Minor degree of hypohydration adversely influences cognition: a mediator analysis

Because the assumption that small changes in hydration status are readily compensated by homeostatic mechanisms has been little studied, the influence of hypohydration on cognition was examined.

We assessed whether a loss of <1% of body mass due to hypohydration adversely influenced cognition, and examined the possible underlying mechanisms.

A total of 101 individuals were subjected to a temperature of 30°C for 4 h and randomly either did or did not consume 300 mL H2O during that period. Changes in body mass, urine osmolality, body temperature, and thirst were monitored. Episodic memory, focused attention, mood, and the perceived difficulty of tasks were measured on 3 occasions. The data were analyzed with the use of a regression-based approach whereby we looked for variables that mediated the influence of hypohydration on psychological functioning.

Drinking water improved memory and focused attention. In the short-term, thirst was associated with poorer memory. Later, a greater loss of body mass was associated with poorer memory and attention (mean loss: 0.72%). At 90 min, an increase in thirst was associated with a decline in subjective energy and increased anxiety and depression, effects that were reduced by drinking water. At 180 min, subjects found the tests easier if they had consumed water.

Drinking water was shown, for the first time to our knowledge, to benefit cognitive functioning when there was a loss of <1% body mass at levels that may occur during everyday living. Establishing the variables that generate optimal fluid consumption will help to tailor individual advice, particularly in clinical situations. This trial was registered at clinicaltrials.gov as NCT02671149.