In an analysis of nearly 100 studies that included approximately three million adults, relative to normal weight, overall obesity (combining all grades) and higher levels of obesity were both associated with a significantly higher all-cause risk of death, while overweight was associated with significantly lower all-cause mortality, according to a study in the Jan. 2 issue of JAMA.

“Estimates of the relative mortality risks associated with normal weight, overweight, and obesity may help to inform decision making in the clinical setting,” according to background information in the article.

Katherine M. Flegal, Ph.D., of the National Center for Health Statistics, Centers for Disease Control and Prevention, Hyattsville, Md., and colleagues conducted a study to compile and summarize published analyses of body mass index (BMI) and all-cause mortality that provide hazard ratios for standard BMI categories.

For the review and meta-analysis, the researchers identified 97 studies that met inclusion criteria, which provided a combined sample size of more than 2.88 million individuals and more than 270,000 deaths.

Regions of origin of participants included the United States or Canada, Europe, Australia, China or Taiwan, Japan, Brazil, Israel, India and Mexico.

All-cause mortality hazard ratios for overweight, obesity, grade 1 obesity, and grades 2 and 3 obesity were calculated relative to normal weight.

The researchers found that the summary hazard ratios indicated a 6 percent lower risk of death for overweight; a 18 percent higher risk of death for obesity (all grades); a 5 percent lower risk of death for grade 1 obesity; and a 29 percent increased risk of death for grades 2 and 3 obesity.

The authors note that the finding that grade 1 obesity was not associated with higher mortality suggests that that the excess mortality in obesity may predominantly be due to elevated mortality at higher BMI levels.

The researchers add that their findings are consistent with observations of lower mortality among overweight and moderately obese patients. “Possible explanations have included earlier presentation of heavier patients, greater likelihood of receiving optimal medical treatment, cardioprotective metabolic effects of increased body fat, and benefits of higher metabolic reserves.”

The use of predefined standard BMI groupings can facilitate between-study comparisons, the authors conclude.