Predicting a person’s risk of death from their body mass index or BMI, which takes both their weight and height into consideration, may be less accurate than previously thought, according to a new report.

The study findings suggest that men, at least, who are classified as overweight according to their body mass index (weight in kilograms divided by weight in meters squared), may actually have a lower risk of death than those classified as normal weight.

“The BMI is the most widely used indicator of relative weight status - too high, too low, just right,” said study author Jerome Timothy Gronniger, an analyst at the Congressional Budget Office in Washington, DC.

However, “It is not especially good at predicting mortality for a large fraction of the population, the broad group of people who are not very obese and not underweight,” he told Reuters Health. Gronniger conducted the researcher while a graduate student at the University of Michigan.

For years, healthcare professionals and researchers have considered a BMI of 20 to 25 to be ideal, and representative of the range for normal-weight individuals. People with a BMI of 25 to 30 are considered overweight, and those with a BMI of 30 or higher are classified as obese.

This definition of obesity was the result of a consensus among the World Health Organization, the Centers for Disease Control and Prevention and various other health bodies. They determined that disease risks increase as an individual’s BMI rises above 25 and that those risks become serious at a BMI of around 30. Still, research has not yet shown a direct link between BMI and mortality risk.

In his study, Gronniger assessed whether the conventional BMI categories are adequate for planning public health programs to reduce the associated risks of death. His analysis was based on data from 33,558 respondents to the 1987 and 1989 National Health Interview Survey.

Gronniger found that for men the optimal BMI with the lowest mortality risk was in the “overweight” range of 26 to 27, he reports in the American Journal of Public Health. The risk of death for men with a “normal” BMI was just as high as for those whose BMI fell within the “mildly obese” range of 30 to 35.

For women, death risks appeared to be lowest for those with BMIs of 23 to 24, and steadily increased for those with BMIs above 27.

Overall, overweight individuals - those with BMIs 25 of to 30 - seemed to be healthier than both their normal-weight and obese peers, the study findings show.

Thus, the risks of death for overweight individuals, a category that comprises about 75 million adults, “have typically been overstated in the health literature,” according to Gronniger.

He concludes that his study “does not support the idea that reducing weight alone would result in any large mortality risk reduction for most of the population.”

Still, Gronniger emphasizes that he does “not wish to downplay the importance of the health risks involved.”

“Though there are ongoing debates about the effects of weight loss programs, exercise programs, and surgical interventions on mortality and health outcomes, there is little doubt that most people could benefit from moderation in diet and increased exercise,” he told.

SOURCE: American Journal of Public Health, January 2006.