The studies used Body Mass Index (BMI), a measurement that divides a person's weight in kilograms by their height squared in meters to determine obesity. Researchers found that death rates were lowest in people who had a BMI of 23 to 24, on the high side of the normal range.
Health officials generally define overweight people as those with a BMI from 25 to 29, and obese people as those with a BMI above 30.
The study was published online Wednesday in the medical journal, Lancet. It was paid for by Britain's Medical Research Council, the British Heart Foundation, Cancer Research UK and others.
"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.
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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