CHICAGO, Sept 20, 2007 (AFP) - Scientists have known for decades that drastically reducing calories -- but not nutrients -- can prolong the lives of everything from yeast to mice and monkeys, but they didn't know why, until now.

In a study released Thursday, US researchers suggest that the link between food restriction and longevity may be a molecular response to the stress from cutting back calories.

That reaction preserves critical cellular functions, helping the body to fight off age-related diseases.