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Chapter: Medical Physiology: Sports Physiology

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Body Fluids and Salt in Exercise

As much as a 5- to 10-pound weight loss has been recorded in athletes in a period of 1 hour during endurance athletic events under hot and humid conditions.

Body Fluids and Salt in Exercise

As much as a 5- to 10-pound weight loss has been recorded in athletes in a period of 1 hour during endurance athletic events under hot and humid conditions. Essentially all this weight loss results from loss of sweat. Loss of enough sweat to decrease body weight only 3 per cent can significantly diminish a person’s per-formance, and a 5 to 10 per cent rapid decrease in weight can often be serious, leading to muscle cramps, nausea, and other effects. Therefore, it is essential to replace fluid as it is lost.

Replacement of Sodium Chloride and Potassium. Sweat con-tains a large amount of sodium chloride, for which reason it has long been stated that all athletes should take salt (sodium chloride) tablets when performing exercise on hot and humid days. However, overuse of salt tablets has often done as much harm as good. Fur-thermore, if an athlete becomes acclimatized to the heat by progressive increase in athletic exposure over a period of 1 to 2 weeks rather than performing maximal athletic feats on the first day, the sweat glands also become acclimatized, so that the amount of salt lost in the sweat becomes only a small fraction of that lost before acclimatization. This sweat gland acclimatization results mainly from increased aldosterone secretion by the adrenal cortex. The aldosterone in turn has a direct effect on the sweat glands, increasing reabsorption of sodium chloride from the sweat before the sweat itself issues forth from the sweat gland tubules onto the surface of the skin. Once the athlete is acclimatized, only rarely do salt supplements need to be considered during athletic events.

Experience by military units exposed to heavy exer-cise in the desert has demonstrated still another elec-trolyte problem—the loss of potassium. Potassium loss results partly from the increased secretion of aldos-terone during heat acclimatization, which increases the loss of potassium in the urine as well as in the sweat. As a consequence of these findings, some of the supple-mental fluids for athletics contain properly propor-tioned amounts of potassium along with sodium, usually in the form of fruit juices.


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