Excretion trends in men undergoing deacclimatization to heat

¹ USAF School of Aerospace Medicine, Brooks Air Force Base, San Antonio, Texas

In an effort to characterize metabolic changes associated with heat deacclimatization under natural conditions, urinary determinations were made on 21 healthy men during the transition period from summer to fall. Two overnight urine specimens per subject per week were analyzed for sodium, potassium, phosphate, urea, uric acid, creatinine, and 17-hydroxycorticosteroids (17-OHCS). No trends were found for urine output, uric acid, urea, or sodium. Creatinine excretion rate and Na/K ratio fell as environmental temperature fell; potassium/creatinine ratio and potassium excretion rate increased. These four variables showed linear trends, but phosphate/creatinine ratio increased in a stepwise manner and urea/creatinine ratio increased only after temperature had fallen far below the summer level. Excretion of 17-OHCS (as rate or ratio) declined (two-stage shift). Subjects who did not exercise regularly showed upward shifting in phosphate excretion; nonexercising subjects as a group showed a cyclic tendency; and those who exercised daily showed essentially no change in phosphate excretion. This retention of one summer characteristic in the latter group is consonant with the literature on heat acclimatization in that it shows that physical fitness is a factor that delays heat deacclimatization.