This document provides a brief review of relevant scientific literature pertaining to the value of adequate fluid and nutrient intake before, during, and after vigorous physical activity in which sweat loss occurs. The reader desiring additional detailed information is referred to more extensive scientific reviews.^{(Maughan & Murray, 2001; Sawka & Pandolf, 1990; Sawka, 1992;Sawka et al. 2005)}

Introduction

Physical activity is widely recognized as an essential part of a healthy lifestyle because inactivity is a major risk factor for chronic disease and a threat to quality of life.^{(NIH, 1996)}

Physical activity is typically accompanied by sweating because the evaporation of sweat is an effective way for the body to lose the heat produced by muscle contractions and thereby help maintain a safe internal body temperature. As sweat evaporates from the surface of the skin, heat is lost to the environment (0.58 kcal/gram of water), reducing the rise in body temperature. Unless sufficient fluid is ingested during exercise, sweat loss will result in progressive dehydration. (Although the terms hypohydration or volume depletion are preferred by some scientists, dehydration will be used throughout this document to refer to a loss of body fluid that results in a reduction of total body water from normal – euhydrated – levels.)

Physiological consequences of dehydration

Dehydration reduces the physical and mental capacity for vigorous exercise, especially for prolonged exercise in warm or hot environments.^{(Cheuvront 2005)} The decrement in performance is associated with compromised cardiovascular and thermoregulatory function, including a lower cardiac output and higher core temperature at any given workload. ^{(Coyle, 1998; Gonzalez-Alonso et al., 1997)} Because dehydration is associated with higher body temperatures, it can increase the risk of heat-related disorders such as heat exhaustion during vigorous exercise.^{(Casa et al, 2005)} However, dehydration often attenuates the risk of heat illness by causing a reduction in exercise intensity or sometimes complete cessation of exercise.^{(Coyle & Montain, 1992)} Dehydration is most dangerous to health when athletes ignore the warning signs and continue vigorous exercise.

Performance consequences of dehydration

There is little doubt that dehydration can impair exercise performance.^{(Montain & Coyle, 1992; Sawka & Coyle, 1999)} The level of dehydration at which performance can be impaired has been identified as 2% of normal, euhydrated body weight (e.g., a fluid loss of 3 lb in a 150-lb athlete; 1.4 kg in a 68-kg athlete)^{(Casa et al., 2005)}, although there is some evidence that lower levels of dehydration will impair performance in warm environments.^{(Murray 2007)} The effect of dehydration on performance will vary depending upon factors such as the environmental conditions, exercise intensity, and exercise duration. Accordingly, it is possible that there are some conditions in which performance might be impaired by low levels of dehydration and other conditions in which performance is not affected until higher levels of dehydration are reached. Scientific quibbling aside, regardless of the conditions, it is better to be well hydrated during physical activity than to be dehydrated.^{(Maughan & Shirreffs, 1998; Montain & Coyle, 1992)}

It is possible that under some circumstances, dehydration of less than 2% of body weight might impair performance, although more research is required before a scientific consensus can be reached. It is well established that dehydration has a progressively negative impact on physiological function and performance, even when the loss of body water expressed as a percentage of body weight is low, e.g., 1 – 2%. (ACSM, 2007; Armstrong et al.,1985; Dougherty et al., 2006; Ekblom et al., 1970; Walsh et al. 1994)

A hot environment by itself impairs performance and magnifies the adverse effect of dehydration.^{(Sawka & Pandolf, 1990)} Time-to-fatigue at sub-maximal intensities is shortened when exercise is performed in the heat and is further impaired by dehydration.(Armstrong et al., 1997) Prolonged efforts are more likely to be negatively influenced by dehydration than brief, high-power exercise tasks that rely heavily on anaerobic metabolism.^{(Armstrong et al., 1985)} One reason for this difference is that dehydration is associated with a decrease in cardiac output and in the flow of blood and the delivery of oxygen to muscle.^{(Gonzalez-Alonso & Calbet, 2003)}

