It has been suggested that lactation evolved as the most advantageous way to provide infant nutrition when food was unavailable or patchy, despite inefficiencies associated with converting nutrients from food to reserves and milk (14). One of the best nutrient-related examples of human genetic variation maintained via natural selection is given by our ability to digest milk lactose. Like most mammals, humans slowly lose their ability to digest lactose after weaning and have a low digestive capacity, which leads to abdominal discomfort when
Molecular Nutrition and Genomics: Nutrition and the Ascent of Humankind, Edited by Mark Lucock Copyright © 2007 John Wiley & Sons, Inc.
Human diseases resulting from incompatibility between genetic/metabolic programming and modern diet
Figure 2.1. Incompatibility between programming in our ancestral genes and the dietary consequences of a modern diet.
in excess of 500 mL of milk is consumed. By contrast, many northern Europeans, northern Africans, and Arab populations maintain their ability to digest lactose into adulthood, and they have a high digestive capacity. The dominant LAC P gene controls adult lactose digestion: Either one or two alleles confer high digestive capacity. By contrast, homozygosity for the recessive allele LAC R confers low digestive capacity. The genetic distribution of LAC P may stem from the environmental pressures of a nomadic lifestyle in some northern African and Arab populations.
Nomads from this part of the world probably came to rely on goat and camel milk during the drier months; to obtain all their fluid, energy, and protein requirements, they would need to exceed the threshold 500-mL value by a factor of at least 5x. This large volume of milk would have to be consumed while fresh, and as a consequence, the LAC P allele has a high frequency in today's nomadic tribes from this region. Quite simply, this gene probably facilitated survival. The Beja of the desert region between the Nile and the Red Sea exhibit milk dependence sufficient to result in selective pressures in favor of the lactase persistence allele. The proportion of lactose malabsorbers was 16.8% in the Beja and 74.5% in the Nilotes of Sudan who are semi-nomadic cattle herders. The high prevalence of lactose malabsorption among the Nilotes fits into a converging gradient of lactase gene frequencies along the Nile Valley (15). Rationalizing this simple nutrient-gene interaction that permits survival in an extreme environment is straightforward, but why is the lactase persistence allele common in the United Kingdom and other northern European countries? It has been suggested that substantial geographic coincidence exists among (1) high diversity in dairy cattle genes, (2) locations of the European Neolithic cattle farming sites (>5000 years ago), and (3) present-day lactose tolerance in Europeans. This suggests a gene-culture coevolution between cattle and humans (16).
Another hypothesis suggests that having an ability to digest lactose increases vitamin D absorption. At northern European latitudes, the level of ultraviolet (UV) exposure is insufficient to manufacture vitamin D all year round. Lactose tolerance may therefore help prevent the vitamin D deficiency disease rickets as well as maintain key biochemical interactions involving vitamin D within the cell nucleus (see below).
It is interesting to note the irony that, after returning from his voyage in the Beagle in 1836, Charles Darwin suffered from long bouts of abdominal discomfort that perplexed his physicians for 40 years. Darwin only recovered, when, by chance, he refrained from milk and cream. Darwin's malady highlights the importance of lactose in mammalian and human evolution in a somewhat obtuse, but nonetheless salient, manner (17).
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