Lactose intolerance
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 the sugar in milk, lactose. In order to be digested, the disaccharide lactose must be broken down to the simpler sugars, glucose and galactose. The excess lactose causes enteral bacteria to adapt and lead to fermentation in the intestines.
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 is caused by environmental factors. It occurs in populations where dairy products are not commonly available or consumed.
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 is present at birth. It is a genetic disorder that prohibits the production of lactase.
Geographic Variance
Lactose intolerance is one 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.
Most other populations tend to exhiit lactose intolerance. 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.