The operation shrinks the stomach and reroutes the digestive tract. Postsurgery, patients eat less and absorb fewer nutrients, so they lose weight. Some people also show metabolic improvements in just a few days. In the wake of his surgery, Treff’s blood sugar plummeted, and for a long time afterward, he didn’t need to take insulin. Today, he takes just a fraction of the dose he once required. “These operations are better for treating diabetes than they are for treating obesity,” says Carel Le Roux, a physician at the University College Dublin. New research may help explain why: The body increases production of bile acids, which bind to a receptor called FXR, prompting the release of hormones that help regulate blood sugar.