Acute Changes in Body Water in a mild environment, the average adult gains and loses aboutL of water each day. The majority of fluid loss in this situation is due to urine, but feces, respiratory water, and water diffusion through skin other than sweat also contribute. In a hot environment, most fluid is lost as sweat, which may exceed L if heat exposure lasts for an entire day. Such long-term heat exposure, without air-conditioning, may result in gradual dehydration, heat illness see below, or changes in disposition. Because the rate of dehydration is slow, these effects require several hours to develop.
It is important for athletes, and anyone who lives or works continuously in a hot environment, to remain aware of dehydration as it develops. Perhaps the simplest way to accomplish this is to measure body weight before and after an activity or lengthy exposure to heat. A decrease in body weight indicates that body water has been lost; very little of this body weight change will be due to fat loss or other tissue breakdown. If you follow the simple steps in figure you will be able to calculate your hourly sweat rate. Once you know your hourly rate of sweat loss, you will have a handy fluid replacement guide to use whenever you exercise or live in the heat. Remember, however, that sweat rate will change as body temperature, activities, exercise intensities, environmental conditions, and your heat acclimatization status change.
. Record body weight before and after of exercise or rest in heat.
. Calculate the difference D between these weights.
. If clothing is wet, the increase in clothing weight should be noted by weighing it before and after heat exposure. This increase in weight represents sweat that was produced but remained in the fabric, and should be added to D step above. If nude body weight is taken, this step can be ignored.
. The weight of fluid consumed during step should be added to D.
. The weight of urine lost during step should be subtracted from D.
. Your hourly sweat rate in kg/h equals D stepafter correcting for the items in steps -.
. Weight loss should be replaced by consuming L of fluid for each kilogram of body weight that was lost.
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Calculate your hourly sweat rate.
The American College of Sports Medicine ACSM, the world’s foremost professional organization of its kind, has published comprehensive recommendations regarding dehydration, athletic performance, and fluid losses. In the ACSM Position Stand Exercise and Fluid Replacement see appendix A, the following guidelines are offered regarding the optimization of exercise performance:
At least prior to exercise, you should consume ml about oz of fluid; this promotes proper hydration and allows time for the kidneys to excrete excess water, if any exists.
During exercise, you should begin drinking early, before you become thirsty. You should set a goal of consuming fluids at a rate equal to your sweating rate see figure or at the maximal rate that can be tolerated – ml/h, or about – oz/h.