Food smells reduce diet's life-extending benefits.

Researchers have long known that some lab animals live longer than normal when they receive diets sharply reduced in calories. But in a surprising twist, scientists have now cut short that longevity effect in fruit flies by simply tantalizing them with the aroma of yeast, a fruit fly staple.

Some scientists have suggested that calorie limitation prevents a type of cell damage that hastens death. Other researchers suggest that a different mechanism might be at work. Experiments have shown that interfering with the neural circuitry that's responsible for smell changes low-calorie diets' life-extending benefits in worms.

To investigate whether scent can affect life extension, Sergiy Libert of the Baylor College of Medicine in Houston and his colleagues put fruit flies on strict diets but continually provided with some of the insects with the odor of yeast. Flies that were always exposed to the continuous scent had just 82 percent the longevity of flies smelling yeast only at mealtime.

Libert's team genetically altered other flies so that they couldn't smell anything. Even when those insects were permitted to eat as much as they wanted, they lived as long as calorie-restricted flies did.

The scent of yeast didn't affect the life span of fully fed flies with a normal sense of smell.

Libert suggests that the smell of food may give organisms early information on whether they should devote resources to reproducing or to maintaining their bodies. While animals in a lush environment are more likely to reproduce, animals that sense that food isn't available tend to avoid reproduction and live longer, he speculates.

The findings appear in an upcoming Science.