Want to live longer? Stop eating so much. Even if you're not overweight.
So say proponents of caloric restriction, a lifestyle choice where you take in fewer calories than the average person while making sure to get sufficient nutrients.
The relationship between food-curtailing and longevity has been studied for a while, and there is some compelling evidence in animals, including mice and rhesus monkeys.
Other researchers are more skeptical.
How much is "less"?
Well, that depends on where you start.
What the experts say
"The idea is to reduce calorie content in order to alter certain genes so energy is used more efficiently and gets directed toward repair and maintenance, not growth and reproduction. In practice it means eating a health- food diet and eating less than one normally would. The original idea was that this was about altering glucose metabolism. That's a big part of it. But now researchers are focusing on particular genes; we don't fully know what they do. How much you should consume is hard to answer. Research suggests that even a little CR can make a difference."
BRIAN M. DELANEY, president, CR Society, co-author, The Longevity Diet, Stockholm
"If you calorically restrict mice [of a strain not normally weight gainers], their lifespan is not extended. The point is, caloric restriction is a benefit only where there is excessive weight gain. If somebody has normal weight, could decreasing it be deleterious? It used to be thought that all mice, all people, would benefit from caloric restriction. But we find that it does not extend the lifespan of all strains of mice. Caloric restriction does not affect the aging process; it extends life by preventing the toxic effects of lipid deposition. Fat is not a benign tissue. It releases a variety of toxic substances."
RAJINDAR SOHAL, Timothy M. Chan professor of pharmacology, University of Southern California, Los Angeles
"If you calorically restrict a fruit fly and infect it with different pathogens, you get different results for every pathogen. Sometimes you infect a fly and the fly tolerates the infection better than it otherwise would, and that's great; it means caloric restriction is a good thing. But sometimes you calorically restrict the fly and infect it and it drops dead. Even when you look at a simple system like the fly's, you get complicated results."
DAVID SCHNEIDER, professor, microbiology and immunology, Stanford University, Palo Alto, California
"The old idea was that CR slowed metabolism, but that's been proven wrong. Metabolic rate goes up with caloric restriction. There are genetic pathways that respond to adversity and protect the body when times are tough. When you exercise or calorie-restrict, your body turns to these pathways for defence. These mechanisms generate more energy and protect the telomeres, whose erosion is thought to be a cause of aging. They also keep neurons from dying. If you start caloric restriction in midlife on a mouse, you can extend life. We're working on molecules that will mimic the benefit without having to diet. There are clinical trials right now on people."
DAVID SINCLAIR, professor of genetics, Harvard Medical School, Boston
"In a first-phase study of caloric restriction in non-obese people, we found that cardiovascular risk factors and insulin sensitivity improved. The metabolism becomes more efficient. This was associated with a lower core temperature and a decrease in DNA damage. A lot of cancers are caused by mutations of DNA. In each individual we measured how many calories were required not to lose or gain weight. Then we decreased intake by 25 percent. Those of normal weight on restricted calories will, on average, live longer, but not everybody is going to live to 90. You still have genes and biology. Some people are more prone to cancer or cardiovascular disease."
ERIC RAVUSSIN, Douglas L. Gordon Chair in Diabetes and Metabolism, Pennington Biomedical Research Center, Baton Rouge