Many experiments have demonstrated that eating about 30 percent less than needed to keep up with the body's energy needs extends longevity and improves health in animals ranging from the humble earthworm to rhesus monkeys. Humans who try calorie restriction appear to reduce their risk of heart disease and diabetes, and improve their immune function. There's even some evidence that calorie restriction reduces the risk of Alzheimer's disease — in mice and monkeys, at least.
Why does calorie restriction produce this effect? No one knows for sure, but researchers at the University of Alabama at Birmingham have conducted experiments that lead them to suspect that it's the hunger produced by calorie restriction that confers benefits, not the reduction in food intake itself.
The scientists found that hunger triggers a cascade of hormonal signals that may counter age-related mental decline in mice genetically altered to develop Alzheimer's pathology. To test the idea, they gave the mice a synthetic form of ghrelin, the hormone that produces the sensation of hunger. The mice who received the ghrelin avoided the age-related cognitive decline more often than those that didn't receive the treatment.
"Hormonal hunger signaling may represent a new way to combat Alzheimer's disease, either by itself or combined with caloric restriction," said lead author Inga Kadish, assistant professor at the School of Medicine at the University of Alabama at Birmingham.
The mice in the study were divided into three groups. One consumed a diet with about 20 percent fewer calories than typical. The other two groups ate normally, but one group received the synthetic ghrelin to produce a sense of hunger. The memory of the mice was tested by having them swim in a pool of milky water that obscured the view of a platform they could climb on to get out of the water. The mice could find the platform faster if they remembered the visual cues located around the perimeter of the pool. Mice receiving synthetic ghrelin found the platform 26 percent faster than their normally fed peers, and the group on a calorie-restricted diet found it 23 percent faster.
Ghrelin creates hunger signals by acting on the hypothalamus, a part of the brain that influences growth, development, reproduction and metabolism. A new study just published online in Nature has found that the hypothalamus may function as a "fountain of youth" capable of slowing the aging process. "What's exciting is that it's possible — at least in mice — to alter signaling within the hypothalamus to slow down the aging process and increase longevity," said senior author Dongsheng Cai.
The researchers injected gonadotropin-releasing hormone (GnRH) into the brains of the aging mice and found that the hormone promoted the production of new neurons — a process that declines with age. The mice also showed a reduction in other signs of aging, such as a decrease in muscle strength, skin thickness and the ability to learn. They also lived about 20 percent longer.
Might this treatment have affected the activity of ghrelin too? No one knows yet, but you can boost the production of ghrelin in your body simply by eating less. The Cooking Channel may be offering good advice with its slogan: "Stay hungry."
Tom Valeo writes about health matters. He can be reached at firstname.lastname@example.org.