It may be difficult for you and your doctor to tell if an inability to digest lactose is causing your symptoms because other conditions create similar gastrointestinal distress. Keeping a food diary is one way to start narrowing down potential causes. “It’s helpful to keep a food diary for one to five days, recording what you eat, including the amounts of all food and beverages, and how you feel hours later,” Ferrer says. “This information will be useful when you visit a registered dietitian or gastroenterologist for a full evaluation. During your visit (in office or via telehealth), your health care provider will ask for at least one day of a diet recall or a food frequency questionnaire.” Therefore, doing this before your visit will save time and offer insight into which foods or ingredients cause your digestive problems. In addition, your physician may order a lactose tolerance blood test or a hydrogen breath test.