Slashing calories may not extend life

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Two of the oldest rhesus monkeys in a University of Wisconsin study on the effects of slashing caloric intake. Canto (left) is on a restricted diet, while Owen is on an unrestricted diet. (THE ASSOCIATED PRESS ARCHIVES)

Slash your food intake and you can live dramatically longer -- at least if you're a mouse or a nematode. But a major study designed to determine whether this regimen, known as caloric restriction, works in primates suggests it improves monkeys' health but doesn't extend their lives. That outcome contradicts a similar study of monkeys reported three years ago.

Researchers not involved with the new paper say the results are still encouraging. Although the monkeys didn't evince an increase in lifespan, "both studies show a major improvement in 'health span,'" or the amount of time before age-related diseases set in, says physiologist Eric Ravussin of the Pennington Biomedical Research Center in Baton Rouge, La. "I certainly wouldn't give up on calorie restriction as a health promoter" based on these findings, adds molecular biologist Leonard Guarente of the Massachusetts Institute of Technology in Cambridge.

For many species, eating between 10 per cent and 40 per cent less than normal not only prolongs life, but also improves metabolic measures such as insulin sensitivity and defers age-related ailments such as cancer and heart disease.

Scientists haven't nailed down what happens to humans on this extreme diet. The first randomized study, the two-year CALERIE trial, wrapped up earlier this year. Ravussin, who is one of the trial's leaders, says that the final results haven't been analyzed, but in 2011, he and a colleague revealed six months of calorie-cutting reduced the participants' risk of cardiovascular disease by 28 per cent.

Testing whether calorie restriction also stretches human lifespan is impractical, however, so more than 20 years ago, scientists put two groups of rhesus monkeys (which have an average lifespan of about 27 years) on skimpy rations. In 2009, researchers reported in Science that a group of monkeys at the Wisconsin National Primate Research Center in Madison was reaping the benefits of the diet. Eating less cut rates of cancer and heart disease by half, for example. More than 50 per cent of the animals were still alive, but the team detected a survival trend. Although overall mortality was the same, only 13 per cent of the calorically restricted monkeys had died from age-related conditions, versus 37 per cent of the control animals.

Those findings jibe with most of the results from the second group of monkeys, which have been living and dying at the National Institutes of Health Animal Center in Dickerson, Maryland. "I think this (report) is quite positive," says senior author Rafael de Cabo, a gerontologist at the National Institute on Aging in Baltimore.

For instance, the team found none of the Maryland monkeys that started calorie restriction when they were young has developed cancer. De Cabo and colleagues report dieting monkeys also showed lower blood levels of triglycerides, which influence the risk for cardiovascular disease. But a disparity between the groups emerged when the team analyzed longevity. About half of the animals are still alive, so the researchers don't have definitive lifespan data. However, the famished monkeys don't appear to be living longer than control monkeys that ate more.

One possible cause of the discrepancy, de Cabo says, is a difference in the animals' diets. The Maryland monkeys noshed on more healthful food that included plenty of complex plant compounds, whereas the Wisconsin monkeys consumed processed food high in refined sugar. Control animals in Wisconsin also ate more than control animals in Maryland. He notes calorie restriction produces a bigger effect on longevity "if the control group is couch potatoes."

Genetic variability between the groups could also be a factor. The Maryland group was more diverse, including Indian and Chinese animals, whereas the Wisconsin monkeys all came from India. Studies of other organisms have shown genetic differences between individuals can affect the response to calorie restriction, notes molecular biologist Matt Kaeberlein of the University of Washington, Seattle, who wasn't involved with the research.

For now, gerontologist Richard Weindruch, who heads the Wisconsin monkey project, isn't ready to give up on a longevity effect. He notes some control and calorically restricted animals in the Maryland group surpassed by several years what researchers thought was the maximum lifespan for the species, about 40 years. That suggests the animals' diets do increase how long they can live, he says.

Researchers will have better answers in 10 years or so, when the last animals in both groups have died.

-- ScienceNOW online daily news service of the journal Science

Republished from the Winnipeg Free Press print edition September 1, 2012 B4