lactose intolerance

Definition: Lactose intolerance is the inability to metabolize lactose, a sugar found in milk and other dairy products, because the required enzyme lactase is absent in the intestinal system or its availability is lowered.

It is estimated that 75% of adults worldwide show some decrease in lactase activity during adulthood. The frequency of decreased lactase activity ranges from as little as 5% in northern Europe, up to 71% for Southern Europe, to more than 90% in some African and Asian countries.

Lactose digestion

Infant mammals nurse on their mothers to drink milk, which is rich in the carbohydrate Lactose. The intestinal villi secrete an enzyme called lactase (β-D-galactosidase) to digest it. This enzyme cleaves the lactose molecule into its two subunits, the simple sugars glucose and galactose, which can be absorbed.

Since lactose occurs mostly in milk, in most mammals the production of lactase gradually decreases with maturity due to a lack of constant consumption.

Many people with ancestry in Europe, the Middle East, India, and parts of East Africa maintain lactase production into adulthood. In many of these areas, milk from mammals such as cattle, goats, and sheep is used as a large source of food. Hence, it was in these regions that genes for lifelong lactase production first evolved. The genes of lactose tolerance have evolved independently in various ethnic groups.

People who are not lactose tolerant may suffer uncomfortable or socially unacceptable symptoms of too much lactose consumption. In these people, lactose is not broken down and provides food for gas-producing gut flora. This can lead to bloating, flatulence, and other gastrointestinal symptoms.

Disaccharides cannot be absorbed through the wall of the small intestine into the bloodstream, so in the absence of lactase, lactose present in ingested dairy products remains uncleaved and passes intact into the colon.

The operons of enteric bacteria quickly switch over to lactose metabolism, and the resultant in vivo fermentation produces copious amounts of gas (a mixture of hydrogen, carbon dioxide, and methane).

This, in turn, may cause a range of abdominal symptoms, including stomach cramps, bloating, and flatulence. In addition, as with other unabsorbed sugars (such as sorbitol, mannitol, and xylitol), the presence of lactose and its fermentation products raises the osmotic pressure of the colon contents.

Types

There are three major types of lactose intolerance:

 Primary lactose intolerance. Environmentally induced when weaning a child in non-dairy consuming societies. This is found in many Asian and African cultures, where industrialized and commercial dairy products are uncommon.

 Secondary lactose intolerance. Environmentally induced, resulting from certain gastrointestinal diseases, including exposure to intestinal parasites such as giardia.

In such cases the production of lactase may be permanently disrupted. A very common cause of temporary lactose intolerance is gastroenteritis, particularly when the gastroenteritis is caused by rotavirus. Another form of temporary lactose intolerance is lactose overload in infants.

 Congenital lactase deficiency. A genetic disorder which prevents enzymatic production of lactase. Present at birth, and diagnosed in early infancy.