Certain populations in eastern and western Africa who practise the husbandry of milk-producing animals consume large quantities of raw or unprocessed milk. For more than 70% of the world’s non-Caucasian adults, however, fresh milk is indigestible and when consumed produces gastric distress (e.g., abdominal cramps, diarrhoea, and sometimes nausea and vomiting). Among the semi-nomadic cattle herders the Masai, it is estimated that 62% of adults are lactose intolerant, and yet they consume large quantities of milk each day. Apparently, they curdle milk by mixing it with blood taken from their cattle, thereby removing the lactose. However, among the Fulani, another semi-nomadic group of cattle herders (at least in the past), the figure is only 20%. Bearing in mind 99% of the Yoruba are lactose intolerant, this speaks against the argument that it is mainly restricted to nomadic populations, as does the rate of lactose malabsorption among the Chinese (and to a lesser extent among Southern Europeans, especially Italians). Some 40-50% of the calories in human milk are contained in carbohydrates, the main one of which is lactose or milk sugar. While its function is to absorb calcium and phosphorous, lactose is not used directly by the body, but must be hydrolysed into the absorbable monosaccharides glucose and galactose. The agent responsible for this conversion is the gut brush border enzyme lactase, which is produced in the villi of the small intestine. Many of the world’s adult populations do not produce enough lactose to hydrolyse the lactase in milk and related products and hence are termed lactase deficient or intolerant. One of the interesting features of lactase is that its production begins before birth during the third month of gestation and then rapidly declines some time between the third and fifth year in most non-Caucasoid populations. In those populations where it does not decline, it is held that the continuation of lactose tolerance into adulthood is a consequence of a single autosomal dominant allele. This allele is frequent in populations with a long history of drinking unprocessed milk. Assuming that the domestication of milk-producing animals started during the Neolithic period about 6500 years BP, it has been suggested that lactose tolerance is a recent genetic adaptation confined to certain populations. The rapidity of this divergent evolution is difficult to explain in terms of random mutations and is assumed to be an example of culturally-determined selection pressures acting on human gene frequencies: those members best adapted to the rules of the culture prescribing diet are those who would be the most successful reproductively. The presence of lactase activity in all human infants and its subsequent decline in many populations who do not drink large quantities of dairy milk is relevant to understanding the nature of human infancy. The ability of infants to tolerate lactose in maternal milk is essential for the production of an acid flora in the stool that facilitates the absorption of calcium necessary for bone growth. Lasting up to the third or fifth year, this growth-related tolerance, together with the effects of lactational amenorrhea, may contribute to regulation of birth spacing. In most hunter-gatherer societies at least, the time between births and onset of lactose intolerance is strikingly close. Thus, it would of interest to know if the age of weaning in cultures classified as lactase deficient is closely related to the ages at which lactose intolerance appears in those cultures.
See Adaptation, Alcohol tolerance, Allele, Carbohydrates, Cultural selection, Enzyme, Fructose intolerance, Intolerance for cow’s milk protein, Mutation