What is the primary reason for lactose intolerance?

The absence of enzyme lactase.

Explanation:

Lactose intolerance may be defined as the condition in which the body is unable to digest the milk and milk related products. The symptoms of lactose intolerance are abdominal pain, diarrhea, gas problem and nausea.

The individual's ability to digest the lactose has been decreased in lactose intolerance. The primary and foremost reason of lactose intolerance is the deficiency of the enzyme lactase in the small intestine.