What is lactose intolerance?

Lactose intolerance (or hypolactasia) is the term for the normal decline in the level of lactase, an enzyme needed for proper metabolization of lactose (a sugar that is a constituent of milk and other dairy products), in human beings after weaning.

Lactose intolerance is normal in all adult mammals, including human beings. However, a few ethnic groups—particularly certain European groups—continue to produce lactase throughout their lives, and in these populations lactose intolerance is usually treated as a medical disorder.

Lactose intolerance is physiologically normal for most adults.

With lactose intolerance, the result of consuming too much lactose is excess gas production, stomach aches and often diarrhea. Most adults can drink about 250 mL (8 oz) of milk per day without severe symptoms.

In human beings, significant production of lactase ends sometime between the ages of two and five. However, a relatively recent genetic change has caused some populations, including many northern Europeans, to continue producing lactase into adulthood.

Biology

The normal mammalian condition is for the young of a species to lose the ability to digest milk sugar (lactose) effectively after the end of the weaning period (a species-specific length of time often equal to roughly 3% of lifespan). In humans, lactase production usually drops about 90% during the first four years of life, although the exact drop over time varies widely. However, certain human populations have undergone a mutation on chromosome 2 which results in a bypass of the common shutdown in lactase production, making it possible for members of these populations to continue consumption of fresh milk and other milk products throughout their lives.

There is some debate on exactly where and when the mutation(s) occurred. Some argue for separate mutation events in Sweden (which has one of the lowest levels of lactose intolerance in the world) and the Arabian Peninsula around 4000 BCE. However, others argue for a single mutation event in the Middle East at about 4500 BCE which then subsequently radiated. Some sources suggest a third and more recent mutation in the East African Tutsi.

Whatever the precise origin in time and place, most modern Western Europeans and people of Western European ancestry show the effects of this mutation (that is, they are able to safely consume milk products all their lives) while most modern East Asians, sub-Saharan Africans and native peoples of the Americas and Pacific Islands do not (making them lactose intolerant as adults).

Coeliac Disease

Pathological lactose intolerance can also occur due to Coeliac disease, as coeliac disease damages the villi in the small intestine that produce lactase. This lactose intolerance is temporary. Lactose intolerance associated with coeliac disease ceases after the patient has been on a gluten-free diet long enough for the villi to recover.

Late onset of lactose intolerance in an adult who has otherwise remained lactose-tolerant, also known as secondary or acquired lactose intolerance, sometimes occurs following Gastrointestinal diseases, including exposure to intestinal parasites such as giardia. In such cases the production of lactase may be permanently disrupted.

Etiology

Without lactase, the lactose in milk remains unabsorbed. Lactose cannot pass easily through the intestinal wall into the bloodstream, so it remains in the intestines. Soon, enteral bacteria adapt to the relative abundance of lactose (relative to other sugars like glucose) and switch over to metabolizing lactose. Along the way they produce copious amounts of gas by fermentation.

The gas causes a range of unpleasant abdominal symptoms, including stomach cramps, bloating, flatulence and diarrhea. Like other unabsorbed sugars, e.g. mannitol, the lactose raises the osmotic pressure of the colon contents, preventing the colon from reabsorbing water and hence causing a laxative effect to add to the excessive gas production.

Additional information

About 44% of lactose intolerant women regain the ability to digest lactose during pregnancy. This might be caused by slow intestinal transit and intestinal flora changes during pregnancy.
Many global cat breeds (Asian breeds in particular) share the mammalian lactose sensitivity, unlike many European breeds that have a mutation similar to the European human mutation.
Although many Maasai are found by standard tests to have substantial amounts of lactose intolerance, they are able to consume large amounts of dairy products including milk without exhibiting any symptoms.
Infection with the common intestinal parasite Giardia lamblia, can cause lactose intolerance. This may persist after infection has ended, more commonly in ethnic groups inclined toward lactase deficiency.