Finally, a problem among some athletes is exercise-associated muscle cramping, “painful spasmodic involuntary contractions of skeletal muscle that occur during or immediately after muscular exercise.” ^{(Schwellnus et al., 1997)} It is likely that muscle cramps are caused by a variety of etiologies, but severe, whole-body cramps are associated with dehydration and sodium loss.^{(Bergeron, 2003; Stofan et al., 2005; Horswill et al., 2009)} Maintaining proper hydration and ingesting adequate electrolytes appears to reduce the risk of whole-body cramping.^{(Stofan et al., 2005; Horswill et al., 2009)}

Sweat loss during physical activity

Sweating rates vary substantially among athletes, due not only to the inherent biological variability in the human sweating response, but also to variability in factors such as exercise intensity, environmental temperature and humidity, fitness level, extent of heat acclimation, amount and type of clothing and equipment, and hydration status.^{(Casa et al., 2005)} For example, one athlete might sweat at a rate of only 750 ml/h during a light workout, but over 2 L/h during an intense training session in the heat, while another athlete doing exactly the same tasks could lose 400 ml/h in the first occasion and 950 ml/h in the second. This large variability in sweat loss among athletes precludes a one-size-fits-all recommendation for fluid intake during exercise. For that reason, athletes must be willing to determine their personal fluid intake requirements during training and competition to minimize the risk of dehydration and maintain proper fluid balance. The factors that influence sweat rate and determine fluid intake are described below.

In brief, sweating rates among athletes can range from less than 100 ml (3 oz) per hour to over 3,000 ml (roughly 100 oz) per hour.^{(Rehrer & Burke, 1996)} As athletes become more fit and better acclimated to the heat, physiological adaptations occur that result in a greater sweating rate.^{(Casa et al., 2005; Maughan & Shirreffs, 1998)} Consequently, fluid intake must increase as athletes become acclimated to the heat if dehydration is to be minimized. In fact, an increase in voluntary fluid consumption during the day is a vital adaptation that occurs early in the process of heat acclimation to allow for expansion of plasma volume (Wenger, 1988). However, during exercise, athletes often do not drink enough to adequately replace sweat loss and, as a result, “voluntarily” dehydrate.(Bar-Or et al., 1980; Greenleaf, 1992; Greenleaf & Sargent, 1965; Passe, 2001; Rothstein, 1947) That is not to say that voluntary dehydration occurs each and every time athletes exercise; there are certainly occasions when athletes do ingest enough fluid to minimize dehydration and other occasions when some athletes over-consume fluid and gain weight.

The simple act of comparing pre- and post-exercise body weights is an indispensable tool in helping athletes understand their hydration needs. The loss of 1 kg (2.2 lb) of body weight equals about 1 L (34 oz) of body water loss. Weight loss after exercise represents dehydration – the result of drinking too little, while weight gain is indicative of hyperhydration – the result of drinking too much. It should be noted that the difference in pre- and post-exercise body weight is a reliable and accurate measure of the change in body water because metabolic water gain (~0.13 g/kcal) is balanced by respiratory water loss (~0.12 g/kcal.)^{(Cheuvront & Haymes, 2001)} Loss of body weight due to the oxidation of muscle glycogen, liver glycogen, and fatty acids stored in adipocytes is a non-sweat source of weight loss during exercise, but this value rarely exceeds roughly 1 lb (0.45 kg) even during endurance exercise. For these reasons, pre- and post-exercise body weights can be confidently used to assess the adequacy of drinking during exercise, especially during exercise lasting less than two hours.

Electrolyte loss during physical activity

Sweat is mostly water, but also contains a range of electrolytes (salts; minerals) and metabolites. As with sweating rates, there is a large biological variability in the electrolyte content of sweat, particularly in the concentrations of sodium and chloride in sweat.^{(Costill, 1977)} Increased fitness and improved heat acclimation promote conservation of sodium chloride by the sweat glands, an important adaptation that helps increase plasma volume at rest and helps maintain higher plasma volume during exercise.^{(Wenger, 1988)}. The increased sweating rate that accompanies increased fitness and regular heat exposure may mean that total salt losses before and after acclimation are similar.

