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Like hospital patients, US farm animals tend to be confined to tight spaces and dosed with antibiotics. But that’s where the similarities end. Hospitals dole out antibiotics to save lives. On America’s factory-scale meat farms, the goal is to fatten animals for their date at the slaughterhouse.

And it turns out that antibiotics help with the fattening process. Back in the 1940s, scientists discovered that regular low doses of antibiotics increased “feed efficiency”-that is, they caused animals to put on more weight per pound of feed. No one understood why, but farmers seized on this unexpected benefit. By the 1980s, feed laced with small amounts of the drugs became de rigueur as US meat production shifted increasingly to factory farms. In 2009, an estimated 80 percent of the antibiotics sold in the United States went to livestock.

This year, scientists may have finally figured out why small doses of antibiotics “promote growth,” as the industry puts it: They make subtle changes to what’s known as the “gut microbiome,” the teeming universe populated by billions of microbes that live within the digestive tracts of animals. In recent research, the microbiome has been emerging as a key regulator of health, from immune-related disorders like allergies and asthma to the ability to fight off pathogens.

In an August study published in
Nature, a team of New York University researchers subjected mice to regular low doses of antibiotics-just like cows, pigs, and chickens get on factory farms. The result: After seven weeks, the drugged mice had a different composition of microbiota in their guts than the control group-and they had gained 10 to 15 percent more fat mass.

Why? “Microbes in our gut are able to digest certain carbohydrates that we’re not able to,” says NYU researcher and study coauthor Ilseung Cho. Antibiotics seem to increase those bugs’ ability to break down carbs-and ultimately convert them to body fat. As a result, the antibiotic-fed mice “actually extracted more energy from the same diet” as the control mice, he says. That’s great if you’re trying to fatten a giant barn full of hogs. But what about that two-legged species that’s often exposed to antibiotics?