Could treating an infant with antibiotics cause them to gain weight decades later? It seems far-fetched, but a research team at New York University’s Langone Medical Center says yes. Antibiotics given to a child during a critical period of early development may radically change the child’s gut microbiome and disrupt their metabolism.

Dr. Martin Blaser and postdoctoral researcher Laura Cox, Ph.D., wanted to know how the trillions of bacteria that have evolved to live on and in our bodies — called the microbiome — react to antibiotics.

Birth through age 3 appears to be a particularly important time in the development of the microbiome.

“Infancy is a time of growth and development where stem cells are dividing — they’re choosing if they’re going to become muscle or fat or bone. There have been other studies that show changes early in infancy can impact body composition later on,” Cox told Healthline.

“We know microbes can affect metabolism, so potentially these metabolic early life interactions could be either speeding up a child’s growth or not,” she said.

Off to a Bad Start
To test their theory, the researchers exposed groups of mice to low doses of penicillin in a series of experiments.

One group was given antibiotics starting during the last week of development in the womb and ending after the mouse pups were weaned. A second group received antibiotics starting in the womb and continuing for life. A third group started receiving antibiotics after weaning and also stayed on them for life. A fourth group received no antibiotics.

The two groups that received antibiotics during the last week in the womb and during nursing were much more likely to gain weight and to have metabolic issues than the mice exposed to antibiotics after weaning or not at all.

“We knew that low-dose antibiotics would modulate the microbes [in the gut]. We found that giving low-dose antibiotics just in infancy, even if we stopped them, the mice would still develop obesity later in adulthood,” Cox said.

“One of the big surprises to us … is we found that the microbiome rebounded about four weeks after we stopped the antibiotics, but the mice got obese about 20 weeks later,” she added. “So even though the bacteria got back to normal, there were still lasting changes in body composition.”

When the scientists fed the mice a high-fat diet, those that had been treated with antibiotics in early life got “very, very fat,” Blaser reported. These mice packed on a third of their body weight in fat. They also had higher-than-normal resting insulin levels and changes in genes that control liver metabolism. These are classic signs of metabolic disease in obese humans.

The researchers published their findings today in the journal Cell.

Chicken or the Egg?
The final question for researchers was whether the metabolic changes they saw were caused by the antibiotics or by changes in the mice’s gut bacteria.

To find out, the scientists took bacteria from the guts of mice treated with antibiotics and transferred them into the guts of mice specially bred to have no native gut bacteria. These “germ-free” mice also became fat in adulthood, indicating that altered gut bacteria — not direct exposure to antibiotics — are at the root of the problem.

The researchers had one more surprise in store. Conventional wisdom says that antibiotics reduce the total number of microbes in the gut, allowing the microbes that survive to thrive with less competition.

The team found that penicillin did not reduce the total number of gut bacteria. Instead, the drug suppressed four specific types of microbes: Lactobacillus, Candidatus Arthromitus, Allobaculum, and an as-yet-unnamed member of the Rikenellaceae family. These microbes may well play a part in the development of a person’s metabolism.

Should You Give Your Child Antibiotics?
Does this new research mean parents shouldn’t give their babies antibiotics? Not so fast, Cox says.

“The decision should really be based on what the doctor is recommending and also the severity of the illness,” she said. “Antibiotics can have an effect on the microbiome and that might have health consequences, but a life-threatening infection could have health consequences as well.”

Antibiotics are the cornerstones of modern medicine. Without them, it would be nearly impossible to cure common infections or perform surgery safely. But the overuse of antibiotics in hospitals and on farms in recent decades has led to a rise in antibiotic-resistant bacteria that are very difficult to treat.

“Antibiotics can still be very useful and shouldn’t be completely avoided, but this new risk we’ve identified should be taken into account,” Cox said. “There probably is over-prescription of antibiotics in the United States, so if your doctor is saying, ‘It’s a viral illness and I don’t recommend an antibiotic,’ you should really go with that.”

Sometimes, prevention is the best medicine.

“Good hygiene can help you avoid antibiotics. We know certain practices reduce bacterial infection and illness, and a lot of that is proper sanitation, proper food handling,” Cox said. “I think sometimes people look at it too broadly — they say either all dirt and bacteria is good or all cleaning is good. Really, we want the right bacteria.”

Are Probiotics the Solution?
Could researchers someday create a probiotic with all the “right” bacteria to re-seed a gut that antibiotics have disrupted? Cox thinks so.

“There’s a couple of probiotics available now, but when you consider the huge diversity in the gut, we really have very few options,” she said. “We want to try these new organisms that we think could be beneficial and we want to see if we can maybe speed up recovery [from taking antibiotics].”

The ultimate goal is to restore the natural microbial community in the gut and improve a person’s metabolic health. But many of the organisms the researchers found have yet to be named, let alone studied, so it will be years before they know which are important for metabolic health and which may be dangerous.

“The big question that people need to look at is: ‘Is it safe to get these organisms back?’ Can you reverse the antibiotic effects and can you give them to kids without causing problems?” Cox said.