HealthDay News: Being obese can shorten your life, a new study shows.

"Moderate obesity typically shortens life span by about three years," said researcher Gary Whitlock, from the Clinical Trial Service Unit at the University of Oxford in the United Kingdom. "By moderate obesity, I mean weighing about a third more than is ideal, which for most people would mean being about 50 or 60 pounds overweight."

More than one in three middle-aged Americans are now in this category, Whitlock said. "By contrast, weighing twice your ideal weight -- say, an extra 150 pounds -- shortens life span by about 10 years," he added.

This obesity level is still not common, but it equals the known 10-year reduction in life span caused by smoking. "So, smoking is about as dangerous as being severely obese, and about three times as dangerous as being moderately obese," he said.

The report is published in the March 18 online edition of The Lancet.

For the study, Whitlock and other members of the Prospective Studies Collaboration collected data on 894,576 men and women who participated in 57 studies. The people in these studies came primarily from western Europe and North America. Their average body-mass index (BMI) was 25.

BMI is a calculation that expresses a relationship between height and weight. People are considered underweight if their BMI is less than 18.5, normal weight when the BMI is between 18.5 and 24.9, overweight when BMI is between 25 and 29.9, and obese when BMI is 30 or more, according to the U.S. National Heart, Lung, and Blood Institute.

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