Lactose is the sugar found in milk. Normally, an enzyme called lactase—which is made in your small intestine—breaks it down so your body can absorb it. Nearly all babies produce plenty of lactase, but most people make less of the enzyme as they get older. Only some of them go on to develop symptoms.
“Because lactose is not broken down by lactase, the sugar that was supposed to be absorbed into your bloodstream instead sits in your small bowel and makes its way through, pulling in water in the gut, which leads to bloating and diarrhea,” explains Hamita Sachar, MD, a Yale Medicine gastroenterologist.
Many people develop lactose intolerance in their 20s and 30s, Dr. Sachar says. “Young people show up in our office and say, ‘I used to drink so much milk and now I can hardly have any without running to the bathroom,’” she says. It can start later in life too, though that’s less common.
Genetics play a role as well. Lactose intolerance is most common in people of East Asian, Native American, and West African descent, and least common in people of Northern European descent—a pattern that traces back to which populations historically raised dairy animals.
Lactose intolerance isn’t the same as a milk allergy, sometimes called a dairy allergy. A milk allergy is an immune reaction to proteins in milk, not the sugar, and it usually shows up in infancy with symptoms like hives, vomiting, trouble breathing, eczema, abdominal pain, or diarrhea. Developing a new milk allergy as an adult is uncommon. Lactose intolerance doesn’t involve the immune system at all—one reason it’s uncomfortable rather than dangerous.
Fortunately, Dr. Sachar says, it’s one of the more manageable conditions she sees. “It’s not harmful,” she adds. “Most people don’t have to fully eliminate dairy from their diets to manage this condition.”
