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Lactose Intolerance
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What is lactose intolerance?

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Lactose intolerance is the inability to digest and absorb lactose (the sugar in milk) that results in gastrointestinal symptoms when milk or food products containing milk are consumed.

What causes lactose intolerance?

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Lactose is a large sugar molecule that is made up of two smaller sugars, glucose and galactose. In order for lactose to be absorbed from the intestine and into the body, it must first be split into glucose and galactose. The glucose and galactose are then absorbed by the cells lining the small intestine. The enzyme that splits lactose into glucose and galactose is called lactase, and it is located on the surface of the cells lining the small intestine. Lactose intolerance is caused by reduced or absent activity of lactase that prevents the splitting of lactose (lactase deficiency). Lactase deficiency may occur for one of three reasons, congenital, secondary or developmental.

Congenital causes of lactose intolerance

Lactase deficiency may occur because of a congenital absence (absent from birth) of lactase due to a mutation in the gene that is responsible for producing lactase. This is a very rare cause of lactase deficiency, and the symptoms of this type of lactase deficiency begin shortly after birth.

Secondary causes of lactose intolerance

Another cause of lactase deficiency is secondary lactase deficiency. This type of deficiency is due to diseases that destroy the lining of the small intestine along with the lactase. An example of such a disease is celiac sprue.

Developmental causes of lactose intolerance

The most common cause of lactase deficiency is a decrease in the amount of lactase that occurs after childhood and persists into adulthood, referred to as adult-type hypolactasia. This decrease in lactase is genetically programmed, and the prevalence of this type of lactase deficiency in different ethnic groups is highly variable. Thus, in Asian populations it is almost 100%, among American Indians it is 80%, and in blacks it is 70%; however, in American Caucasians the prevalence of lactase deficiency is only 20%. In addition to variability in the prevalence of lactase deficiency, there also is variability in the age at which symptoms of lactose intolerance appear. Thus, in Asian populations, the symptoms of lactase deficiency (intolerance) occur around the age of 5, among Blacks and Mexican-Americans by the age of 10, and in the Finnish by age 20.

It is important to emphasize that lactase deficiency is not the same as lactose intolerance. Persons with milder deficiencies of lactase often have no symptoms after the ingestion of milk. For unclear reasons, even persons with moderate deficiencies of lactase may not have symptoms. A diagnosis of lactase deficiency is made when the amount of lactase in the intestine is reduced, but a diagnosis of lactose intolerance is made only when the reduced amount of lactase causes symptoms.

What are the symptoms of lactose intolerance?

The common primary symptoms of lactose intolerance are gastrointestinal include:

• abdominal pain,
• diarrhea, and
• flatulence (passing gas).

Less common symptoms of lactose intolerance include:

• abdominal bloating,
• abdominal distention, and
• nausea.

Unfortunately, these symptoms can be caused by several gastrointestinal conditions or diseases, so the presence of these symptoms is not very good at predicting whether a person has lactase deficiency or lactose intolerance.

Symptoms occur because the unabsorbed lactose passes through the small intestine and into the colon. In the colon, one type of normal bacterium contains lactase and is able to split the lactose and use the resulting glucose and galactose for its own purposes. Unfortunately, when they use the glucose and galactose, these bacteria also release hydrogen gas. Some of the gas is absorbed from the colon and into the body and is then expelled by the lungs in the breath. Most of the hydrogen, however, is used up in the colon by other bacteria. A small proportion of the hydrogen gas is expelled and is responsible for the increased flatulence (passing gas). Some people have an additional type of bacterium in their colons that changes the hydrogen gas into methane gas, and these people will excrete only methane or both hydrogen and methane gas in their breath and flatus.

Not all of the lactose that reaches the colon is split and used by colonic bacteria. The unsplit lactose in the colon draws water into the colon (by osmosis). This leads to loose, diarrheal stools.

The severity of the symptoms of lactose intolerance varies greatly from person to person. One reason for this variability is that people have different amounts of lactose in their diet; the more lactose in the diet, the more likely and severe the symptoms. Another reason for the variability is that people have differing severities of lactase deficiency, that is, they may have mild, moderate, or severe reduction in the amounts of lactase in their intestines. Thus, small amounts of lactose will cause major symptoms in severely lactase deficient people but only mild or no symptoms in mildly lactase deficient people. Finally, people may have different responses to the same amount of lactose reaching the colon. Whereas some may have mild or no symptoms, others may have moderate symptoms. The reason for this is not clear but may relate to differences in their intestinal bacteria.

http://www.medicinenet.com/lactose_intolerance/article.htm

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