Review articleFluids and hydration in prolonged endurance performance
Introduction
Fluid intake and adequate hydration during exercise are essential and, more importantly, critical during prolonged training sessions and competition events. Fluid intake maintains hydration and helps to maintain body temperature (thermoregulation), avoid dehydration, and maintain adequate plasma volume. For events lasting longer than 1 h, athletes should consume fluids containing carbohydrates (CHOs) and electrolytes rather than water alone. Reduction in body water, availability of CHOs, and an inadequate electrolyte balance during prolonged exercise events will hamper performance and may lead in some cases to serious medical problems (heat exhaustion or heat stroke). A 1% reduction in body weight due to water loss may evoke an undue stress on the cardiovascular system accompanied by increases in heart rate and inadequate heat transfer to the skin and the environment, increase plasma osmolality, decrease plasma volume, and may affect the intracellular and extracellular electrolyte balance.1
Fluid in the body is contained mainly in two areas (compartments). These are the extracellular (fluid outside the cells) and the intracellular (fluid in the red blood cells) fluids. The extracellular fluid is subdivided into interstitial fluid and blood plasma. In humans, body water constitutes about 60% of an individual's body weight, with a few minor differences depending on sex, age, training status, percentage of body fat, etc. About 40% of body water is housed in the intracellular fluid. On average, blood volume in an adult accounts for about 7% of a person's body weight, or about 5 L. Blood volume is distributed between plasma (60% of blood plasma) and red blood cells (40%). When these levels are challenged during prolonged training sessions and competition, they will singly or collectively result in reduced performance and in some cases may cause serious injury, medical emergency, or even death. The sequence of physiologic events affecting the loss of water is via breathing, sweating, feces, and urine output. During prolonged performance, most water is lost in sweat especially during high environmental temperatures. About 580 kcal is lost for every liter of sweat that is evaporated.2 Loss of body fluid can be determined by changes in body weight resulting from exercise. Each kilogram of body weight loss accounts for about 1 L of fluid loss. Development of sports drinks with appropriate and adequate concentrations of electrolytes and CHOs promotes maintenance of homeostasis, prevents injuries, and maintains optimal performance.3 Water balance in the body is regulated by various means. Changes in osmotic pressure or circulating blood volume stimulate the osmoreceptors in the hypothalamus and baroreceptors in heart and blood vessels. Water balance also is regulated by several hormones. The renin-angiotensin-aldosterone system regulates sodium retention, and vasopressin (antidiuretic hormone) regulates water retention in kidneys. Vasopressin also participates in thermoregulation in hypohydrated subjects. Atrial natriuretic peptide secreted by the heart participates in water balance regulation. Atrial natriuretic peptide does not participate or only minimally participates in water balance regulation during cold exposure at 10°C to 12°C. However, exposure to a −20°C environment while wearing warm clothing elicits a two-fold increase in atrial natriuretic peptide. Cold-stimulated atrial natriuretic peptide release has been found to be inhibited by a 3% level of dehydration.4
Section snippets
Electrolyte balance, plasma volume, and fluid shift during exercise
Numerous investigators have reported changes in plasma electrolyte concentrations, plasma volume changes, fluid shifts, delayed fatigue, thermoregulation, and hormonal and catecholamine responses during short, prolonged, and low-, moderate-, and high-intensity exercise.4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32 Prolonged exercise may induce Na+ and K+ losses and excretion of electrolyte free water in subjects with otherwise
Carbohydrate and electrolyte fluid intake during exercise
Many studies have been conducted with sports drinks, CHO and electrolyte solutions, and their effects on hydration/dehydration status, plasma volume changes, gastric emptying, and performance times to name a few.10, 36, 38, 41, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67 Murray at al.60 investigated the effect of CHO content on exercise. They reported that beverages containing too much CHO (8% to 10%) delayed gastric emptying, reduced fluid
Medical reasons for fluid replacement during exercise
Noakes71 historically and very eloquently described medical reasons for fluid replacement during marathon running. He addressed several crucial factors that determine the risk of heatstroke, stating that heatstroke is not associated with the levels of dehydration during exercise but, more importantly, with the rate at which the athlete produces heat and the capacity of the environment to absorb that heat. He further reported that several factors more important than dehydration are responsible
Conclusion
Exercise, athletic competition, prolonged workouts, and long training sessions in hot, hot and humid, or cold environments challenge physiologic function. Dehydration, thermoregulation, fluid balance, rehydration, electrolyte changes, plasma volume, and cardiovascular challenges, to name a few, accompany most physical activities, exercise, training, and competition. This is especially true during prolonged endurance exercise and competition. Loss of body fluids inherently leads to a performance
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