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

You need to provide replacement of Salt AND Potassium. Sweat contains a large amount of salt, for which reason it has long been stated that all athletes should take salt tablets when performing exercise on hot humid days. Unfortunately, overuse of salt tablets has often done as much harm as good. Furthermore, 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 sweat is only a small fraction of that prior to acclimatization. This sweat gland acclimatization results mainly from increased aldosterone secretion by the adrenal cortex. The aldosterone in turn has direct effects on the sweat glands, increasing the reabsorbtion of NaCl (salt) from the sweat before it 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.

On the other hand, recent experience by military units exposed to heavy exercise in the desert has demonstrated still another electrolyte problem- the problem of potassium loss. Potassium loss results partly from the increased secretion of aldosterone 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 newer supplemental fluids for athletes and soldiers are beginning to contain properly proportioned amounts of potassium and NaCl.

Note about patients experiencing hypokalemia. When the normal K+ concentration falls below approximately 1/2 to 1/3 the normal value (4.5 mEq/L) , muscle paralysis or at least severe muscle weakness often develops. This is caused by effects on the nerve and muscle fibers membranes that prevent transmission of the normal action potentials.