From Scientific American:
Bacteria living naturally within the gut provide a gateway to flab, according to a few reports this week. These bacteria may explain how antibiotics fatten farm animals and perhaps people too, and how certain genes predispose organisms to obesity.
In a study published 22 August in Nature, researchers mimicked what farmers have been doing for decades to fatten up their livestock: they fed young mice a steady low dose of antibiotics. The antibiotics altered the composition of bacteria in the guts of the mice and also changed how the bacteria broke down nutrients. The bacteria in treated mice activated more genes that turn carbohydrates into short-chain fatty acids, and they turned on genes related to lipid conversion in the liver. Presumably, these shifts in molecular pathway enable fat build-up. Just as farm animals get fat, the antibiotic-fed mice put on weight. Martin Blaser, a microbiologist at New York University in New York, says that parents might unknowingly be promoting a similar phenomenon when they treat common ailments and ear infections in their children. To back that idea up, he points to another study he authored. The study, published on 21 August, found that a disproportionate number of 11,000 kids in the United Kingdom who were overweight by the time they were 3 years old had taken antibiotics within their first 6 months of life.