Lack of Strong Thirst Signals Leads Elderly to Drink Too Little

Mar. 23 -- TUESDAY, Dec. 18 (HealthDay News) -- Older adults don't drink enough water and become dehydrated during heat waves because their brains and bodies don't coordinate sensory signals about thirst, a new Australian study suggests.

The researchers aren't sure whether thirst signals from the body or the interpretation of these signals by the brain cause the problem, said study author Gary Egan, an associate professor at the University of Melbourne.

Knowledge of this lack of coordination may make it easier to motivate older people "to make sure they are actively re-hydrating because there is a clear reason why they are not necessarily aware of their own need to drink," Egan said.

Deaths of elderly people from dehydration is a well-known public health problem, Egan said. During a French heat wave in 2003, the deaths of 14,000 mostly older people were attributed largely to not drinking enough water, he said. "This issue becomes of paramount public health significance," he added.

For the study, Egan and his colleagues recruited a group of 10 younger men (mean age 23.7) and a group of 12 healthy older men (mean age 68.1). The researchers injected saline solution into the volunteers to make them thirsty. Then they were permitted to drink as much water as they liked, Egan said.

The older men drank less water to quench their thirst. PET scans of areas of their brains activated by thirst showed reactions -- particularly in the cingulate cortex.

"In the elderly, drinking a much smaller volume of water is needed to cause that area of brain activation to subside," said Egan, who is an expert on neuro-imaging. "For some reason, elderly people's attention of awareness of the need to drink to re-hydrate rapidly dissipates after a small amount of ingested water."

One cause of this could be the result of weaker signals from the body, Egan explained. For example, as people age their stomach muscles weaken. So, when they consume water or food their stomach expands more in comparison to volume, he said.