Hot weather, exercise, old age, and the kidneys.
Thirst is not a very accurate measure of water need; optimal fluid replacement may lag 2 to 3 days behind actual needs, particularly for persons unaccustomed to a hot climate. If an older person begins an exercise program during warm weather or travels to a warmer climate, the intensity and duration of exercise should be restrained initially, then gradually increased over 10 to 14 days to allow for acclimatization.
D000368:Aged; D000375:Aging; D001833:Body Temperature Regulation; D003681:Dehydration / Q000503:physiopathology / Q000517:prevention & control; D006359:Heat Exhaustion / Q000503:physiopathology / Q000517:prevention & control; D006439:Hemodynamics; D006358:Hot Temperature; D006801:Humans; D007668:Kidney / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D005082:Physical Exertion; D014882:Water-Electrolyte Balance; D014887:Weather