March 22, 2019
Woman helping young girl take medicine in bedroom

Antibiotics, Obesity, and the Gut Microbiome

by Jeanine Barone  

Obesity can be blamed on many factors, including family history, sedentary lifestyle, and a calorie-rich diet. These have long seemed fairly obvious variables. But scientists are training their focus on a cause that is literally under the cover of darkness: the effect of the ubiquitous use of antibiotics in our society on the balance of the different bacteria (as well as other microorganisms) that live in our gut. There are trillions of these bacteria, which are collectively referred to as the gut microbiome. The theory is that antibiotics may be fueling the obesity epidemic by disturbing the balance of these bacterial populations.

Antibiotics and weight gain

It’s been long known that antibiotics have weight-promoting effects in a variety of farm animals, which are typically given antibiotics as a way to fend off infectious disease (and to speed up weight gain). And studies dating back to the 1950s found that malnourished childrenall over the world, from Jamaica to Africa, who were treated with antibiotics gained weight. But these observations didn’t prove causation, nor what mechanism might account for any weight gain.

Scientists have since become interested in how gut bacteria affect the way the body derives energy from food and responds to hormones that affect satiety—and more specifically, how a change in the balance of these microbes may tilt the scales in favor of obesity. Some 90 percent of the bacteria in the gut fall into two groups: Bacteroidetes and Firmicutes. Firmicutes can cause us to absorb more calories from the same amount of food compared with Bacteroidetes. In sophisticated animal experiments, researchers discovered that obese mice had a preponderance of Firmicutes, while lean mice had a greater proportion of Bacteroidetes. Scientists then experimented with transplanting microbes from either obese or lean mice into a special population of lean mice who were raised from birth in a germ-free environment, so that their gut contained no bacteria at all. Despite the researchers’ controlling for calorie intake, the mice given the “lean microbes” remained lean, while those given the “obese microbes” became heavier and accumulated more body fat.

The relationship between these gut microbes and obesity is complicated, however, in part because being obese may itself affect the bacterial balance in the gut. For example, when obese people lose weight, their proportion of Bacteroidetes increases relative to Firmicutes. Genetic factors may also play a role in determining the balance between these two bacterial groups.

Adding to this complexity is the effect of antibiotics. Even a short course of antibiotics can alter the gut’s microbial populationfor up to a year before it returns to normal. This microbial change is referred to as dysbiosis. Increasing research is looking at how dysbiosis, most notably in early childhood, can affect the risk of obesity as well as of other health conditions later in life.

Antibiotics and childhood obesity

Antibiotics are widely prescribed in the U.S., especially during infancy and childhood. In a paper published in 2015 in Nature, researchers determined that by the time the average child is 2 years old he or she has already received close to three courses of antibiotics, typically to treat ear and upper respiratory tract infections. By age 10, the typical child has had 10 courses of antibiotics. (The authors based their conclusions on national prescription data from 2010.)

A study in 2014 in JAMA Pediatrics analyzeddata from more than 64,000 children who visited clinics and private practices in Philadelphia over a 12-year period. Children who took antibiotics—especially broad-spectrum antibiotics—at least four times before age 2 had a higher risk of becoming obese by age 5. Broad-spectrum antibiotics, such as amoxicillin, can knock out a wide range of bacteria compared with narrow-spectrum antibiotics, such as erythromycin, which target only specific bacteria. For that reason, narrow-spectrum antibiotics are considered a better first-line treatmentfor typical childhood infections.

Since there’s evidence that humans acquire some of their gut bacteria in utero from the mother, the relationship between prenatal antibiotic use and childhood obesity is also being investigated. In a study in 2014 in the International Journal of Obesity, the investigators followed more than 400 pregnant women and, subsequently, their children until they were 7 years old. The researchers found that, after controlling for numerous variables including birth weight, women who took antibiotics during their second or third trimester of pregnancy had a significantly higher risk of having obese children.

Breastfeeding, on the other hand, may help protect children from obesity; one theory that’s been proposed for this is that breastfeeding helps promote a healthy balance of gut bacteria (though this depends on the mother’s health, including whether she’s obese). A study in 2016 in JAMA Pediatrics found that breastfeeding was less likely to protect a child from becoming overweight if the child was given antibiotics when they were very young, or if the mother took antibiotics while she was breastfeeding. An accompanying editorial noted that research has been inconsistent in terms of the link between antibiotic use in young children and obesity, and that this may be because studies have not taken into account whether or for how long children were breastfed.

Bottom line: The field of how the gut’s population of bacteria affects health is still in its early stages,and that includes any possible effect of how antibiotics may alter these bacterial populations and the risk of obesity. Animal research and epidemiological studies of humans point to an association between receiving antibiotics in early life—especially in the first year—and obesity, especially if the child receives multiple antibiotic courses, and if the antibiotics are broad-spectrum ones. But more research is needed—including investigating why the gut bacteria of boys may be more affected by antibiotics than those of girls.

Inthe meantime, the existing evidence offers another good reason to take antibiotics only when necessary, and if possible to receive a narrow-spectrum rather than a broad-spectrum drug. Remember also that antibiotics only work against bacterial infections; they aren’t effective against viruses, the cause of most childhood acute infections. Don’t hesitate, however, to allow your child to be prescribed antibiotics, when they are necessary, out of concern that the drug may promote weight gain. Antibiotics are truly wonder drugs—all the more reason to use them appropriately.

Also see Are Antibiotics Making Us Fat? and When to Be Anti-Antibiotics.