Objectives: The link between dehydration and fluid intake with brain function in elderly individuals has produced conflicting findings. The aim of this study was to examine the effects of water supplementation on fluid intake, cognitive and motor performance, and serum brain-derived neurotrophic factor (BDNF) levels in euhydrated and dehydrated elderly.

Methods: Twenty participants 60 to 89 y of age were randomly divided into four groups, namely; euhydration + control (EC), euhydration + treatment (ET), dehydration + control (DC), and dehydration + treatment (DT) groups. The treatment groups were personally supplemented with plain water for 7 d. The volume of water intake was assessed with food recalls. The digit span test and finger tapping test were used to examine cognitive and motor performances. Peripheral BDNF levels were determined using enzyme-linked immunosorbent assay of serum samples.

Results: Water supplementation increased 2.5 times the volume of water and total fluid intake in the ET and DT groups. The mean of the volume water intake on day 7 in ET and DT was significantly different from EC (P < 0.01) and DC (P < 0.001) groups. Serum BDNF levels were higher on preintervention in dehydration compared to euhydration groups (P < 0.05) and significantly negative correlated with the volume of water (r = -0.59, P = 0.006) and fluid intake (r = -0.60, P = 0.004).

Conclusions: The present study demonstrated that 7 d of water supplementation increased the water and the total fluid intake in elderly individuals regardless of dehydration status. Serum BDNF levels increased during dehydration and reverted to physiologic levels after appropriate rehydration.