated temperatures during an exercise bout in the heat (50,58). Ingestion of methylphenidate, a medication commonly prescribed to children and adults for the treatment of attention deficit hyperactivity disorder (ADHD) resulted in increased exercise performance as well as increased core temperatures only in the warm condition (30°C) compared to a temperate condition (18°C) (50). The subjects' perceptual responses of ratings of perceived exertion and thermal stress were not influenced by the drug treatment (50). This could be potentially dangerous during intense exercise in the heat, as athletes taking this medication may be susceptible to higher body temperatures while signals from the central nervous system to decrease intensity or cease exercise may be inhibited.

CONCLUSION

Exercising in the heat can predispose an athlete to an exertional heat illness if preventative measures are not heeded. Given that supplements are easily available and the veracity of their contents are tainted, it is imperative to remain abreast of the most recent evidence regarding supplementation to better educate and advise athletes. One also must rely on empirical evidence when making conclusions about the efficacy of a supplement while not ignoring significant anecdotal reports that more closely may resemble real-life situations. There continues to be an immense need for more randomized, controlled trials on the influence of nutritional supplements on hydration and thermoregulation in order to determine their clinical application in the sports arena.

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