The report, published in the Journal of the American Medical Association, reviewed 97 studies involving more than 2.88 million people globally. It found the lowest risk among those who are overweight though not obese, according to generally accepted health standards. Those just over the obesity threshold had the next lowest risk.
The findings, by scientists at the U.S. Centers for Disease Control and Prevention, were immediately challenged because the report didn't consider gender, age, fat distribution, or fitness levels, all factors that influence the risk of disease and death. Because of this, the researchers agreed no recommendations should be made based on their findings.
"The implication, if there is any, is that this may be a more complicated issue than meets the eye," said Katherine Flegal, a senior CDC scientist who is the report's lead researcher.
The report also shouldn't be viewed as a free pass to overindulge, the researchers said. The heaviest participants were 29 percent more likely to die from any cause during the course of the studies, according to the data.
"Being mildly obese or just overweight doesn't increase your risk on average, but severe obesity is still a killer," said Philip Schauer, director of the Cleveland Clinic's Bariatric and Metabolic Institute, who wasn't involved in the study. "Fifteen to 20 million Americans are severely obese, and they need to adhere to New Year's resolutions to lose weight or get bariatric surgery."
The researchers divided participants into categories based on the body mass index, a ratio of weight to height that's commonly used to measure fatness. A six-foot man is considered overweight at 185 pounds, mildly obese at 225 and severely obese at 260 pounds. A 5-foot-4-inch woman is overweight when she's above 145 pounds, obese at 175 pounds and severely obese at 205 pounds.
"For many types of people, especially those with chronic diseases such as diabetes, pulmonary disease, heart failure and even cancer, it may be better to carry a little extra weight," said Schauer, who suggested it may be time to re-evaluate the "somewhat arbitrary" cutoffs used to evaluate body mass index.
"Clearly some people with a BMI above normal may be very healthy or even at lower risk than those who are in the lower range," Schauer said.
Using only body mass index as a measure for health risk lumps together people with widely divergent nutritional status, disability and disease into one weight category, wrote Steven Heymsfield and William Cefalu, from the Pennington Biomedical Research Center in Baton Rouge, Louisiana, in an editorial accompanying the study.
It also overlooks the harm from illnesses that weren't deadly, they wrote.
Still, some people may indeed gain from having a little extra weight, the editorial said, including those with are ill and may benefit from having additional energy reserves during an illness, according to the editorial.
"Not all patients classified as being overweight or having grade 1 obesity, particularly those with chronic diseases, can be assumed to require weight loss treatment," Heymsfield and Cefalu wrote. "Establishing BMI is only the first step toward a more comprehensive risk evaluation."
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