Another type of lactase deficiency comes on gradually over time. It also seems to run in families. It affects certain ethnic groups more than others. The enzyme lactase is present in the human intestine from about the middle of pregnancy onward. The amount of lactase in the intestine begins to drop after weaning.
Some people lose their ability to digest lactose.
This varies by ethnic group. Any disorder that affects the intestines can lead to a temporary deficiency of lactase. When lactose is not digested, water is retained in the bowel. This results in bloating and watery diarrhea. Lactose that passes into the large intestine is fermented by bacteria.