The new report on nearly 3 million people, 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, experts said.

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. But 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.

While obese people had a greater mortality risk overall, those at the lowest obesity level (BMI of 30 to 34.9) were not more likely to die than normal-weight people.

The study did show that the two highest obesity categories (BMI of 35 and up) are at high risk.

“Once you have higher obesity, the fat’s in the fire,” Blackburn said.