A new report on nearly 3 million people found that those whose body mass index ranked them as overweight had less risk of dying than people of normal weight. The report, although not the first to suggest this relationship between BMI and mortality, is by far the largest and most carefully done, analyzing nearly 100 studies.
But don't scrap those New Year's weight-loss resolutions and start gorging on fried Belgian waffles or triple cheeseburgers.
Experts not involved in the research said it suggested that overweight people need not panic unless they have other indicators of poor health and that depending on where fat is in the body, it might be protective or even nutritional for older or sicker people. However, overall, piling on pounds and becoming more than slightly obese remains dangerous.
"We wouldn't want people to think, 'Well, I can take a pass and gain more weight,' " said Dr. George Blackburn, associate director of Harvard Medical School's nutrition division.
Rather, he and others said, the report, in the Journal of the American Medical Association, suggests that BMI, a ratio of height to weight, should not be the only indicator of healthy weight.
"Body mass index is an imperfect measure of the risk of mortality," and factors like blood pressure, cholesterol and blood sugar must be considered, said Dr. Samuel Klein, director of the Center for Human Nutrition at Washington University School of Medicine in St. Louis.
Dr. Steven Heymsfield, executive director of the Pennington Biomedical Research Center in Louisiana, who wrote an editorial accompanying the study, said that for overweight people, if indicators like cholesterol "are in the abnormal range, then that weight is affecting you," but that if indicators are normal, there's no reason to "go on a crash diet."
Experts also said the data suggested that the definition of "normal" BMI, 18.5 to 24.9, should be revised, excluding its lowest weights, which might be too thin.
But the study did show that the two highest obesity categories (BMI of 35 and up) are at high risk.
It is possible that overweight or somewhat obese people are less likely to die because they, or their doctors, have identified other conditions associated with weight gain, like high cholesterol or diabetes.
Some experts said fat could be protective in some cases, although that is unproved and debated. The study did find that people 65 and over had no greater mortality risk even at high obesity. And studies on specific illnesses, like heart and kidney disease, have found an "obesity paradox," that heavier patients are less likely to die.
Still, death is not everything. Even if "being overweight doesn't increase your risk of dying," Klein said, it "does increase your risk of having diabetes" or other conditions.
Ultimately, said the study's lead author, Katherine Flegal, a senior scientist at the Centers for Disease Control and Prevention, "the best weight might depend on the situation you're in."