Ever wonder if fat cells just get bigger or if you actually gain more fat cells? To find out, researchers at the Mayo Clinic fed 23 lean young men and women 400 to 1,200 extra calories a day by padding their diets with Snickers bars, milkshakes, and Boost Plus drinks. After two months, the volunteers had gained about eight pounds. But that surplus got stashed in different depots. “Almost all the weight gain in abdominal body fat was an increase in fat cell size,” says Michael Jensen, director of the Obesity Specialty Council at the Mayo Clinic. In contrast, “when people gained leg fat, they actually gained new fat cells.” Jensen’s earlier study estimated that when people gained 3½ pounds of new leg fat, they acquired roughly 2.6 billion new fat cells.