The average sodium concentration of sweat is 40 – 50 mmol/L (920 – 1,150 mg/L). Highly fit, well-acclimated athletes with a genetic predisposition for low sweat sodium might exhibit a sodium concentration of 20 mmol/L (460 mg/L) or even less. On the other extreme are some athletes who, regardless of their fitness and acclimation, are “salty sweaters” and excrete sweat with a sodium concentration over 80 mmol/L (1,840 mg/L.)^{(Stofan et al., 2005)}.

Even for those athletes with “normal” sweat sodium concentrations, the amount of sodium lost through sweat can be high by virtue of large sweat losses. For example, an athlete with a sweat sodium concentration of 45 mmol/L (1,035 mg/L) who loses 6 L of sweat during two-a-day practices will lose 6,210 mg of sodium (15.5 g NaCl; roughly 3 teaspoons of salt). Ingesting a sports drink during exercise helps replace some of the electrolyte loss, but most of the electrolytes will be replaced in the food eaten between training sessions.

Sodium ingested during exercise helps maintain the osmotic drive to drink, often resulting in greater voluntary fluid consumption,^{(Nose et al. 1988)} helps protect plasma volume (Below et al., 1995), helps maintain plasma sodium concentration,^{(Vrijens & Rehrer, 1999)} and reduces urine production,(Vrijens & Rehrer, 1999) helping stimulate rapid and complete rehydration.^{(Maughan & Shirreffs, 1998)} These responses aid in keeping athletes well hydrated, help sustain cardiovascular function,^{(Coyle & Montain, 1992)} and reduce the risk of fluid-electrolyte imbalances such as hyponatremia (dangerously low blood sodium level.)^{(Montain et al., 2006)} However, athletes who use sports drinks should not assume that they are immune to hyponatremia if they drink substantially more fluid than they lose in sweat. Although sports drinks contribute sodium to the blood and thereby help reduce the risk of hyponatremia, the sodium concentration of sports drinks is lower than that of the blood, so zealous over-drinking of a sports drink might eventually result in hyponatremia.

The results of research studies suggest that athletes should ingest at least 450
mg of sodium per hour of exercise when sweating occurs to help protect plasma volume and plasma sodium concentration.^{(Baker et al, 2005; Barr et al., 1991; Below et al., 1995; Montain et al., 2006; Twerenbold et al., 2003; Vrijens & Rehrer, 1999)} By comparison, one liter (34 oz) of sports drink containing 20 mEq/L of sodium will provide 460 mg of sodium. Athletes who lose large volumes of sweat should consider ingesting additional sodium in the form of sports drinks with greater sodium content or in bars, gels, or electrolyte powders or tablets that provide extra sodium.

Fuel use during physical activity

Muscular contractions that last for longer than 10 seconds rely on the oxidation of carbohydrate and fat to produce the adenosine triphosphate (ATP) required to sustain muscular activity. Fatty acids stored in muscle and released from adipocytes represent an important fuel source during all types of exercise, but compared to carbohydrate, fatty acids are oxidized more slowly and require more oxygen. Amino acids from the breakdown of proteins in muscle and other tissues can also serve as a fuel source, although their overall contribution to ATP production rarely exceeds 5% of the total energy requirement, even in extreme endurance exercise. ^{(Hargreaves, 1996; Maughan, 2001)}

For those reasons, contracting muscles rely on carbohydrate as the primary source of fuel,^{(Hargreaves, 1996)} especially during vigorous exercise when the metabolic demand for energy is high. Research shows that physical, mental, and motor skill aspects of performance can be improved when an adequate amount of carbohydrate is ingested during exercise lasting in excess of 45 minutes.^{(Jeukendrup, 2004; Shi & Gisolfi, 1998)} Research is needed to determine if similar benefits occur during exercise of shorter durations but, in general, the impact of carbohydrate feeding appears to be dependent on the combination of exercise intensity, duration, and environmental conditions that determine the body’s reliance on carbohydrate as fuel. Improved performance associated with carbohydrate feeding has been reported to occur in conditions that mimic the demands of basketball and soccer,^{(Nicholas et al., 1999; Sigiura & Kobayashi, 1998; Dougherty et al., 2006)} tennis,^{(Ferrauti et al., 1997; Vergauwen et al., 1998)} running, ^{(Nicholas et al., 1995)} and cycling.^{(Coyle et al., 1983; Jeukendrup et al., 1997)}

