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Further, the risk of dying increases in proportion to the amount of excess weight you gain, the researchers found.
The results undercut the so-called "obesity paradox" -- a theory that obesity could protect the health of some people and even give them a survival advantage, said senior study author Andrew Stokes. He's an assistant professor of global health with the Boston University School of Public Health.
In the study, Stokes and his colleagues tracked the weight history of more than 225,000 participants in three large studies, gauging the maximum body mass index (BMI) of each person across an average of 16 years.
"We found that after considering weight history, the apparent paradoxical association between overweight/obesity and the risk of dying completely disappeared," Stokes said.
Morbidly obese people were twice as likely to die from any cause, more than three times as likely to die from heart disease, and 50 percent more likely to die from cancer compared with normal-weight folks, researchers concluded.
Previous studies with results supporting the obesity paradox have only checked participants' BMI at one point in time, producing a weight "snapshot" that might not reflect the person's actual excess pounds over their lifetime, Stokes said.
This can bias the results, when you consider that many people with a fatal illness frequently lose a lot of weight prior to death, he said.
"Some people have unintentional weight loss driven by the onset of a chronic disease like cancer or a heart condition," Stokes said. "When you just consider the snapshot, some people in the normal-weight category are those who developed a disease and are losing weight on the pathway to dying. That acts as a bias."
Tracking the subjects' weight every couple of years via questionnaires, researchers were able to categorize them based on the highest BMI they reached during the study period -- underweight (less than 18.5 BMI), normal weight (18.5-25 BMI), overweight (25-30 BMI), obese (30-35 BMI) and morbidly obese (greater than 35 BMI).
They then tracked participants an average of 12 years, noting which ones died and the cause of their deaths.
A person's overall risk of dying increased based on their maximum BMI, the researchers found: 10 percent increased risk for overweight people, 34 percent for the obese and 98 percent for the morbidly obese.
The same sliding scale held for risk of death from heart disease (23 percent increased risk for overweight people, 71 percent increased risk for the obese and more than triple for the morbidly obese) and cancer (5 percent for overweight, 20 percent for obese and 50 percent for morbidly obese).
However, the study cannot prove that the additional weight caused the increased death risk, and it can't say whether or not losing the weight would reduce the extra risk, Stokes added.
"That's a really important question, and it's a question I aim to address in future research," Stokes said. "In this paper, we have not distinguished between intentional and unintentional weight loss. We cannot say anything at this point about whether having a history of overweight and obesity sticks with you even after you lose the weight through lifestyle change."
Dr. Scott Kahan, director of the National Center for Weight and Wellness in Washington, D.C., said he's not surprised that the obesity paradox does not appear to stand up to close scrutiny.
"We have no biologically plausible reason to think that carrying excess weight would be protective in any way," Kahan said, noting that extra pounds place added stress on the body while larger fat cells produce harmful inflammatory chemicals and hormones.
At the same time, Kahan thinks ultimately it will be proven that overweight and obese people could reduce their risk by losing weight.
"Many, many other studies have shown that even moderate weight loss leads to improvement of a wide range of health problems," said Kahan, a spokesman for The Obesity Society.
Stokes agreed. "We have quite compelling evidence from trials of bariatric surgery that weight loss is hugely beneficial in reducing your risk of disease or dying," he said.
The study was published in the April 3 issue of the Annals of Internal Medicine.
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