Exercise and Thermoregulation-1

Exercise Thermoregulation Fluid Balance and Re-hydration Patricia Davidson DCN, RDN, CDE, LDN, FAND Exercise Generates Heat Core temperature normally increases during exercise. The relative stress of exercise determines the magnitude of the increase. A well-regulated temperature increase creates a more favorable environment for physiologic and metabolic functions. Exercise Generates Heat Cutaneous and muscle blood flow increase during exercise in the heat, while other tissues temporarily compromise their blood supply. Core Temperature Humans tolerate relatively small variations in core temperature. Exposure to heat or cold initiates thermoregulatory mechanisms that generate and conserve heat at low ambient temperatures and dissipate heat at high temperatures. Temperature Regulation The hypothalamus serves as the “thermostat” for temperature regulation. The hypothalamus initiates adjustments from: Thermal receptors in the skin. Changes in hypothalamic blood temperature. Heat Loss Warm blood diverts from the body’s core to the shell in response to heat stress. Heat loss occurs by radiation, conduction, convection, and evaporation. Evaporation provides the major physiologic defense against overheating at high ambient temperatures and during exercise. Sweating and Exercise Increased sweating strains fluid reserves, creating a relative state of dehydration. Excessive sweating without fluid replacement decreases plasma volume and causes core temperature to rise precipitously. Dangerous Conditions Warm, humid environments dramatically decrease the effectiveness of evaporative heat loss. Exercise in a hot, humid environment poses a thermoregulatory challenge because a large sweat loss in high humidity contributes little to evaporative cooling. Dangerous Conditions Wet-bulb thermometers are used to determine the influence of humidity and ambient temperature. Dangerous Conditions Fluid loss in excess of 5% of body mass significantly impedes heat dissipation, compromises cardiovascular function, and diminishes exercise capacity. Adequate Fluid Replacement Drinking enough fluid maintains plasma volume so circulation and sweating progress at optimal levels. The ideal replacement schedule during exercise matches fluid intake with fluid loss. This is effectively monitored by changes in body weight. Adequate Fluid Replacement The small intestine absorbs about 1000 mL of water per hour. Prime factors that affect absorption rate include stomach volume and the osmolality of the beverage. Adequate Fluid Replacement Excessive sweating plus ingesting large volumes of plain water during prolonged exercise sets the stage for hyponatremia (water intoxication) caused by a decrease in extracellular sodium concentration. Adequate Fluid Replacement A small amount of electrolytes in the rehydration beverage facilitates fluid replenishment more than drinking plain water. Heat Acclimatization Repeated heat stress initiates thermoregulatory adjustments that improve exercise capacity and reduce discomfort on subsequent heat exposure. Acclimatization triggers favorable redistribution of cardiac output and increases sweating capacity. Full acclimatization generally occurs in about 10 days of heat exposure. Heat Acclimatization Aging affects thermoregulatory function, yet acclimatization to moderate heat stress does not appreciably deteriorate with age. Gender Differences When controlling for fitness and acclimatization levels, women and men show equal thermoregulatory efficiency during exercise. Women produce less sweat than men do at the same core temperature. Heat-Stress Indices Various practical heat-stress indices use ambient temperature and relative humidity to evaluate the environment’s potential thermal challenge to an exercising person. Proper Attire The ideal warm-weather clothing consists of lightweight, loose-fitting, and light-color clothes. Moisture-wicking fabrics against the skin optimize heat and moisture transfer from the skin to the environment. Athletic Uniforms Some uniforms impose a significant barrier to heat dissipation because they effectively seal off the body’s surface from the benefits of evaporative cooling. They also add to the metabolic load imposed by exercise. Heat-Related Illness The major forms of heat illness: Heat cramps Heat exhaustion Heat stroke Heat stroke represents the most serious and complex of these maladies. Oral Versus Core Temperature Oral temperature underestimates core temperature following strenuous exercise. This discrepancy results from evaporative cooling of the mouth and airways during high levels of pulmonary ventilation.