Antibiotics can be lifesaving, but they can have serious downsides — including increasing the risk of obesity when they are given early in life, according to a recent study.

Antibiotics kill bacteria. That can be a very good thing when the bacteria are causing a serious infection. But antibiotics don’t limit themselves to killing infection-causing bacteria; they kill other bacteria in the body, too. And that can be a very bad thing.

Our bodies are full of bacteria. These bacteria, part of our microbiome, are important. Along with other micro-organisms in our body, they play a role in how we digest foods, in normal growth, and in our immune system. When we take antibiotics, we inadvertently kill some of those bacteria. At first glance, it seems like this wouldn’t be such a big problem; after all, the world is full of bacteria, can’t we replace them? But as we learn more about our microbiome, it appears that the way it gets started — meaning the bacteria that we gather and grow early in life — is very important and can have lifelong effects.

Studies have shown that babies who are born by caesarean section are more likely to be obese as they grow, and part of the reason is thought to be that because they aren’t born through the birth canal, they don’t get that natural birth dose of bacteria to get them started in the right direction.

In the study, researchers looked at more than 300,000 infants born into the military health system. They looked at whether they were given antibiotics during the first two years of life. They also looked at whether they were given either of two medications used to decrease stomach acid, commonly prescribed to treat stomach reflux in babies. Giving antacids can alter bacteria, both by allowing the bacteria from the mouth and nose that usually get killed by stomach acid to move into the intestine, crowding out other species — and by killing bacteria themselves.

In the study, children who got antibiotics had a 26% higher chance of obesity. Taking one or both of the two kinds of antacid also increased the risk of obesity, although to a lesser extent. Taking antibiotics along with one or both kinds of antacid increased the risk, as did being on the antacids for longer periods of time.

Interestingly, farmers have been using this to their advantage for some time. Giving livestock antibiotics early in life makes the animals heavier, which means there is more meat on them. This use of antibiotics in livestock may mean more profits for farmers, but it has been a significant contributor to the problem of antibiotic resistance.

It’s not just obesity; giving antibiotics and antacids early in life increases the risk of food allergies and other allergic disease like asthma. Bottom line: we need to be very careful before we do anything that messes with the bacteria in our bodies.

As I said before, antibiotics can be lifesaving, and messing with the bacteria in our bodies is a risk absolutely worth taking — sometimes. But too often we use antibiotics when they aren’t really needed: many prescriptions, for example, are written for the common cold, something caused by viruses. Some infections, like ear infections, can get better without antibiotics — and even when we do need to use antibiotics, we often use them for longer than is necessary, or use stronger antibiotics than are necessary.

Since doctors write the prescriptions, it’s mostly doctors that need to make the change. But parents can play an important role, by asking if a prescription for antibiotics (or antacids) is truly necessary. If the answer is yes, parents should give it — but they should also ask about giving the shortest course possible.

We are in the midst of an obesity epidemic, one with so very many health implications for us and our children. As with any epidemic, we have to fight it in every way possible.

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