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Hydrating in the Heat ©1995 Joe Friel With the return of warm weather in the spring the need for fluid intake during exercise becomes increasingly important. During exercise in hot weather blood is shunted to the working muscles to supply oxygen and fuel. As blood passes through the muscles it absorbs heat. To cool the working body, the warmed blood is transferred to the skin where some of the fluid goes into the production of sweat. This combined with fluid losses through heavy breathing causes a loss of body fluids. For every 2.2 pounds of water (about a quart) lost during exercise in the heat heart rate will increase by about eight beats per minute, heart stroke volume (the amount of blood pumped per beat) will decline by about one quart per minute, and core temperature will rise by approximately 0.5 degrees. If a 150-pound man loses two percent of his body water, about six cups, his speed will drop off by 20 percent. A four-percent loss forces a 30-percent decline in performance and places the athlete at risk for heat exhaustion.
Fluid requirements increase dramatically during exercise in the heat. To determine how much you need to take in per hour during running multiply your weight in pounds by the average speed in miles per hour and divide by 28.5. For example, a 150-pound man running at 8 mph would need about 42 ounces (150 x 8 + 28.5 = 42.1). Another way to determine your normal fluid requirements is to weigh without clothes on before and after a workout. Every pound lost is a pint of water lost. If you drank during the workout, add one pound for every pint consumed. The result is the amount of fluid you needed to take in during that type of workout.
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