"If you are heading towards obesity, it may be a good idea to lose weight," said Sir Richard Peto, the study's main statistician and a professor at Oxford University.

An obese woman stands outside a sandwich shop. Health officials have ordered Russians to adopt a back-to-basics diet for the  economic crisis to stop their weight ballooning after a decade of indulgence during the boom years.[Agencies]

Peto and colleagues found that people who were moderately fat, with a BMI from 30 to 35, lost about three years of life. People who were morbidly fat — those with a BMI above 40 — lost about 10 years off their expected lifespan, similar to the effect of lifelong smoking.

Moderately obese people were 50 percent more likely to die prematurely than normal-weight people, said Gary Whitlock, the Oxford University epidemiologist who led the study.He said that obese people were also two thirds more likely to die of a heart attack or stroke, and up to four times more likely to die of diabetes, kidney or liver problems. They were one sixth more likely to die of cancer.

"This really emphasizes the importance of weight gain," said Dr. Arne Astrup, a professor of nutrition at the University of Copenhagen who was not linked to the Lancet study. "Even a small increase in your BMI is enough to increase your risks for cardiovascular disease and cancer."

Previous studies have found that death rates increase both above and below a normal BMI score, and that people who are moderately overweight live longer than underweight or normal-weight people.

Other experts said that because the papers used in the study mostly started between 1975 and 1985, their conclusions were not as relevant today.

Astrup worried that rising obesity rates may reverse the steep drops in heart disease seen in the West.

"Obesity is the new dark horse for public health officials," he said. "People need to be aware of the risks they're taking when they gain weight."

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Editor:Yang Jie