Age-related changes in natural spontaneous fluid ingestion and thirst in humans.

Elderly humans have impaired responses to dehydration and overhydration when tested under experimental conditions. This study investigated whether under ad libitum conditions age-related changes in fluid intake and thirst could be detected. Adult humans (n = 262) maintained 7-day diaries of everything they ingested, the timing and conditions present, and subjective thirst, anxiety, depression, and activity. No differences were found in overall fluid intakes of subjects from 20 to 80 years of age, including equivalent fluid intake responses to solid food ingestion, subjective thirst, social stimuli, and stomach factors. However, fluid ingested in bouts increased slightly over the day for younger groups, whereas it declined markedly for elderly groups, producing a blunted diurnal rhythm. Also, elderly subjects ingested more coffee and less soda and alcohol. The findings indicate that elderly individuals, under ad libitum conditions, are able to obtain normal levels of fluid by the coingestion of fluids with solids.