American scientists have discovered the key to how eating less can lead to a longer, healthier life — an enzyme that curbs cell damage.

The discovery, by scientists at the University of Wisconsin-Madison, could lead to new drugs designed to slow aging and to enhance health in the golden years.

Scientists have studied the relationship between caloric intake and life expectancy for decades, but this study, published Thursday in the journal Cell, unveils the cause.

"This study is the first direct proof for a mechanism underlying the anti-aging effects we observe under caloric restriction," said Tomas A. Prolla, a UW-Madison professor of genetics and a senior author of the study.

The research provides important insight into the only external ways to prolong lifespan, says Russ Hepple, an associate professor at the University of Calgary and former member of the Institute on Aging.

"It's not just about extending life, it's extending the quality of life," he said. "You live to be 80 and you might expect to live 70 of those years in good health. Caloric restriction would allow you to live . . . to 100 for example, and 95 of those years in good health."

While studying caloric restriction in hearing-impaired mice, the team found that sirtuins, a type of enzyme, are key determinants of the aging process. Of particular importance is a sirtuin called Sirt3.

This particular enzyme works on the cell's mitochondria, which produce energy and a form of oxygen known as free radicals.

Free radicals are naturally occurring in the body, but as people age their levels become excessive, Prolla said. The free radicals damage cells and promote aging, but Sirt3 can curb their release under conditions of reduced caloric intake.

Currently, people wanting to lengthen their lifespan have to decrease their caloric intake by the same percentage. For example, an extra 10 years requires a reduction of 10 per cent. The scientists say this discovery could provide more comfortable options.

"If we can find drugs or nutritional interventions that activate Sirt3, we may be able to slow down the aging process in mammals," Prolla said, adding that it could take years to get these products on pharmacy shelves.

That is promising, says Hepple, because studies show that humans are only able to tolerate a 20 per cent calorie reduction because their brains don't adapt to hunger like their bodies do.

Scientists first started studying how caloric intake impacts longevity in 1934 when researchers at Cornell University discovered that lab rats who were fed less than normal, while still getting the vitamins and minerals they needed, lived longer than rats eating regularly. These findings were later explored in the 1980s by Roy Walford and Richard Weindruch, who found that restricting the caloric intake of rats proportionally prolonged their lives. The impact of caloric reduction in humans has not been formally tested, but in a study of primates it was found to increase lifespan.

Gaining a few extra years through a strict diet may not be fun warns Dr. Yoni Freedhoff, an Ottawa-based doctor who specializes in weight management and obesity.

"Food is a pleasure in life and denying yourself pleasure from food is certainly a very large price to pay when you've got a whole life to live and now the life is going to be longer," he said.

"Cutting back something, somewhere, makes sense," says Patricia Chuey, chair-elect of the Dietitians of Canada. "But I think it's really important to make the point of quality of calories versus quantity."

Chuey said it is important that dieters remember to get key nutrients in the calories they continue to eat.

She also suggests taking it slow. "Seriously cutting back is creating a feeling of over hunger which is almost always going to set you up for binge eating."

Most people can afford to find some calories to cut, Freedhoff said, advising people interested in cutting some of their food intake to keep a journal of what they eat.

"A slight reduction in calories is something that is achievable for most people and could probably result in benefits," Prolla said, adding that people still need the proper vitamins and minerals.

He warned that a major reduction would be unlikely in humans and could entail side effects such as a reduction in hormone levels and muscle mass.