Lactose intolerance

Eduzendium article

Lactose intolerance is due to a decreased production of the enzyme lactase in the cells lining the small intestine, resulting in an inability to break down or metabolize lactose, the sugar in milk. In order to be digested, the disaccharide lactose must be broken down to the simpler sugars, glucose and galactose. Excess lactose causes enteral bacteria to adapt and lead to fermentation in the intestines. Due to this, lactose intolerance is often called lactase nonpersistence or lactase restriction, although it is the normal genetic adaptation for most populations in the world.

Symptoms

Nausea, bloating, cramps, diarrhea, and gas are symptoms commonly included with lactose intolerance. They typically occur 30 to 120 minutes after consuming lactose.

Types of Lactose Intolerance

Primary Lactose Intolerance

Primary lactose intolerance caused by genetic and environmental factors and occurs in adulthood. It occurs in populations where dairy products are not commonly available or consumed. The allele that codes for lactose intolerance (as the decline in the ability to produce lactase) is dominant.

Secondary Lactose Intolerance

Secondary lactose intolerance is also caused by environmental factors. It is often temporary and is a result of many gastrointestinal diseases from parasites or other causes.

Congenital Lactase Deficiency

Congenital Lactase Deficiency is present at birth. It is an autosomal recessive genetic disorder that prohibits the production of lactase.

Geographic Variance

Lactose intolerance is possibly an example of biocultural evolution, human variation through multiple generations due to both biological and cultural forces. Lactose intolerance is inherited as the expression of the dominant gene. However, expression of the trait is also environmentally determined. Enteral bacteria can often buffer the effects of lactose intolerance and can be increased with previous exposure, leading to acquired tolerance even in individuals are genetically lactose intolerant.

Varience throughout populations is most likely due to a history of an economic reliance on pastoralism and, therefore, consumption of large quantities of milk after childhood. Historically, the availabiliy of milk disappeared after weaning. This rendered lactase useless and perhaps problematic for digestion of other foods in humans. Therefore, there was a possible genetic advantage to ceasing production of lactase. This would change the allele frequency making the expression of lactose intolerance genetically dominant. In populations with a history of pastoralism, however, there was a likely genetic advantage or allele shift toward lactose tolerance. The descendants of these populations, most European groups, tend to retain lactose tolerance. Distribution of lactose intolerance in Africa seems to be due to the same variation in economic history. Most populations of Africa exhibit lactose intolerance. However, individuals in groups that historically relied on pastoralism, such as the Fulani and Tutsi, tend to show lactose tolerance.

Another theory about the ability of people of Northern European descent has to do with vitamin D. Because the allele that codes for lactose tolerance or continued lactase production in adulthood is recessive, selective forces keep it in existence. One selective force is the necessity of lactose consumption in pastoral communities. Another in Northern Europe is the reduced production of vitamin D due to the infrequnent direct sunlight in the region. The lack of vitamin D would make the population more vulnerable to bone diseases like rickets because of the poor absorbtion of calcium. The advantage of lactase production is the ability to consume milk, which is a good source of calcium, and contains lactose, which allows for better calcium absorbtion. This theory has gained support because of its congruence with the theory that Northern European light skin color allows the most vitamin D production with little sunlight. However, this theory is not relevant south of the equator, nor is it supported with archaeological or osteological evidence.

Lactose intolerance is sometimes seen in groups that frequently consume dairy because they usually consume it as cheese or yogurt, forms in which bacterial action has already broken down the lactose.

Frequency of Lactose Intolerance within Population Groups

Population Group	Percent
United States Whites	2-19
Finnish	18
Swiss	12
Swedish	4
United States Blacks	70-77
Ibos	99
Bantu	90
Fulani	22
Tutsi	7
Thais	99
Asian Americans	95-100
Native Australians	85
Native Americans	99

Genetic Factors

It is likely that selection was a main factor in determining the distribution of lactose intolerance. The gene that codes for the lactase enzyme is located on chromosome 2. This is the case in all populations, but the reguation of the gene varies. The lactase gene has four common haplotypes- A, B, C, and U. The A haplotype is usually found in populations that exhibit lactase persistence. Increased haplotype diversity occurs in populations that exhibit lactose intolerance. While it is still unclear, it is likely that lactase regulation occurs during DNA transcription because there is variation in mRNA levels between lactase persistent and impersistent individuals. Genetic drift probably also played a role in latase gene diversity. The more homogeneous populations are probably as such because of genetic drift, this is why African cultures have more diversity in the haplotypes. Gene flow, mainly due to colonization, is probably the cause of continued frequencies of lactose intolerance and tolerance within populations.