The Truth About Lactose Intolerance and Constipation

Feeling constipated but not sure if it's due to lactose intolerance or something else? It's easy to assume that lactose intolerance could cause the same uncomfortable gastrointestinal tract ailments as other digestive conditions. But constipation is not a typical symptom of lactose intolerance.

"Lactose intolerance leads to diarrhea, bloating and pain — not constipation," says Matthew Ciorba, MD, a gastroenterologist and associate professor at Washington University School of Medicine in St. Louis.

Nausea, gas and sometimes vomiting can occur as well, according to the Mayo Clinic. Symptoms tend to show up about 30 minutes to two hours after eating or drinking lactose-containing foods — milk and other dairy foods, plus packaged items that include milk products, such as whey, in their ingredients.

Read more: Are There Remedies for a Stomachache With Lactose Intolerance?

Lactose intolerance occurs when a person's small intestine does not produce enough lactase, which is the enzyme that digests lactose, the sugar in cow's milk, says Mayo Clinic. With lactose intolerance, the sugar moves along the gut without being digested, and then bacteria in the gut ferment that sugar, causing gas.

Plus, the sugar draws liquid into the gut, causing loose stools, explains Scott Sicherer, MD, a professor of pediatrics, allergy and immunology and director of the Jaffe Food Allergy Institute at the Icahn School of Medicine at Mount Sinai in New York City.

About 65 percent of the world's population has some form of lactose intolerance, to varying degrees, according to the U.S. National Library of Medicine. It can get worse as you go from childhood into adulthood for those with primary lactose intolerance, according to the Mayo Clinic. There's also secondary lactose intolerance, the type that develops as a result of a primary disease, like Crohn's or celiac, or after intestinal surgery.

Your doctor may be able to tell if you have lactose intolerance based on your symptoms, but a lactose intolerance test is available to confirm a diagnosis. This involves drinking a lactose-based liquid and then having bloodwork done, according to Mayo Clinic. The bloodwork will gauge the amount of glucose in your blood. If your glucose levels don't rise, your body did not properly digest and absorb the liquid.

Another test, the hydrogen breath test, involves drinking a lactose-containing liquid and then measuring the amount of hydrogen in your breath, says Mayo Clinic. If you exhale larger amounts of hydrogen than normal, it can show that you aren't absorbing lactose.

In children, a stool acidity test can also diagnose lactose intolerance. A child who is not digesting lactose will have lactic acid, glucose and other fatty acids in the stool, says Stanford Children's Hospital.

Once you know if you definitely have lactose intolerance, you can do things to prevent the symptoms. Many people avoid large servings of lactose-containing products, or use lactose-reduced dairy products. There also are products that can be added to regular milk to help break down the lactose and ease symptoms.

Constipation often occurs when stool moves too slowly in the digestive tract, causing it to become hard or dry and difficult to eliminate. Constipation can be caused by dehydration, certain medications, physical inactivity and many diseases or conditions, Mayo Clinic reports.

Signs of constipation include having fewer than three stools a week and feeling as if you can't fully or comfortably pass stool. It's important to talk to your doctor if you experience constipation on a regular basis or if it gets worse.

Because it is not a known symptom of lactose intolerance, having constipation might indicate that you have another health issue. Medical conditions that can cause constipation include blockages and other problems in the colon or rectum as well as diseases that affect nerves in those areas.

According to the Mayo Clinic, having a milk allergy or eating too much dairy can sometimes lead to constipation in children. Like constipation, bloody stool is another symptom mistakenly linked to lactose intolerance.

Experiencing bloody stool is not a symptom of lactose intolerance, but can be a sign of a milk allergy, according to Mayo Clinic. Check in with a doctor if you or your child is experiencing this.

Read more: How to Avoid Dairy in Your Diet and What to Eat Instead