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A new study published in the International Journal of Obesity suggests that antibiotic use may be associated with increased weight gain in children.

Researchers at Johns Hopkins reviewed the electronic health records of 142,824 children ages 3 to 18 who were treated at Pennsylvania’s Geisinger Health System from 2001 to 2012. They used data on on prior prescriptions for antibiotics and patients’ body weight and height to develop a model of how a child's body mass index or BMI changes as they age and evaluate how that trajectory is altered by antibiotic prescription.

They found that by age 15, children who had been prescribed antibiotics seven or more times weighed on average 3 pounds more than those who had no history of antibiotic use.  

Children initially given an antibiotic would gain a small amount of weight and then return to their baseline. With more antibiotic prescriptions, however, the gain was cumulative and progressive. As the children aged, the effect of antibiotic prescription on BMI became more pronounced.

Scientists have known for years that antibiotics cause rapid weight gain in animals — they have been a mainstay of industrial farming since the 1950s.  More recently, studies have linked their use in newborns and infants to weight gain as well.  This is the first study to find that the effects of antibiotics on weight gain are cumulative and persist throughout childhood.  

Although the new study did not specifically look at the causes of this weight gain, past research suggests that it might be because the antibiotics we take to eliminate infectious bacteria can also kill off the helpful bacteria in our gut. The microorganisms in our digestive tract help process food; their absence impacts how the body digests and absorbs nutrients and calories, possibly leading to weight gain.

Just how big of an impact do antibiotics have on weight gain in children?

To put the study in perspective, a three-pound weight gain generally translates to less than one BMI point, and it is unlikely that antibiotic use is a major player in the high rates of childhood obesity, a public health epidemic that is likely more due to social and environmental causes than medical ones.  But it is possible that these effects have a lifelong impact, and the researchers say they suspect that their reported findings may actually underestimate the true effect of antibiotics on weight gain over a lifetime.

That's partly because information on maternal antibiotic use, a known contributing factor in childhood weight, was not included in the study.  There was also no data available for most children prior to age 3, a time when they may be particularly sensitive to the impact of antibiotic use on obesity.  

Lead researcher Brian Schwartz emphasized that he expects that the pattern that “the BMI trajectories of children who did and did not receive antibiotics are increasingly diverging at older ages … suggests that the effect likely continues into adulthood.”

The CDC estimates that almost half of antibiotics prescribed in outpatient settings to both children and adults are medically unnecessary.  This happens for a variety of reasons, including general lack of understanding as to when antibiotics will actually work as a treatment.  Studies like this should encourage judicious antibiotic use and a renewed emphasis on patient education and health literacy.  With antibiotic-resistant infections on the rise, researchers say it's time to do away with the “just in case” prescriptions from physicians and the “I want some medicine” demands from patients.

Schwartz explained: “Bottom line: if your doctor tells you that you are your child does not need antibiotics, don’t ask for them. … Eat healthy and your gut microbiota will be very good to you. Don’t change your gut microbiota unnecessarily by taking antibiotics if you don’t need them.”

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