Lactose is the most important carbohydrate in milk. It cannot be absorbed in the intestine, but needs to be split in two molecules by the enzyme lactase. In newborns the (very rare) absence of the enzyme is lethal unless special food can be provided.
Until fairly recently it was considered normal by Western medicine that in older children and adults the activity of lactase was maintained. We now know that this is the rule among west Europeans and their descendants in other countries. In many other populations the continuation of lactase excretion in older children and adults is virtually absent, leading to lactose intolerance (Dobzhansky et al., 1997).
Lactose intolerance is manifested by diarrhea, abdominal pain, and flatulence after consumption of, let us say, half a liter of cow’s milk (Braly, 1992). This holds for many East Asian groups, Melanesians, Native Americans and for most Africans. Groups of nomadic pastoralists in Africa, such as the Fulani, form a notable exception with high prevalence of lactose tolerance. In southern Europe and in certain regions of India intermediate values (from 30 percent to 70 percent) are found lactose tolerance, a biological, genetic adaptation that is commonly found in populations with a long history of dairying (www.vegsource.com). Populations with this genetic trait have the ability to digest lactose, a sugar found in milk.
The intestinal enzyme lactase breaks down lactose into simpler sugars that can be absorbed and metabolized as a source of energy. Lactose also plays a part in the absorption of the calcium in milk. This is especially important when Vitamin D deficiency is present ( Durham 1991:226-228). Durham (1991) has pointed out that the full story is more complicated than was originally supposed, since it involves calcium as well as lactose absorption. Nevertheless, the result is the same.
Today, between 70 and 100 per cent of human adults whose ancestors came from long-term dairy-farming areas are lactose-tolerant, whereas the great majority of those who stem from non-dairy-farming areas remain lactose-intolerant. This differentiation must have occurred within the last 10,000 years, and must have been induced by human phenotypes changing their own environments.
When the body is functioning normally, lactase breaks down lactose into two simple sugars, glucose and galactose, which are used by the body. But when there is a lack of sufficient lactase, the unabsorbed lactose migrates to the colon, where it becomes fermented by intestinal bacteria and causes gastrointestinal problems.
In most humans, lactase activity disappears after infancy. That is, they become more or less lactose intolerant after they are weaned. Although they may be able to drink a small glass of milk without much trouble, if they drink large amounts the undigested lactose gives them diarrhea, bloating, and gas.
Even though they do not absorb the lactose in milk as an energy source, they may be able to make use of the protein, calcium, and fat in milk, if they drink small enough amounts to avoid distress and the nutritional losses incurred with diarrhea. Alternately, cultural adaptations such as making cheese or yogurt reduce the lactose content. Presumably, the reason lactose intolerance occurs early in life has to do with the process of weaning. Some children are genetically programmed to stop being able to handle milk once they pass the age of breastfeeding.
Although there is no perfect correlation the relationship between lactose tolerance in adults and animal husbandry is striking. Two explanations have been suggested, one cultural, and the other referring to physical qualities of the environment (Flatz & Rotthauwe, 1977). In the cultural explanation it is postulated that the consumption of milk, because of its nutritional value in proteins, should give a selection advantage. Once there were a few individuals who can tolerate milk, this trait could slowly spread through the population over a large number of generations. The fact that there are cattle farming populations with a low frequency of tolerance weakens this hypothesis. In addition, when milk has fermented it is low in lactose content and is digestible in the absence of lactase in the consumer’s intestinal tract.
The second hypothesis postulates an advantage of lactose tolerance in areas with relatively little ultraviolet sunlight, such as northern Europe. Sunlight plays a role in the production of vitamin D which is needed for calcium metabolism. A too low level of vitamin D leads to rickets, a bone disease. It has been suggested that lactose is an alternative substance to vitamin D in the metabolism of calcium. Another version of this hypothesis bears on the direct absorption of vitamin D contained in milk and milk products.
In practical terms, knowledge of lactose tolerance and intolerance is important when proposing food aid programs. Knowing that milk can cause digestive problems helps us understand why recipients of powdered milk as emergency aid have used the milk to whitewash their buildings and have even accused aid programs of being U.S. plots to poison them ( Lerner and Libby 1976:327).
Health educators also need to be cautious about over-promoting milk products to ethnic groups, such as Asian Americans, who do not tolerate them well. In evolutionary terms, farming is quite recent on the human scene, and most of the adaptations to it have been cultural rather than genetic. Lactose tolerance is particularly interesting because it shows the coevolutionary interaction between biological and cultural adaptation to the farming way of life.
Attwood, Charles R. Calcium Without the Cow. Retrieved on January 29, 2006
Braly, James, M.D., and Torbet, Laura. (1992). Dr. Braly’s Food Allergy and Nutrition Revolution. New Canaan, Conn.: Keats Publishing, Inc.
Dobzhansky, T., Ayala, F. J., Stebbins, G. L., & Valentine, J. W. (1997). Evolution. San Francisco: Freeman.
Durham, William H. (1991). Coevolution: Genes, Culture, and Human Diversity. Stanford, CA: Stanford University Press.
Flatz, G., & Rotthauwe, H. W. (1977). The human lactase polymorphism: Physiology and genetics of lactose absorption and malabsorption. Progress in Medical Genetics, 2, 205–49.
Lerner, Michael, and William J. Libby. (1976). Heredity, Evolution and Society. Second ed. San Francisco: W. H. Freeman.