Temperature regulation and hypohydration: a singular view.

Body temperatures of exercising humans who have been denied water are elevated when compared to hydrated controls. The simplest "explanation" for the elevated temperature is a decrease in sensitivity of the sweating mechanism. This and similar "explanations" do not direct attention to basic causes but only the result(s) of more fundamental aspects of regulatory physiology. Among the items considered in this speculative presentation are influences of changes in osmolarity, specific ions, peptide hormones, fluid shifts, and muscular contractions during exercise. A hypothesis is offered for consideration in explaining elevations of body temperature in exercise with and without water replacement. In general, the hypothesis relates changes in hypothalamic osmotic pressure and/or ionic constituents with fluid and ionic events in muscle during exercise. The fluid and ionic shifts are probably proportional to the amount of lean body mass engaged in dynamic exercise. Since blood volume has also been shown to be related to lean body mass, similar relative work loads should lead to similar changes in the osmotic and/or ionic environment of the hypothalamus, thus resulting in similar increases in body temperature during exercise. Hypohydration is superimposed on this basic response. Increases in body temperature of resting hypohydrated subjects appear to be due to increases in osmotic pressure and/or specific ion concentrations. During exercise, these changes are added to those induced by muscle contraction. The focal point of all such ionic and osmotic changes is thought to be neural processes within the hypothalamus.