In the case of lactose intolerance, the ingested lactose cannot break down completely and remains in the lumen of the small intestine. It binds a lot of water and the intestinal contents travel forward into the colon. A part of undigested lactose is excreted, but the other part is decomposed by the bacteria of our normal intestinal flora. During this process, gases (CO2, hydrogen and methane) and short chain fatty acids are produced. The latter are partly used in the intestinal cells. 5-10% of the ingested lactose is excreted with the faeces. Since lactose binds much water, the faeces can be liquid, and some of the produced fatty acids irritate the intestinal mucosa, which results in diarrhoea and possibly leads to unpleasant burning skin inflammation at the rectum.
The activity of lactase in humans declines within a few years after the cessation of breast-feeding. In environments where lactose intolerance is common, the first symptoms emerge already at the age 2-7, while elsewhere the decrease in lactase activity may be delayed until the age 20.
Despite the disrupted decomposition of lactose, some people do not exhibit any symptoms. It appears that this is in part connected to the bacteria species in the large intestine. Different bacteria produce various acids, which may irritate the intestinal mucosa to varying degrees. Also, different bacteria produce a variety of different gases.
The main symptoms of lactose intolerance are, bloating, gases, flatulence, gurgling, increased urinary frequency, liquid stool, and pains, which depend on the sensitivity of an individual’s gut.
The amount of ingested lactose affects the severity of the symptoms.
Avoiding lactose is of course necessary only when unpleasant symptoms ensue because of it. The fact is that the ingestion of lactose does not cause any long-term health damage or dangerous acute consequences even in those people who are unable to digest it.