Dec. 19 (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.

"When muscles are weaker, there is less sensory input telling you have eaten or drunk as much as you have," Egan said. Signals come from other areas of the body, such as the back of the throat, and that is also less sensitive with age, the study noted.

The study concluded that "scheduled drinking may be a strategy to reduce the risk of dehydration in older people, although care should be exercised to avoid excessive water intake and the associated risks of cerebral swelling."

The findings are published in this week's edition of Proceedings of the National Academy of Sciences.

The work of Egan and his team of scientists from Melbourne and San Antonio, Texas, is one of many studies of thirst in the elderly, said Neil E. Rowland, a professor of psychology at the University of Florida and a thirst researcher. "These studies have had two different results: That elderly people experience less thirst and consequently drink less fluid, or that elderly people experience just as much thirst but still drink less," he said.

"This paper is important because it's really the first study that looks inside the brain to try to find out what might be different" about the thirst mechanism in older people, Rowland said.

The study is also interesting because it looked at the cingulate cortex, a region of the brain that hasn't been studied widely by thirst researchers, he added.

While the findings are important to basic science, they don't have immediate practical consequences, Rowland said. "The authors suggest controlled drinking programs so that [older people] take more drinks across the day. That doesn't follow from this particular research. Those sorts of programs have been around for a long time."

Scheduled drinking isn't always successful with the elderly, added Barb Troy, a clinical assistant professor of dietetics at Marquette University. Anyone who works with the elderly will say that if you prod them to drink beyond their limit, that can be counterproductive, she said.

"They don't ambulate as well. In the middle of the night they don't want to be running to the bathroom, and that catches up with them."

SOURCES: Gary Egan, Ph.D., leader, neuroimagng and neuroinformatics laboratory, Howard Florey Institute, University of Melbourne, Parkville, Australia; Neil E. Rowland, Ph.D., professor, psychology, University of Florida, Gainesville; Barb Troy, M.S., RDCD, clinical assistant professor, dietetics, Marquette University, Milwaukee; Dec. 17, 2007, Proceedings of the National Academy of Sciences, online