Lactose is a type of sugar naturally found in milk and dairy products. In the intestine, lactose is transformed by lactase, an enzyme, into glucose and galactose, both simpler sugars used by our body for energy and various functions. The activity of lactase is high during infancy and slowly declines after weaning. In some individuals, a residual lactase activity is maintained. Undigested lactose subsequently enters the colon, where it is fermented by the resident microbiota (the microorganism population that lives in the digestive tract). Bacterial fermentation is responsible for the formation of gas, lactic and acetic acids, which increase gut transit time and intracolonic pressure, resulting possibly in bloating, diarrhea, and flatulence. Symptoms generally do not occur until there is less than 50% of lactase activity, compared to the level of lactase activity before weaning.
However, the symptoms associated with lactose intolerance are not specific to this condition and can be observed in other frequent gastro-intestinal dysfunctions such as irritable bowel syndrome, inflammatory bowel diseases (Crohn’s disease and ulcerative colitis) and intolerance to FODMAP (Fermentable, Oligo-, Di-, Mono-saccharides And Polyols, which are short-chain carbohydrates poorly absorbed in the small intestine). Psychological factors such as somatic anxiety, stress and depression can also cause the occurrence of these symptoms.
Moreover, a temporary lactose malabsorption, appearing in cases of infectious diarrhea, radiotherapy, mucosal damage due to coeliac disease or some medicine use, can also give rise to similar symptoms.
Thus, self-diagnosis of lactose intolerance based solely on intestinal discomfort and made without proper medical diagnosis, is not possible. Self-diagnosed lactose intolerance can lead to unnecessary avoidance of dairy food.
For more information about lactose intolerance, please go to question 20.