Final answer:
Lactose intolerance is caused by a lactase enzyme deficiency, leading to difficulty in digesting lactose in milk. This results in gastrointestinal symptoms and requires individuals to find alternative sources of nutrition or enzyme supplements.
Step-by-step explanation:
The reason that some people are lactose intolerant is that they have a deficiency in the enzyme lactase, which is necessary to break down lactose, the sugar found in milk. This condition occurs when the absorptive cells of the small intestine do not produce enough lactase. Consequently, undigested lactose moves into the colon where it is fermented by bacteria, causing symptoms like bloating, gas, and abdominal cramps.
Furthermore, lactose intolerance can be due to a genetic defect where lactase production is deficient or absent, leading to an inability to digest lactose properly. In such cases, individuals may need to substitute regular milk with lactose-free alternatives or take lactase supplements to aid digestion.