Maybe it has happened to you: over time, you notice that after you drink a glass of milk or eat a bowl of ice cream, you’re running to the bathroom the rest of the day. Could you suddenly be lactose intolerant as an adult? It’s possible. Most people experience a gradual decline of lactase (the enzyme that helps your body digest milk products) activity as they age. If dairy isn’t sitting with you as well as it used to, try this self-test: Stop consuming dairy products for five days, and on the sixth day, drink a glass of milk. If your symptoms resolve during the dairy-free days and return when dairy is reintroduced, you most likely have developed lactose intolerance.