Body Heat in Exercise

   Almost all the energy released by the body’s metabolism of nutrients is eventually converted into body heat. This applies even to the energy that causes muscle contraction for the following reasons: First, the maximal efficiency for conversion of nutrient energy into muscle work, even under the best of conditions, is only 20 to 25 percent; the remainder of the nutrient energy is converted into heat during the course of the intracellular chemical reactions. Second, almost all the energy that does go into creating muscle work still becomes body heat because all but a small portion of this energy is used for (1) overcoming viscous resistance to the movement of the muscles and joints, (2) overcoming the friction of the blood flowing through the blood vessels, and (3) other, similar effects—all of which convert the muscle contractile energy into heat.

Heatstroke.

During endurance athletics, even under normal environmental conditions, the body temperature often rises from its normal level of 98.6° to 102° or 103°F (37° to 40°C). With very hot and humid conditions or excess clothing, the body temperature can easily rise to 106° to 108°F (41° to 42°C). At this level, the elevated temperature itself becomes destructive to tissue cells, especially the brain cells. When this happens, multiple symptoms begin to appear, including extreme weakness, exhaustion, headache, dizziness, nausea, profuse sweating, confusion, staggering gait, collapse, and unconsciousness. This whole complex is called heatstroke, and failure to treat it immediately can lead to death..

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.

Replacement of Sodium Chloride and Potassium.

Sweat contains 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. 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 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.