Heat Dissipation During Exercise
Except when mechanical energy is produced (eg, stair climbing) in direct application of the first and second laws of thermodynamics, all the chemical energy released by the muscle appears in the form of heat. This heat is carried away from the muscle by blood and, after being pumped across the lungs, is distributed by the left heart to the skin where it is dissipated to the environment via radiation, convection, or conduction. But if the amount of heat produced is very large, the excess is removed by evaporating the water in sweat, which is produced by the eccrine sweat glands in the skin. This is a very efficient mechanism because, when 1 L of water is converted from liquid to vapor form (1244 L), 580 kcal is lost. Thus, water in the body serves both as a carrier of heat and as a refrigerant that removes heat by evaporation.
Dehydration and Heat Dissipation
The best way to appreciate the performance of this cooling system is to look at what happens during exercise in the heat without proper hydration (eg, see Gonzalèz-Alonso et al1). When compared with the control situation with adequate water ingestion during exercise (ie, enough to compensate water loss in sweat), plasma volume is lower. As a consequence, and despite a higher heart rate than in the control situation, the lower filling of the heart during diastole results in a smaller stroke volume and cardiac output: the total amount of blood that is distributed to the tissues is reduced. The lower blood flow to the muscle and to the skin impairs the transfer of heat from the muscle to the environment. In addition, marked dehydration reduces the production of sweat. The end result is a higher body temperature than in the control situation (eg, 39.5°C vs 38°C in the study by Gonzalèz-Alonso et al2). Because of both the exaggerated increase in body temperature and the lower amount of fluid in the body, and particularly in the circulatory system, the exercise is more difficult to sustain as indicated by a higher rating of perceived exertion. Fatigue will develop more quickly.3
Reduction in Performance With Dehydration
The average reduction in the capacity to exercise depends on the level of dehydration (expressed in percent body mass) and is different in a temperate and hot environment. The sweating rate depends on the workload sustained as well as on the environmental conditions (Figure 1).4 As shown in Figure 1, a subject jogging (12-14 km/h) in hot and humid conditions can lose sweat at a rate up to 1.5 L/h (2% of the mass for a 70-kg subject). This modest level of dehydration results in a decrease in the ability to perform submaximal exercise that averaged 20% in a temperate climate but can reach 40% in a hot climate (Figure 2).5 The impairment in performance quickly increases with the level of dehydration, for example, −50% and −70% in a temperate and hot climate with a 5% to 6% reduction in body mass.
The lesson to learn from these observations is that, during exercise, we should keep well hydrated by ingesting fluids.6 This is not only for the sake of a better performance, which is not the goal of everyone. Most people exercise regularly as part of a healthy lifestyle for their well-being and, for a large segment of the population, to help maintain body mass. For these people, the goal should be to minimize the pain and increase the pleasure.
Hydration Before, During, and After Exercise
The amount of fluid that can reasonably be ingested during moderate exercise is around 0.6 L/h for a 70-kg subject. This is enough to fully compensate for water loss in sweat only a low workload (eg, brisk walking, slow jogging) in a cool environment. For a higher workload, it becomes difficult to compensate for the sweat lost during the exercise period, particularly in warm conditions. Thus, the individual should be well hydrated before the beginning of exercise and should ingest fluids not only during but also after the exercise period. A recent position paper from the American College of Sports Medicine7 suggests to ingest slowly, 5 to 10 mL/kg of fluid, 4 hours before exercise (350-700 mL for a 70-kg subject); 6 to 12 mL/kg per hour during exercise (400-800 mL/h for a 70-kg subject, in 3-4 fractions); and 1.5 L of fluid after exercise for each 1-kg reduction in body mass (Table).
Minerals and Carbohydrate in Hydrating Fluids
Except for people engaging in very prolonged endurance exercise (>2-3 hours) in warm environment, there is no need to add large amounts of minerals to the fluids that are ingested before, during, and after exercise. In fact, because the amount of minerals in sweat is much lower than in the plasma (eg, sodium 20-90 mmo/L in sweat vs 140 mmol/L in plasma),8-10 sweating increases the osmolality of plasma. Unlike the horse, which produces sweat that is very rich in salts,11 when humans sweat, more water is lost than minerals. A proper replacement of the fluid lost thus should focus on water, not on minerals. Interestingly, sport drinks do not contain a lot of mineral salts, as reviewed by Coombes and Hamilton.12 The concentrations of the mineral salts in these drinks range only between 4 and 19 mmol/L for sodium (vs 140 mmol/L in plasma), 0 to 9 mmol/L for chloride (100 mmol/L in plasma), and 3 to 16 mmol/L for potassium (5 mmol/L in plasma). When compared with plasma, however, these drinks are iso-osmotic or hyperosmotic because they contain large amounts of carbohydrates. Indeed, consistent evidence shows that carbohydrate ingestion postpones fatigue and increases endurance performance for exercise lasting about 90 minutes and more,13 the beneficial effect of carbohydrate being larger during exercise lasting longer than 2 hours.14 However, for shorter periods of exercise, such as 60 minutes or less, there is no convincing evidence that ingestion of carbohydrates either before or during exercise has any effect on fatigue and performance. A study by Clark et al15 actually showed that, for a 60-minute exercise period, the beneficial effect of carbohydrate ingestion, when present, could be a placebo effect. No improvement in performance was observed when (a) subjects were told that they received a placebo, but actually consumed carbohydrate or the placebo and (b) when they were told that they received carbohydrate and actually consumed carbohydrate. An improvement in performance was observed only when they were told that they received carbohydrate but were given the placebo. A recent study by Chambers et al16 using functional magnetic resonance imaging suggests a neurophysiological basis for the placebo effect of glucose. In this study, performance for an exercise period of about 60 minutes was better when a glucose or glucose polymer solution (vs a saccharin solution) was swished in the mouth but not swallowed. This effect of the mere presence of carbohydrate in the mouth was associated with an activation of reward zones in the brain.
Maximizing Energy Deficit During Exercise
A large number of people exercise mainly for 1 hour or less as part of a healthy lifestyle, to stay in shape, help delaying some effect of aging, and prevent some degenerative disease. For them, unlike endurance athletes who exercise for a longer period, there is no need to include carbohydrate in the fluids ingested before, during, or after exercise. If they do so, the added calories will counteract one of the effects of exercise, which is to contribute to the reduction or the maintenance of body weight-often one of the main objectives of a regular exercise program. In this type of program, for an average person, the amount of energy spent during an exercise session generally does not exceed about 400 to 500 kcal. If this individual ingests a drink with about 65 g of carbohydrate per liter,12 following the recommendation of the American College of Sports Medicine to drink before, during, and after exercise (Table), the total amount of energy provided by fluids will be about 650 kcal. This is ∼35% more than spent during the exercise period. Thus, instead of helping create the energy deficit needed to lose weight or maintain a healthy body weight, because of a poor choice of the rehydration fluid ingested, the exercise session can actually further contribute to the excess caloric intake, without providing any benefit to the participant in terms of reduction in fatigue. In this situation, water offers a clear advantage to sport drinks in terms of energy balance.
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