If you get gas or stomach pains when you drink a glass of milk or eat an ice cream cone, it might be because your body doesn't make enough of the enzyme it needs to digest lactose, the sugar found in milk. That could mean you're lactose intolerant. But it's also possible that your digestive tract can't handle the fat in the milk, says Sheila Crowe, MD, vice president of the American Gastroenterological Association.
She says a simple test can tell the difference: "Drink a glass of skim milk, which has lactose but no fat. If that causes problems, you're lactose intolerant. Then eat a piece of brie cheese, which is high in fat but low in lactose. Can't stomach that? Then you have an intolerance to the fat."