NEJM: NCI study identifies ideal body mass index

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A study looking at deaths from any cause found that a body mass index (BMI) between 20 and 24.9 is associated with the lowest risk of death in healthy non-smoking adults, according to research in the Dec. 2 New England Journal of Medicine.

Previous studies that examined the risks from being overweight were inconclusive, with some reporting only modestly increased risks of death and others showing a reduced risk. In addition, the precise risks for different levels of obesity were uncertain.

To help clarify the issue, researchers from the National Cancer Institute (NCI) and collaborators from a dozen other research institutions worldwide pooled data from 19 long-term prospective studies designed to follow participants over time, from five to 28 years, depending on the study. The study encompassed 1.46 million non-Hispanic white adults, 19 to 84 years of age (median, 58), with a median BMI of 26.2.

The investigators gathered information about BMI and other characteristics from questionnaires participants completed at the beginning of each study. Causes of death were obtained from death certificates or medical records.

Led by Amy Berrington de Gonzalez, D.Phil., from the division of cancer epidemiology and genetics at the NCI in Bethesda, Md., researchers found that healthy women who had never smoked and who were overweight were 13 percent more likely to die during the study follow-up period than those with a BMI between 22.5 and 24.9.

Women categorized as obese or severely obese had a dramatically higher risk of death. As compared with a BMI of 22.5 to 24.9, the researchers reported a 44 percent increase in risk of death for participants with a BMI of 30 to 34.9; an 88 percent increase in risk for those with a BMI of 35 to 39.9; and a 2.5 times higher risk of death for participants whose BMI was 40 to 49.9.

Results were broadly similar for men. Overall for men and women combined, for every five unit increase in BMI, the researchers observed a 31 percent increase in risk of death.

"By combining data on nearly 1.5 million participants from 19 studies, we were able to evaluate a wide range of BMI levels and other characteristics that may influence the relationship between excess weight and risk of death," said de Gonzalez. "Smoking and pre-existing illness or disease are strongly associated with the risk of death and with obesity. A paramount aspect of the study was our ability to minimize the impact of these factors by excluding those participants from the analysis."

The investigators observed similar patterns of risk even after accounting for differences in alcohol consumption, physical activity and education level. The increased risk of death for a BMI of 25 or greater also was seen in all age groups, although it was more prominent for those who were overweight or obese before age 50.

"The strengths of our study include the very large and diverse study population, long-term follow-up with the majority of deaths occurring in the last decade and the broad age range. This permitted statistically precise estimates of the relationship between BMI and mortality across a wide range of BMI categories even in analyses restricted to healthy participants who never smoked," the authors concluded.