Primary lactose intolerance mostly affects your digestive system. It begins in the small intestine, where cells don't produce enough of the enzyme that helps break down lactose, the sugar in dairy products. This means that the body can't absorb that sugar, so it moves on to the large intestine, where it's fermented by bacteria. This fermentation causes symptoms like gas and bloating. It might also lead to abdominal cramps and diarrhea. Most of the time, these symptoms are mild. For some people, diarrhea is severe enough to prevent the intestines from properly absorbing nutrients in food, which can lead to weight loss.