Calorie restriction has long been viewed as a way to extend life for organisms ranging from yeast to mammals -- including, possibly, humans.

Now new research in yeast suggests that cutting calories may lengthen life span by boosting an organism's oxygen consumption -- findings that are in complete contrast to a major theory on why eating less might tack some time onto our lives.

That bit of "conventional wisdom" held that calorie restriction, by lowering metabolism, cut down on the amount of oxygen free radicals circulating in the body. These cell-damaging forms of oxygen are normal byproducts of metabolism.

That idea has been around for a long time, but researchers believe the conventional wisdom is wrong. They had previously found that a gene called Sir2 appears vital in longevity.

In yeast, deleting the gene shaves the life span, while adding an extra Sir2 extends it. The researchers were also able to show that the activity of Sir2 -- which helps regulate the expression of other genes -- is tied to metabolism. To do its job, Sir2 is dependent on a chemical known as NAD that assists in breaking down food and aids metabolism.

In this latest study, they looked at how calorie restriction might boost Sir2 activity and thereby extend the life span of yeast. The investigators found that, in contrast to the slowed-metabolism theory, sugar-deprived yeast cells boosted their oxygen consumption -- which would, in theory, increase oxygen free radicals in a human.

They further found that overexpressing a gene believed to rev up oxygen consumption extended the life span of yeast cells -- an effect that, like calorie restriction, required the Sir2 gene.

All of this suggests that calorie restriction does not make yeast longer-lived by fighting oxidative damage.

It isn't that metabolism slows. It changes qualitatively. According to the researcher, the relationship among calorie restriction, the Sir2 gene and life span appears "much deeper" than the theory on slowed metabolism and reduced oxidative damage would indicate.

But exactly how cutting calories might act upon Sir2, and thereby life span, remains unknown.

In an accompanying editorial, researchers at Harvard University in Boston point out that what applies to yeast won't necessarily apply in animals.

Nature July 18, 2002;418:287-288, 344-348