Body mass index (BMI), the dreaded math calculation used for decades to tell us whether we are merely fat or morbidly obese, suggests that over 41 million or 35% of us are in the latter category. As bad as that may seem, it’s about to get a whole lot worse. Researchers from New York University found that BMI underestimates the obesity numbers, especially for women.
In a study published on PLoS ONE, a peer reviewed open access journal (Lucine supports open source publishing), researchers suggest that when more accurate measures of adiposity (fat) are used, at least 74 million Americans (64%) should be deemed obese. Whoa.
BMI and Women’s Health
It appears that BMI (weight in pounds/(height in inches)2×703) while a quick and easy indicator of obesity, ignores several important factors that tend to underestimate obesity levels the female population. Most importantly, BMI doesn’t account for the relationship between lean body mass and fat mass. Sarcopenic obesity, the loss of lean mass or muscle combined with the increase of fat mass, plagues women more frequently than men, especially as we age. And as we age and lose more lean mass, BMI measures of obesity become less and less accurate. According to the research, BMI underestimates the obesity levels in women by 40% across all age groups, but for the older age groups, >60 years, the number approaches 60-70% error. This is striking, not only because of the high misclassification rate in women (remember medical decisions are made based upon BMI assumptions) but also, because BMI potentially underestimates the number of women who should be considered obese. Or does it?
While I agree that many of us are not as slim as we should be, I wonder if we might not need new measures of health and fitness. I am thinking of the female athletes in the recent Olympics – not the gymnasts or divers (although even as petite as those athletes are, their weight, because of muscle mass, to height ratio could be skewed by BMI standards), but the female weight lifters, boxers, wrestlers, judokos, and even the water polo players. Many of these women would be considered overweight by current BMI standards and yet, they are at the pinnacle of health and fitness. What does that say of the BMI standards when those at the height of health and fitness can be considered fat while those at edge of illness, who are noticeably overweight are considered normal weight because of skewed lean to fat mass ratios?
The Paradox of Obesity: Why BMI Doesn’t Predict Health
And here we have the paradox of obesity (and the problem with BMI), obesity doesn’t correlate with mortality. Indeed, with many conditions and under many circumstances-stroke, cardiovascular disease, hemodialysis, cancer and others-being overweight increases survival. Maybe it’s not the fat, or even the lean muscle to fat ratio, but the fitness level that should be measured. Research shows that individuals who are metabolically healthy regardless of weight, have no increased risk of mortality from cardiovascular disease than their normal weight counterparts.
Resting Heart Rate Better than BMI
It is well known that athletes, no matter their BMI, have lower resting heart rates than couch potatoes. Perhaps resting heart rate might be a better indicator of overall health. Indeed, several studies have demonstrated that a low resting heart rate may be a better indicator of cardiovascular and metabolic health in women than BMI. So before we go starving ourselves to reach some physical notion of health (and beauty) based upon a faulty metric, embrace your inner athlete and exercise.
To the researchers who bemoan the growing obesity epidemic and associated health costs, it’s time to move beyond what we look like as a matter of predicting health and move toward how our bodies function. Resting heart rate may be one solution, biomarkers may be another, but BMI is not an effective metric for evaluating women’s health.