Expanding waistlines may be caused by more than bad diets and sedentary habits. Antibiotics could be disrupting our gut bacteria, helping people pack on fat like farm animals.

This scenario is, for now, a hypothesis, but one that's fleshed out in two new studies. In the first, mice given antibiotics experienced profound changes to internal microbe communities that process food and regulate metabolism. In the other study, body weight in children rose with antibiotic exposures as infants.

"Early life antibiotics are changing the microbiome, and its metabolic capabilities, at a critical time in development," said microbiologist Martin Blaser of New York University. "These changes have downstream effects on metabolism, including genes related to energy storage."

Blaser was among the first researchers to investigate what's become one of the hottest areas in biology: the microbiome, or the vast community of bacteria, viruses and even fungi that live inside our bodies, breaking down food and regulating physiological processes.

Indeed, with 10 microbial cells for every human cell in our bodies, people might from a certain perspective be seen as mobile scaffolds for microbial ecosystems – and these ecosystems are changing, not least because they're under indiscriminate assault by antibiotics.

The changes have been linked to a host of diseases, from cancer, autism and heart disease to obesity. Blaser calls the latter association "microbe-induced obesity," and sees an obvious example of it in farm animals, which receive some 80 percent of all antibiotics used in the United States.

Farmers don't just use the drugs to fight infections, but to enhance growth. For some reason, animals given steady low doses of antibiotics grow larger and faster than usual. Blaser wondered why this was, whether it was related to the animals' microbiomes, and whether something similar could happen in humans.

In an Aug. 22 Nature paper, researchers led by Blaser and fellow NYU microbiologist Ilseung Cho added steady, low doses of antibiotics to the diets of lab mice, mimicking the exposures received by farm animals.

Their mice didn't gain weight, but their body fat swelled by 15 percent, an effect that held for different types of antibiotics. When the researchers looked at their microbiomes, they found markedly different types of bacteria. Changes were also found in the genetic profiles of their mice, with unusual activity in genes linked to breaking down carbohydrates and regulating cholesterol levels.

Representative changes in body fat (yellow) accumulation between mice unexposed to antibiotics (top) and exposed to continual low doses (bottom). Image: Cho et al., Nature

According to Blaser, this could be a version of what happens inside people. That's difficult to study in gut-level detail, but an Aug. 21 International Journal of Obesity paper by Blaser and NYU pediatrician Leonard Trasande looked for population-level trends in 11,000 British children.

Among children exposed to antibiotics before they were six months old, the researchers found small but consistent average increases in body mass years later. Antibiotics appeared to have altered their metabolic trajectories. Blaser called the data preliminary, but said it fits the patterns seen in both farm animals and his mice.

Andrew Gewirtz, a Georgia State University immunologist who studies the connection between microbiome and metabolism, called it "reasonable to speculate" about a link to obesity. The new Nature paper strengthens that notion, he said.

One important caveat is that Blaser's mice, like farm animals, received steady low doses of antibiotics. Children receive them in bursts of treatment. "A better model than this constant low dose might be giving the mice a course of antibiotics early in their lives, then seeing how that affected their development," Gewirtz said.

Blaser said he's now studying exactly that question. He's also looking at whether extremely low antibiotic doses, such as people might receive by eating meat and dairy products from antibiotic-fed animals, have metabolic effects. Another question is how microbiome alterations are passed between generations. The effects might be cumulative.

"Our microbiome is part of human physiology. We are doing things to change it, and those changes have consequences," said Blaser.

Citations: "Antibiotics in early life alter the murine colonic microbiome and adiposity." By Ilseung Cho, Shingo Yamanishi, Laura Cox, Barbara A. Methe , Jiri Zavadil, Kelvin Li, Zhan Gao, Douglas Mahana, Kartik Raju, Isabel Teitler, Huilin Li, Alexander V. Alekseyenko & Martin J. Blaser. Nature, Vol. 488 No. 7412, 23 August 2012.

"Infant antibiotic exposures and early-life body mass." By L. Trasande, J. Blustein, M. Liu, E. Corwin, L.M. Cox and M.J. Blaser. International Journal of Obesity, 21 August 2012.