Being substantially obese, with a body mass index of 35 and higher, does raise the death risk 29 percent, researchers at the Centers for Disease Control and Prevention's National Center for Health Statistics said in the study, published in the Journal of the American Medical Association.
But overweight people with a BMI of 25 to 30 -- who make up more than 30 percent of the U.S. population -- have a 6 percent lower risk of dying than those with a BMI in the normal range of 18.5 to 25, said the CDC study, which analyzed 97 studies involving nearly 3 million people and 270,000 deaths around the world.
The report is the largest to examine the "obesity paradox," stemming from statistics indicating heavier patients in some cases are less likely to die than normal-weight patients, particularly elderly people or those with certain chronic diseases.
To this point, Drs. Steven Heymsfield and William Cefalu of the Pennington Biomedical Research Center in Baton Rouge, La., wrote in an editorial accompanying the study, "Not all patients classified as being overweight or having Grade 1 obesity [with a BMI from 30 to 35], particularly those with chronic diseases, can be assumed to require weight loss treatment."
BMI is usually defined as the individual's body mass in kilograms divided by the square of his or her height in meters. It can be measured in pounds and inches too, but that requires multiplying the quotient by 703.
The study points out BMI is an inexact measure of health, in large part because it doesn't differentiate between fat and muscle mass. Nor does it indicate where fat is located in the body, which researchers say may be more important than total fat overall.
"What is bad is a type of fat that is inside your belly," Dr. Kamyar Kalantar-Zadeh, professor of medicine and public health at the University of California, Irvine, told The New York Times.
"Non-belly fat, underneath your skin in your thigh and your butt area -- these are not necessarily bad," he said.
No matter where the fat is located, the study is not a license to eat more, experts said.
"That would be a mistake -- and this study did show an increase in mortality for people who are obese," CDC Director Thomas Frieden told The Wall Street Journal.
"I don't think anyone would disagree with the basic fact that being more physically active and eating a healthier diet is very important for your health," he said.
And even if "being overweight doesn't increase your risk of dying," it "does increase your risk of having diabetes" and other conditions, Dr. Samuel Klein, director of the Center for Human Nutrition at Washington University School of Medicine in St. Louis, told the Times.