Will antibiotics make our kids fat? Nope. / by Methods Man


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.