The mechanisms by which carbohydrate feeding improves performance are not firmly established, but are likely to include central and peripheral effects involving the brain, oropharyngeal reflexes, gastrointestinal reflexes, the liver and the active muscles. For example, there is evidence that carbohydrate intake is quickly “sensed” by the central nervous system,^{(Carter et al., 2004; Nybo et al., 2003; Pénicaud et al., 2002; Chambers et al., 2009)} that carbohydrate feeding spares liver glycogen, ^{(Jeukendrup et al., 1999)} and that the ratio between exogenous and endogenous carbohydrate oxidation is increased by carbohydrate feeding,^{(Jentjens et al., 2006)} a response that enhances total carbohydrate oxidation. Future research is likely to uncover other mechanisms by which carbohydrate feeding may improve performance.

The various types and amounts of carbohydrates found in sports drinks can positively or negatively affect gastric emptying and intestinal absorption.^{(Gisolfi et al., 1992; Ryan et al., 1998)}. Research shows that mixtures of simple carbohydrates (e.g., combinations of sucrose, glucose, fructose, maltose, and galactose) of roughly no more than 7% weight:volume (i.e., 70 grams of carbohydrate per liter of water) optimizes gastric emptying and intestinal absorption, in addition to enhancing exogenous carbohydrate oxidation.^{(Hargreaves, 1996; Jeukendrup, 2004; Maughan & Murray, 2001)}

While it is well established that carbohydrate intake during exercise can delay fatigue, some studies^{(Ivy et al., 2003; Saunders et al., 2004)}
reported consuming a carbohydrate- protein mixture during exercise further improves endurance capacity. While these findings are noteworthy, there is no compelling physiological explanation for the observations. For example, only a few amino acids can be used by muscles to produce energy and their oxidation accounts for only 2-5% of the total energy expenditure, even during prolonged, intense exercise. Furthermore, research has been unable to show that branched-chain amino acid ingestion during exercise benefits performance.^{(Cheuvront et al. 2004, Davis et al., 1999)} At odds with the earlier carbohydrate-protein studies, a study by van Essen and Gibala⁽²⁰⁰⁶⁾ showed that ingesting carbohydrate at a rate of 60 g/h improved 80-km cycling time-trial performance, but ingestion of protein along with carbohydrate provided no additional performance benefit. It appears that when a suboptimal amount of carbohydrate is consumed during exercise, as was the case in the Ivy et al.⁽²⁰⁰³⁾ and Saunders et al.,^{(2004 )} additional energy in the form of protein might provide a benefit.

Summary

Extensive research has clearly shown the benefits of ingesting proper amounts of water, carbohydrates, and electrolytes before, during, and after vigorous physical activity. These benefits include positive effects on physiological function, performance, and health. Adequate fluid and carbohydrate intake have independent and additive effects on exercise performance.^{(Below et al., 1995; Jeukendrup, 2004)} That is, performance is improved by proper hydration and by adequate carbohydrate intake, and the combination of the two provides an additive benefit. Compared to water ingestion, the ingestion of a properly formulated sports drink* provides superior hydration and performance benefits without impaired gastric emptying or intestinal absorption^{(Ryan et al., 1998).} Ingesting electrolytes (particularly sodium) during and following exercise that is associated with a large sweat loss helps maintain plasma sodium concentration and speeds rehydration because replacement of the electrolytes lost in sweat is just as important as replacing the water lost in sweat.^{(Maughan & Shirreffs, 1998)}

For more information on sports drink science, the interested reader is referred to the text by Maughan & Murray.(2001)

* The American College of Sports Medicine position stand (2007) indicates that when “… both fluid and carbohydrate delivery are going to be met with a single beverage, the carbohydrate concentration should not exceed 8%, or even be slightly less.”

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