Why Seniors Say "When" Too Soon

By Benjamin Lester
ScienceNOW Daily News
17 December 2007

Thirsty old folks drink less water than parched whippersnappers because their brains are more easily satisfied, according to a new study. The findings suggest that the brain's satiation mechanisms malfunction as we age and might help explain why seniors are at greater risk for dehydration.

Feelings of thirst stem from two main changes in the blood: a higher concentration of salts and a lower concentration of water. Ordinarily, the feeling of thirst prompts us to drink enough water to restore a balance. However, researchers have known for several years that older people tend to drink less, and the reason has remained unclear.

Curious about whether the fault lay in the brain, a team led by neuroscientists from the University of Melbourne in Australia induced thirst in 10 men in their 20s and in 12 men in their 60s or 70s by injecting them with a salty solution. The scientists then spied on the subjects' brains using positron emission tomography, which measures changes in blood flow.

The salt cocktail prompted similar feelings of thirstiness in the young and old subjects, as well as similar changes in cerebral blood flow. However, the older men drank nearly 50% less water than did the younger men. A possible explanation showed up in the brain scans. In the older subjects, blood flow to a brain region called the anterior midcingulate cortex--a regulatory area that was previously linked to feelings of thirst--decreased much more per sip than it did in the younger group. According to the team, the observed difference might mean that in older people, the brain misinterprets signals from the digestive tract about how much water has been drunk, leading to a false sense of satiation, the team reports online this week in the Proceedings of the National Academy of Sciences.

Nina Stachenfeld, a physiologist at Yale University, says previous studies had hinted that the phenomenon might have a cerebral explanation and that the scanning information "adds significantly to the field." However, she notes that the team made some potential errors, such as allowing the subjects to drink for only 8 minutes. The 8-minute window was sufficient to study brain function, she says, but it may not tell the full story. "In our studies," she says, "we found in this early period, the older subjects drank less than younger subjects, but they quickly caught up."