Lactose is a sugar found in dairy products. It is a disaccharide sugar, meaning that is composed of two simple sugars: glucose and galactose.
Our bodies normally produce an enzyme called lactase that breaks down lactose into these simple sugars to be able to transport them across the cell membranes into the bloodstream. If the lactase enzymes are missing or insufficient to help with the absorption process, then lactose remains in the intestine.
Lactose in the intestine starts to attract fluids into the bowel through a process called osmosis. (1) The sudden increase of fluids inside the intestine results in diarrhea.
Unabsorbed lactose present in the colon is also fermented by the bacteria to produce gas (especially hydrogen), fatty acids and monosaccharides, which cannot be absorbed by the colon.
Since monosaccharides are not absorbed, they attract more fluids to the intestine, again through the process of osmosis.
The following are the common symptoms that usually occur within a few hours after consuming lactose-containing products in the case of lactose intolerance: flatulence, diarrhea, bloating, stomach cramps and nausea. (2)
Lactose intolerance of different levels of severity occurs in about 70% of the world population with the highest prevalence in African, African-American, and Asian populations. Lactase persistence (30% of the world population) which is the ability to break down lactose is mostly common in people with European ancestry but also common to some areas in Western Africa, Middle East and South East Asia.