Background: Older adults are susceptible to dehydration and fluid overload due to a reduced ability to maintain homeostatic control of fluid and electrolyte balance.

Purpose: To assess fluid and electrolyte balance responses in young and older men following ingestion of commonly consumed beverages differing in composition.

Methods: 12 young and 11 older men were recruited. Euhydrated body mass was recorded. Participants consumed 1L (250 ml every 15 min) of water, fruit juice, a sports drink or low-fat milk in a randomized cross-over design. Urine and blood samples were obtained before and after the drinking period and every hour thereafter for 3-h. Samples were used to determine osmolality, electrolytes (Na(+) and K(+)), water clearance, and glomerular filtration rate.

Results: Free water clearance was significantly higher in Young than Older at 1 and 2 h after the ingestion of W and S (p < 0.05). Net Na(+) and K(+) balance were not different between Young and Older (p = 0.91 and p = 0.65) adults, respectively. At 3 h Na(+) balance was negative after ingesting water and fruit juice, but neutral after sport drink and milk. Net K(+) balance was neutral at 3 h after ingesting milk, but negative after water, fruit juice and sport drink.

Conclusions: Milk was retained longer than other beverages in Young, but not in Older, despite similar net electrolyte balance responses. Older had higher fluid retention in the first 2 h after the ingestion of all beverages, except for milk when compared to Young, indicating an age-related loss of ability to regulate fluid balance under current study conditions.