Fluid and Electrolyte Requirements
Physical Activity and Individual Variability
Participation in physical activity exposes individuals to a variety of factors that influence sweat losses; these include the duration and intensity of exercise, the environmental conditions and the type of clothing/equipment worn. Sometimes, these factors are standardized for a specific activity or event within a sport (e.g., the temperature of an air conditioned indoor stadium or the uniform worn by a sporting team). In other cases, these factors occur in a predictable manner (e.g., running speeds in a 10,000 m race are higher than in a marathon, Nordic skiing, and other outdoor winter sports are undertaken in colder environments than summer sports). Nevertheless, in most activities, there is considerable variability in exposure to the factors that contribute to sweat rates between participants.
Individual characteristics, such as body weight, genetic predisposition, heat acclimatization state, and metabolic efficiency (economy at undertaking a specific exercise task) will influence sweat rates for a given activity. As a result, there is a large range in sweat rates and total sweat losses of individuals between and within activities, and in some cases even in the same event on a given day. For example, elite marathon runners may have higher sweating rates but similar total sweat losses (run for shorter duration) as recreational runners who finish the race at the rear of the field. In a soccer match, sweat rates will vary between players according to their position and playing style as well as the total time spent on the field. Likewise, American football players (large body mass and wearing protective clothing) will have markedly greater daily sweat losses (~8.8 L·d−1) than cross country runners (~3.5 L·d−1) training in the same hot environmental for the same duration.
Table 2 summarizes sweat rates observed among serious competitors across a range of sports, both in training and in competition.[14–16,18,21,22,41,62,89,130,133] These data show that individuals often achieve sweating rates from 0.5 to 2.0 L·h−1. The differences in sweat rates between individuals, different sports and climatic season demonstrate the difficulties in providing a single one size fits all recommendation. Sweating rate differences between persons for a given event and environment are likely reduced when body size (body mass or surface area corrections) is considered, but marked individual differences still persist.
Muscular contractions produce metabolic heat that is transferred from the active muscles to blood and then the body core. Subsequent body core temperature elevations elicit physiologic adjustments that facilitate heat transfer from within the body core to the skin where it can be dissipated to the environment. Heat exchange between the skin and the environment is governed by biophysical properties dictated by the surrounding temperature, humidity and air motion, sky and ground radiation, and clothing. In temperate and cooler environments, the high capacity for dry heat loss (radiation and convection) reduces evaporative cooling requirements, so sweat losses are relatively small. As the environmental heat stress increases, there is a greater dependence on sweating for evaporative cooling. The wearing of heavy or impermeable clothing, such as a football uniform, greatly increases the heat stress and evaporative cooling requirements while exercising in temperate to hot environments. Likewise, wearing heavy or impermeable clothing while exercising in cold weather can elicit unexpectedly high sweat rates.
The following calculations give the minimal sweating rate needed to provide evaporative cooling for persons performing high (e.g., metabolic rate ~1000 W) intensity exercise in hot weather. If the exercise task is 20% efficient, then 80% of metabolic energy is converted to heat in the body. Therefore, high intensity exercise will require ~800 W (0.8 kJ·s−1 or 48 kJ·min−1 or 11.46 kcal·min−1) of metabolic energy to be dissipated to avoid heat storage. Since the latent heat of evaporation is 2.43 kJ·g−1 (0.58 kcal·g−1), the individual will need to evaporate ~20 g·min−1 (48 kJ·min−1 ÷ 2.43 kJ·g−1 or 11.46 kcal·min−1 ÷ 0.58 kcal·g−1) or ~1.2 L·h−1. If the environment is cooler and allows greater dry heat loss the required sweating rates would be lower. If secreted sweat drips from the body and is not evaporated, higher sweating will be needed to achieve the evaporative cooling requirements.[32,120] Conversely, increased air motion (wind, movement velocity) will facilitate evaporation and minimize wasted (dripping) sweat.
Heat acclimatization enhances an individual to achieve higher and more sustained sweating rates, if needed.[120,121] Similarly, aerobic exercise training has a modest effect on enhancing sweating rate responses.[120,121] Other factors, such as wet skin (e.g., from high humidity) and dehydration can act to suppress the sweating rate response.
Sweat electrolyte losses depend on the total sweat losses and sweat electrolyte concentrations. Sweat sodium concentration averages ~35 mEq·L−1 (range 10–70 mEq·L−1) and varies depending upon genetic predisposition, diet, sweating rate, and heat acclimatization state.[3,17,40,60,130,144] Sweat concentrations of potassium averages 5 mEq·L−1 (range 3–15 mEq·L−1), calcium averages 1 mEq·L−1 (range 0.3–2 mEq·L−1), magnesium average 0.8 mEq·L−1 (range 0.2–1.5 mEq·L−1), and chloride averages 30 mEq·L−1 (range 5–60 mEq·L−1). Neither sex, maturation, or aging appear to have marked effects on sweat electrolyte concentrations;[92,99] although dehydration can increase the sweat concentrations of sodium and chloride. Sweat glands reabsorb sodium and chloride, but the ability to reabsorb these electrolytes does not increase proportionally with the sweating rate. As a result, the concentration of sweat sodium and chloride increases as a function of sweating rate.[3,40] Heat acclimatization improves the ability to reabsorb sodium and chloride, thus heat acclimatized individuals usually have lower sweat sodium concentrations (e.g., >50% reduction) for any given sweating rate.
Evidence Statement. Exercise can elicit high sweat rates and substantial water and electrolyte losses during sustained exercise, particularly in warm-hot weather. Evidence Category A. There is considerable variability for water and electrolyte losses between individuals and between different activities. Evidence Category A. If sweat water and electrolyte losses are not replaced, then the person will dehydrate. Evidence Category A.
Cite this: Exercise and Fluid Replacement - Medscape - Feb 01, 2007.