Lactose Intolerance

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Lactose IntoleranceWhat Is Lactose Intolerance?

Lactose intolerance is a well known symptom of carbohydrate malabsorption characterized by inability to properly digest lactose, the sugar in milk, due to low lactase digesting activity in the small intestine.

Lactase is an enzyme produced by specialized cells in the tips of villi within the brush border of the small intestinal lining that digests lactose.

Lactose is a disaccharide, or double sugar, made up of a molecule of glucose and a molecule of galactose (simple sugars). Lactose cannot be absorbed as is into the body unless it is broken down by lactase into glucose and galactose.

Q: How does lactose cause diarrhea?

A: If lactose is not broken down, undigested lactose moves into the colon where it acts osmotically to draw water from the walls of the colon into the lumen, which causes diarrhea by the unnatural amount of water. How much water? Can be 2 liters! Meanwhile, the normal population of colonic bacteria ferment the undigested lactose, generating short-chain fatty acids and hydrogen gas, resulting in bloating pain and flatulence.

Testing for intolerance is based on the action of lactose to increase intestinal permeability.1 Positive response to a breath hydrogen test (BHT), involving 1 – 3 hours of time post ingestion of lactose test dose, signifies malabsorption in the small intestine and fermentation in the colon. If BHT is positive before 60 minutes, the result implies bacteria is abnormally present in the small intestine, causing fermentation there.

Endoscopy is used to measure activity of lactase in a tissue sample.

Treatment is avoidance of milk and dairy products that contain lactose. Nevertheless, milk treated with lactase enzyme can be safely consumed. Fermented milk products like yogurt or kefir are safe because the lactose has been digested by organisms. If there is also a problem with milk protein, then no milk is acceptable that comes from the cow or animal that is causing the allergy.

What Is Lactose Intolerance In Celiac Disease and/or Gluten Sensitivity?


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  1. Farhadi A, Banan A, Fields J, Keshavarzian A. Intestinal barrier: an interface between health and disease. Journal of Gastroenterology and Hepatology. 2003;18:479-91. []