Why Does Everyone Hate BMI?
Everyone seems to hate BMI and yet none of the myriad alternatives get much traction. Two new studies look at one of the core problems — a “U-shaped” relationship between BMI and mortality. At the extremes, low BMI and high BMI are both associated with the risk of an early death. But in the middle, the relationship gets muddled.
Cynthia Thompson and colleagues analyzed data from the Women’s Health Initiative, comparing the ability of BMI to predict mortality with two other indices: a body shape index (ABSI) and body adiposity index (BAI). They found that ABSI might be a better predictor of mortality risk because the relationship is linear. ABSI seems to do a better job than BMI of factoring in the effect of visceral fat because it uses waist circumference in addition to height and weight. Thompson explains:
ABSI was developed to integrate central adiposity with the health risk assessment equation, knowing that there is a subgroup within the population who maintain a healthy BMI but disproportionately carry weight in the form of central adiposity.
In Annals of Internal Medicine, Raj Padwal and colleagues provide some new insights on the risk of visceral fat that is independent of a high BMI. They found that the combination of a low BMI and a high body fat percentage can be predictive of considerable mortality risk. It underscores the fact that obesity is a disease of unhealthy visceral adiposity and BMI is far from being a perfect proxy for it.
And that brings us back to the contempt for BMI. The problem and the virtue of BMI is that it’s a simple screening tool. Height and weight are easily measured and usually readily available in medical records. Direct measures of adiposity — like DXA scans — are not so readily available. We’re not even close to routinely measuring waist circumference. So BMI will continue to be useful for screening. And it’s useful for epidemiology.
When we see the prevalence of BMI in the range of obesity and severe obesity growing, there can be no doubt that it’s a problem. When someone has a BMI in the range of obesity, it’s a signal of something that is worth a closer look. But BMI by itself cannot diagnose the health of an individual.
If people hate BMI it’s because they are expecting too much from it.
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March 19, 2016