How Body Mass Index (BMI) Affects Longevity

Body Mass Index

Body-mass Index (BMI), a measure of body fat based on height and weight, has long been thought to be a reliable predictor of disease risk and, by extension, longevity. According to the CDC, scoring in the ‘Obese’ category puts you at greater risk for cardiovascular disease (coronary heart disease, stroke, and high blood pressure), type 2 diabetes, cancer (endometrial, breast, and colon), high cholesterol, liver and gallbladder disease, and other disorders. Doctors may often use BMI as a diagnostic measure of obesity and to recommend alternative diets and/or an exercise regimen to those who qualify as overweight or obese.

However, the correlation between higher BMI and increased risk of mortality hasn’t always been definitive. Several recent meta-analyses highlight BMI’s lack of qualitative merit as a measure of mortality risk. The first, an April 2014 study published in the American Journal of Clinical Nutrition, found no association between an ‘overweight’ BMI classification and an increased risk of mortality. The second study, appearing in the January 2014 issue of Progress in Cardiovascular Diseases, showed that unfit individuals carried twice the risk of mortality than those who are normal-weight and fit, regardless of BMI. That is, overweight or obese, but fit, individuals held similar risk for overall mortality as those who were of normal weight and fit. Bottom line: BMI classifications carry less weight than previously thought in the story of longevity, but ‘fitness’ seems to play a more prominent role.

The hallmark study of this review, recently published in The American Journal of Medicine, took these findings one step further—finding what may be better than BMI in predicting length of life—and possible altering the landscape of how physicians diagnose and treat weight-related health issues. The study’s authors suggested that muscle mass may be a better way of predicting longevity than BMI. Researchers assessed all-cause mortality in 3,659 participants (men aged 55 years and women aged 65 years or older at study’s start) 10 years after final measurements were collected. All participants were followed and regularly checked for 6 years. Turns out, overall muscle mass was a much more accurate predictor than BMI-based classifications.

Total body mass is a measure of both fat and muscle, tissues with very different metabolic processes and consequences for the body. The study’s authors concluded that physicians may be better off narrowing their focus on muscle-mass index when recommending a healthier lifestyle. Instead of structuring healthy diets around limiting calorie intake, health-improving regimens diets should try to maximize exercise , particularly resistance workouts (squats, bench-press, etc.) that are known to help build muscle. As a general rule, indulging in an extra chocolate might not be that big of a deal, but skimping out on workouts should now make you think twice about the consequences of your decisions.