Severely restricting calories over decades may add a few years to a human life span, but will not enable humans to live to 125 and beyond, as many have speculated, evolutionary biologists report.

"Our message is that suffering years of misery to remain super-skinny is not going to have a big payoff in terms of a longer life," said UCLA evolutionary biologist John Phelan. "I once heard someone say caloric restriction may not make you live forever, but it sure would seem like it. Try to maintain a healthy body weight, but don't deprive yourself of all pleasure. Moderation appears to be a more sensible solution.

"With mice, if you restrict their caloric intake by 10 percent, they live longer than if they have unlimited access to food," Phelan said. "If you restrict their intake by 20 percent, they live even longer, and restrict them to 50 percent, they live longer still; but restrict their intake by 60 percent and they starve to death.

"Humans, in contrast, will not have rodent-like results from dramatically restricting calories," he said. "Caloric restriction is not a panacea. While caloric restriction is likely to be almost universal in its beneficial effects on longevity, the benefit to humans is going to be small, even if humans restrict their caloric intake substantially and over long periods of time."

Phelan developed the first mathematical model demonstrating the relationship between caloric intake and longevity, using representative data from controlled experiments with rodents, as well as published studies on humans, diet and longevity. He and Michael Rose, professor of ecology and evolutionary biology at the University of California, Irvine, published their findings in a journal article titled, "Why dietary restriction substantially increases longevity in animal models but won't in humans," published in the August issue of the peer-reviewed journal Ageing Research Reviews.

Their mathematical model shows that people who consume the most calories have a shorter life span, and that if people severely restrict their calories over their lifetimes, their life span increases by between 3 percent and 7 percent -- far less than the 20-plus years some have hoped could be achieved by drastic caloric restriction. He considers the 3 percent figure more likely than the 7 percent.

"The trade-off between calories and longevity appears to be close to a linear relationship, but the slope isn't very steep," said Phelan, whose model predicts the relationship between calories consumed and life span.

Phelan's conclusion is that the few extra years of life are not worth the suffering necessary to achieve them.

"Do you want to spend decades severely limiting what you eat to live a few more years? You will be unhappy and then your life will end shortly after mine ends," Phelan jokes.

Scientists have known for six decades that cutting the caloric intake of rodents by 40 percent or 50 percent results in dramatically longer lives for them.

"You can practically double their life span," Phelan said. "The same result has been found in fish, spiders and many other species. If it works for them, some thought, it should work for us; I'm here to tell you it doesn't."

Phelan, co-author of the book, "Mean Genes," conducted his dissertation at Harvard University 10 years ago on caloric restriction and on why it works in extending the lives of rodents.

"When you restrict the caloric intake of rodents, the first thing they do is shut off their reproductive system," said Phelan, citing a finding from his dissertation. A normal rodent reaches maturity at one month of age, and begins reproducing its body weight in offspring every month and a half. If humans shut off reproduction by severely limiting calories, "our reduction in wear and tear on the body is minimal," he said.

The rodents placed on severely restricted diets bit people who tried to hold them, and had an unpleasant demeanor, unlike the more docile animals given more "normal" amounts of food, Phelan said.

"I think about food all the time," he said. "I'm not going to be so extreme that I become the mouse that bites anyone who touches me. My advice about food is be sensible, and don't be a fanatic about it because the payoffs are not worth it."

While the relationship between how much you eat and your life span is not so dramatic, there are very real costs of being overweight -- including greater risk for heart disease and other life threatening illnesses, Phelan said.

The human data factored into the mathematical model include the caloric intake of people in Japan, and their longevity, compared with sumo wrestlers, who consume more than twice the normal male diet, and men in Okinawa, Japan, who consume less than the average Japanese male.