The causes of lactose intolerance

Lactose intolerance or rather the lack of sufficient lactase (which is the enzyme that is necessary for the digestion of lactose) can have a number of different origins :

Congenital lactase deficiency
There are some rare cases of congenital lactose intolerance which is an inherited metabolic disorder rather than an allergy. It is due to the body’s inability to synthesize lactase.

Primary lactase deficiency
In the uterus, the lactase activity in the intestine of the embryo incrases during the gestation period. It reaches its maximum during the final third of the pregnancy and at birth. This production of lactase allows the baby to hydrolyze the lactose contained in the mother’s milk (70g/l in human milk, 45g/l in cow’s milk) or the baby formula. When breast feeding is stopped the production of lactase diminishes progressively in many populations. It stabilizes at 10-20% of its level at birth. This situation is known as primary lactose intolerance. The age at which it is established is different in different the ethnic groups. For example in Africa and Asia, primary lactose intolerance appears as early as 2 years of age, while in Japan it appears at the age of 6-8 years and in Finland after 10 to 15 years. The reduction in lactose production is genetically programmed and physiological in many populations. The primary deficit in lactase is the most common cause of lactose intolerance. However the distribution of lactose intolerance around the globe shows that not everybody is born identical as far as this disaccharide is concerned (see Geographical distribution).

Secondary or temporary lactose deficiency
Lactose intolerance can appear on a temporary basis in people who have suffered a severe gastroenteritis, specifically if the gastro is caused by the rotovirus which attacks the mucous membrane of the upper part of the intestine, where the lactase is produced. Diseases that affect the mucous membrane of the intestine (for example celiac disease) or the motility of the small intestine (intestinal pseudo-obstruction or contaminated small intestine syndrome) can also have negative effects. If the mucous membrane is destroyed, the production of lactase stops. One then has to wait for it to heal (normally a few weeks) before the symptoms of the intolerance disappear. Chemotherapy and radiotherapy can also induce a temporary lactose intolerance through cellular damage that they produce.