What’s the Ideal BMI for Longevity?

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How fat is too fat? A large new government-funded study found that being even a bit overweight can increase your risk of death from heart disease, stroke or cancer. The findings counter some data that have suggested that carrying a few extra pounds can be protective of health. So what should your ideal weight be? 

After analyzing data on 1.46 million mostly white, non-Hispanic adults who participated in 19 long-term studies — each designed to follow participants for between 5 to 28 years — researchers determined that, overall, a BMI (or body mass index, a ratio of height and weight that determines overweight and obesity) between 20.0 and 24.9 was associated with the lowest risk of death in healthy non-smoking adults. (More on Time.com: The ‘Other’ Salt: 5 Foods Rich in Potassium)

Current guidelines from the 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 25 or higher; obesity is 30 or higher; and severe obesity is 35 and up.

The National Institutes of Health (NIH), which funded the study, reports:

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.

The patterns of risk persisted, even after researchers accounted for differences in alcohol consumption, physical activity and education. The increased risk of death for a BMI of 25 was also seen in all age groups, the NIH reports, but it was more prominent in people who became overweight or obese before age 50. (More on Time.com: Fitness Tech: 10 Cool Ways to Get in Shape)

What about those at the low end of normal BMI? A healthy BMI ranges as low as 18.5, but longevity was associated with a minimum BMI of 20 in this study. “For women in the BMI category of 15 to 18.5 we observed the increased risk of death was 47%, and for women in the BMI category of 18.5 to 20 the increased risk of death was 14% compared to those in the BMI range of 22.5 to 24.9 (our reference group),” said the study’s lead researcher, Dr. Amy Berrington de Gonzalez of the National Cancer Institute. “However these risks decreased with longer follow-up suggesting they might at least partly be due to weight loss from pre-existing conditions rather than causal.”

The researchers excluded from the study people with heart disease or cancer at the start of the study, and those who smoked, since these factors influence risk of death. (More on Time.com: Photos: From Farm to Fork)

Studies are underway to determine the relationship between BMI and risk of death in other ethnic groups.

To calculate your own BMI, check out the NIH’s dedicated website.

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