Lactose malabsorption and nutrition

Lactose needs to be digested by the small intestinal enzyme lactase into its constituent monosaccharides, glucose and galactose, before transport across the epithelial cell membranes. Lactase activity is therefore essential for the development of young mammals, since their sole source of nourishment is their mother's milk. This chapter summarizes what is known about the genetics and evolution of the lactase persistence (LP) trait. Symptoms of lactose intolerance can arise after an individual with hypolactasia has ingested lactose. Many dairy products contain only small amounts of lactose, allowing their consumption by most lactase non-persistent individuals. Bacteria and yeast fermentation convert the lactose in milk into various by-products, reducing the content of lactose. Nutritional epidemiology studies, and the staggering selective advantages that have favoured LP-associated alleles over the last 10,000 years, clearly show that adult milk consumption can be highly beneficial.

The onset of lactase persistence in Europe

The genomic region containing the lactase (LCT) gene shows one of the strongest signals of positive selection in Europeans, detectable using a range of approaches including haplotype length, linked microsatellite variation and population-differentiation-based tests. Lactase is the enzyme that carries out the digestion of the milk sugar lactose. Its expression decreases at some point after the weaning period is over in most mammals and in around 68% of all living adult humans. However, in some humans, particularly those from populations with a history of dairying, lactase is expressed throughout adulthood. This trait is called lactase persistence (LP), and in people of European ancestry, it is associated with a single mutation (-13910*T). Evidence from the detection of dairy fat residues in potsherds, and allele frequencies in ancient DNA samples suggest that LP arose after dairying practices had developed. However, the reasons why LP may have been advantageous are still debated, and the respective contribution of demography and natural selection remains to be disentangled. This paper discusses various studies, from archaeology to population genetics, that have shed some light on the subject by investigating the evolution of LP in Europe.