BMI is Dead. Long Live BMI!

For the video version of this post, click here. Body mass index. Since the term entered the medical consciousness in 1972, it has served admirably as a proxy measure for body fat, since body fat itself is sort of tough to measure. But it is demonstrably imperfect. Since it relates weight to height, it has no ability to distinguish between fat mass and muscle mass, leading to so called “obesity paradoxes” that are really no such thing. We need something better than BMI.

That something better, according to an article appearing in the Annals of Internal Medicine, may be the ratio of waist to hip circumference.

The study has some dramatic results.  Using data from the National Health and Nutrition Examination Survey, researchers examined around 15,000 participants. Each participant had a BMI and a waist-to-hip ratio.  It turned out that waist-to-hip ratio was a much better predictor of mortality and cardiovascular disease than BMI.

In fact, once you accounted for waist-to-hip ratio, BMI didn’t predict mortality at all.  What this suggests is that all those studies linking BMI to bad outcomes were secretly studies linking central obesity to bad outcomes (because BMI and central obesity are correlated).  But when you introduce a better measure of central obesity, the utility of BMI goes out the window. It’s a proxy measure without a home.

It turned out that, among men with normal BMIs, those with a high waist-to-hip ratio had an 87% higher risk of death. Women with normal BMIs and central obesity had a 50% higher risk of death. Perhaps more interesting, men with normal BMIs and central obesity had around twice the risk of death of men who were overweight or obese by BMI but who didn’t have central obesity. Women’s results went in the same direction, though the magnitude wasn’t as great.

So, do we give up on BMIs altogether?  Not necessarily.  Waist-to-hip ratio does seem to be the superior risk marker, but it’s not as easy to measure. These data were collected by individuals trained to do these measurements the same way, every time - it may not be possible to do that in the doctor’s office and get reliable results. Though maybe we could start employing tailors.

Also, remember that BMI still captures a lot of this data. The finding that individuals with normal BMI but high waist-to-hip ratio have increased mortality is compelling, but only 11% of men and 3% of women fit in this category. In other words, chances are if you have a normal BMI you’re fine. That said, it seems clear now that we need to find something better than BMI, something that helps distinguish between fat mass and muscle in a way that BMI can not. Whether a technological solution, such as bioimpedance analysis, or an anthropometric solution like the one in this study takes the baton, my intuition is that BMI now has a shelf-life.

Will antibiotics make our kids fat? Nope.

For the video version of this post, click here. The ubiquitous and often inappropriate use of antibiotics is a serious public health problem. So is obesity.  That these two factors could be linked is the conclusion suggested by a paper, appearing in the International Journal of Obesity, which leveraged the huge Geisinger health system database to examine the BMIs of children who had different exposures to antibiotics.

The researchers examined the records of just under 150,000 children ranging in age from 2 to 18 who had BMI measurements in the system. Now, the relationship between antibiotic use and BMI  is complex, so they tried to characterize a couple of metrics. They examined the immediate effect of the antibiotic - how much BMI increase could be expected after exposure to antibiotics in the past year, but they also measured the persistent effect of antibiotics - how much BMI increase would be associated with your lifetime exposure to antibiotics.

In both cases, antibiotics were associated with higher BMIs, but numbers matter. Let’s start with the basics. 59% of the children in the cohort had at least one antibiotic prescription. Shockingly (to me at least), of the kids that had contact with Geisinger in their first year of life, 49% had an antibiotic prescription in that year. We prescribe a LOT of antibiotics.

As to the effects, the short-term effect was relatively modest. Kids who got antibiotics in the past year had a BMI about 0.05 points higher than those who didn’t. The cumulative effect was even smaller - about 0.01 BMI points for one prior antibiotic at any point in life, but more courses led to more BMI gain. Those who had seven or more courses of antibiotics had a BMI about 0.1 points higher than those who never got antibiotics. All of these associations were statistically significant, but this was a huge study - these changes in BMI don’t strike me as clinically meaningful.

Moreover, these children were not randomized to get antibiotics. The researchers adjusted for age, sex, race, and medical assistance, but that’s it. Socioeconomic status could play a major role here, and medical assistance is not a close enough proxy for that. I also wonder about secondhand smoke exposure.

Putting it all together, the great obesity epidemic can’t be tied to antibiotic use. In fact, these small effect sizes make me less worried about the effects antibiotics have on our children's weight.

That said, this is one case where I’m glad there is some media hype around the study. While the headlines warning that antibiotics are making your children fat are completely overblown, perhaps the negative press will reduce the over-prescribing of antibiotics. And that’s good, not because it will cure the obesity epidemic, but because it will impact the emerging epidemic of microbial resistance.