Why does lactose intolerance cause diarrhoea?

Usually, lactase in the small intestine breaks down lactose so it can be absorbed. When lactose remains undigested, it holds water in the bowel. Once it reaches the large bowel, bacteria ferment it and create gas. Extra water and bowel activity can make stools looser and can also cause urgency, cramps, wind, or bloating.

Diarrhoea is a very common presentation of Lactose Intolerance. However, diarrhoea has many causes, including infection, medicines, and other bowel conditions. A repeated relationship with lactose-containing food can be informative, but it is sensible to have ongoing or unexplained symptoms assessed.

Drink regularly to replace fluid lost in loose stools. If dairy seems connected, keep a note of foods, drinks, and symptoms, then discuss a structured lactose reduction with a clinician or dietitian. This can help clarify the trigger without unnecessarily cutting out major food groups. Wash hands carefully if an infection is also possible, because diarrhoea is not always food intolerance.

Get medical advice if diarrhoea continues, returns repeatedly, wakes you from sleep, or comes with abdominal pain that is not improving. Seek urgent care for blood in stool, black stool, severe dehydration, confusion, fainting, severe pain, fever with worsening illness, or inability to keep fluids down. Children, older adults, and people with significant health conditions may need earlier advice.

Ask whether the food pattern is consistent with lactose intolerance, how long to monitor symptoms, and whether any tests are appropriate. Mention recent antibiotics, travel, sick contacts, weight change, and whether there is blood or mucus in the stool, as these details can change the assessment.