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Why Does Lactose Intolerance Cause Diarrhea?

author image Jacques Courseault
As a physical medicine and rehabilitation physician I have extensive experience in musculoskeletal/neurological medicine that will benefit the network.
Why Does Lactose Intolerance Cause Diarrhea?
Young woman drinking a glass of milk. Photo Credit: Christopher Robbins/Photodisc/Getty Images

Abdominal cramping, nausea and diarrhea are unsettling symptoms related to lactose intolerance. Lactose intolerance is the inability to digest lactose, a form of sugar, found in milk products. Excessive diarrhea secondary to lactose intolerance can cause unsafe weight loss, dehydration and electrolyte abnormalities. Poor digestion of lactose occurs on the biochemical level in the intestines.

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Lactase Deficiency

For lactose to be digested, the small intestine must produce an adequate amount of lactase. Lactase is an enzyme that is involved in the biochemical reaction responsible for the breakdown of lactose into glucose molecules that can be absorbed and used for energy. When there is not enough lactase to break down lactose, lactose builds in the digestive tract to cause symptoms associated with lactose intolerance. If the small intestine produces small amounts of lactase, you may be able to tolerate small amounts of lactose.

Osmotic Diarrhea

Diarrhea can occur from many different causes. However, the cause of diarrhea in lactose intolerance is related to osmosis, or the movement of water between cells and tissues. Excessive lactose in the small intestine attracts water molecules, which prevents them from being properly absorbed into the bloodstream. Therefore, a reduction in lactose intake will result in better water absorption and resolving of diarrhea.

Excessive Gas

Not only can lactose intolerance cause diarrhea, it may also cause excessive gas production. According to the Colorado State University, bacteria in the large intestine ferment excess lactose, which results in excessive gas formation. Similar to diarrhea symptoms, excessive gas should resolve once there is no additional intake of lactose. No anti-gas treatments are necessary.


To prevent diarrhea, you must pay close attention to your lactose intake. MedlinePlus, a service of the National Institutes of Health, recommends drinking 2 to 4 oounces of a milk product to see how your body responds. Closely monitor how much lactose your body can effectively process without causing diarrhea. Further, you may try other milk products that may not cause diarrhea such as soy milk, goat's milk, fermented milk products, buttermilk, cheeses, lactose-free milk and milkshakes. If your lactose intolerance is severe, you may see your doctor for further testing to rule out other serious gastrointestinal problems.

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