They found that healthy mice have plenty of bacteria from a genus called Clostridia, but few from Desulfovibrio, and that their guts let most fat pass right through. Those with an altered immune system had fewer Clostridia and more Desulfovibrio, and this microbial balance helped the gut absorb more fats from food. These mice gained more weight and exhibited signs of type 2 diabetes. “Whether this applies in humans, we don't know,” Hooper says, “but this is a tantalizing clue.” The role of the immune system in the gut is to maintain balance. Changes to the body’s defenses, which can happen as a result of...