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It’s been known since the 1950s that feeding low doses of antibiotics to livestock increases their weight gain. The practice, dubbed subtherapeutic antibiotic therapy (STAT), lowers feed costs while increasing meat production. Nearly 80 percent of the antibiotics purchased in the United States are used for this purpose. The practice is suspected of facilitating evolution of antibiotic-resistant “superbugs” like MSRA, which infects both pigs and people and is known to be especially common at pig factory farms. Meanwhile, it’s finally coming to light that antibiotics can do to people what they do to livestock: make us fat.

Data has supported this hypothesis since a 1955 study in which antibiotics and placebos were given to three groups of Navy recruits. The placebo group showed the least weight gain, significantly lagging behind the two groups given different antibiotics. In general, the rates of obesity have risen with antibiotic use since their discovery, but as they say in science, correlation does not equal causation. Recent research, however, is turning up evidence that this correlation might not be a coincidence. The basic idea is that bacteria alter the body’s microbial communities, also known as the microbiome, and this disruption changes the way our bodies metabolize and store food.

A study published August 27 in Nature looked at how antibiotics impact the microbial balance in mice, and how this might affect weight-gain patterns. One group of mice was exposed to STAT, the other not. Bacteria from the guts of mice from these two groups was compared, and the researchers found antibiotics altered the microbial ecology in the guts of the exposed mice, changing their metabolism.