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TUESDAY, March 17 (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.

The researchers found that men and women whose BMI was between 22.5 and 25 lived the longest. For a person 5 feet 7 inches tall, his or her optimum weight would be about 154 pounds, they noted.

For those with a BMI over 25, every 10 to 12 pound increase translated to about a 30 percent increased risk of dying. In addition, there was a 40 percent increase in the risk for heart disease, stroke and other vascular disease, a 60 percent to 120 percent increased risk of diabetes, liver disease or kidney disease, a 10 percent increased risk of cancer, and a 20 percent increased risk for lung disease, the researchers reported.

"Obesity causes kidney disease, liver disease and several types of cancer, but the most common way it kills is by causing stroke and, most importantly, heart disease. Obesity causes heart disease by pushing up blood pressure, by interfering with blood cholesterol levels, and by bringing on diabetes," Whitlock said.

People who are moderately obese with a BMI in the 30 to 35 range reduced their life span by two and four years. For those who are severely obese with BMIs between 40 and 45, their life span was reduced by eight to 10 years. That's comparable to the effects of smoking, Whitlock said.

In fact, people whose weight was below normal also died earlier, due mainly to smoking-related diseases, the researchers noted.

"If you are obese and smoke, then, above all else, quit smoking," Whitlock said. "If you are obese and don't smoke, then don't start, and do what you can to avoid further weight gain. By avoiding further weight gain, you may well live a few years longer than you otherwise would do. By quitting smoking, a smoker can expect to gain several extra years of life -- about as many as a severely obese person might gain by shedding half of his or her body weight."

Dr. David L. Katz, director of the Prevention Research Center at Yale University School of Medicine, said this study confirms that the obesity epidemic is "the clear and present danger many of us knew it to be."

The association between BMI and mortality has been challenged in the scientific community, due in part to uncertainty about weight estimates and debate about measurement methods. "Here we have an emphatic reaffirmation of the fundamental issue: Overweight and obesity take years from life," Katz said.

"We know that, in many ways, BMI is a crude measure of the health risks associated with obesity, since not all excess body fat is created equal," he said. "Weight gained around the middle tends to be most dangerous, so for those subject to this pattern, risks may indeed be higher than this study suggests. For those with lower body weight gain, risks may be lower."

A study published in the Nov. 13 issue of the New England Journal of Medicine also found that where weight is centered is a risk factor. Men with the largest waist circumference had more than double the risk of death, and women with the largest waist circumference increased their risk of death by 78 percent.