Having a body mass index between 20.0 and 24.9 is associated with the lowest risk of death in healthy non-smoking adults, a new study has revealed.
The research team included investigators from the National Cancer Institute (NCI), part of the National Institutes of Health, and collaborators from a dozen other major research institutions worldwide.
BMI, the most commonly used measure for body fat, is calculated by dividing a person's weight in kilograms by the square of his/her height in meters (kg/m2).
Current guidelines from the U.S. Centers for Disease Control and Prevention, and the World Health Organization define a normal BMI range as 18.5 to 24.9.
Overweight is defined as a BMI of 25.0 to 29.9; obesity is defined as a BMI over 30.0; and severe obesity is defined as BMI 35 or higher.
In this large analysis, investigators pooled data from 19 long-term studies designed to follow participants over time, from 5 to 28 years.
They 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 report a 44 percent increase in risk of death for participants with a BMI of 30.0 to 34.9; an 88 percent increase in risk for those with a BMI of 35.0 to 39.9; and a 2.5 times (250 percent) higher risk of death for participants whose BMI was 40.0 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 NCI's Amy Berrington 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 increased risk of death for a BMI of 25 or greater was also seen in all age groups, although it was more prominent for those who were overweight or obese before age 50.
This analysis was restricted to non-Hispanic whites aged 19 to 84.
The investigators noted the relationship between BMI and mortality may differ across racial and ethnic groups. Other efforts are underway to study the effect of BMI on mortality in other racial and ethnic groups.
The study appears in the New England Journal of Medicine.