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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