Global Geography of Milk Consumption and Lactose (In)Tolerance
The highest ranking non-European country by dairy consumption is Kazakhstan, which ranks #13 with over 260 kg (573 lbs) of milk per capita. For centuries, Kazakhs were herders who raised fat-tailed sheep, Bactrian camels, and horses, relying on these animals for transportation, clothing, and food, in the form of both meat and dairy. Because of their traditional nomadic way of life, the Kazakh prefer sour milk, as it is easier to save. It is drank plain or with tea, and is often added to other dishes, such as soups. Butter, sour cream, and irimzhik, which resembles cottage cheese, are popular as well. Outside of Eurasia, United States ranks #16, Australia #24, Argentina #28, and Canada closes off the list of the top 30 dairy consuming countries (all other countries among the top 30 are in Europe).
This mostly Eurocentric pattern of milk consumption can be partially explained by the spatial distribution of lactose (in)tolerance, or more precisely lactase persistence. The term “lactose intolerance” refers to any form of allergy to lactose, the form of sugar found in milk, while “lactase persistence” means the continued activity of the enzyme lactase in adulthood. In early humans, as in most mammal species, the activity of the enzyme was dramatically reduced after weaning. However, in some populations lactase persistence has evolved as an adaptation to the consumption of non-human milk and other dairy products beyond infancy. The majority of people around the world remain lactase non-persistent, and consequently are affected by varying degrees of lactose intolerance as adults (although the correlation between genetically transmitted lactase persistence and lactose intolerance is not perfect). Most maps on the subject, such as the ones reproduced on the left, represent the spatial distribution of lactose intolerance rather than lactase persistence, though the two notions are often confused. While the figures differ from source to source, the overall pattern is the same, and it complements closely the pattern of global dairy consumption, illustrated above: countries with high levels of lactose intolerance, especially in sub-Saharan Africa, South and East Asia, are also low in dairy consumption. As can be seen from some of these maps, in the U.S. the levels of lactose (in)tolerance differ by race, with Caucasians being most lactose tolerant, followed by Hispanics and African Americans; Native Americans are said to have the highest levels of lactose intolerance.
Since these differences are thought to have been caused by recent natural selection favoring lactase-persistent individuals in cultures in which dairy products are available as a food source (cf. Beja-Pereira et al. 2003), it was first hypothesized that populations in Europe, India, and parts of Africa had high frequencies of lactase persistence because of a particular mutation. More recently, it has been shown that lactase persistence is caused by several independently occurring mutations (cf. Ingram et al. 2009). But even in populations that have eventually developed some degree of lactase persistence, the relevant mutations and consequently the ability to consume dairy products did not spread quickly, nor did they affect the entire populations. For example, the Tyrolean Iceman (see image on the left), who lived several millennia after the arrival of agriculture to southern Europe, was genetically lactose intolerant (cf. Keller et al. 2012). To this day, southern Europe has higher levels of lactose intolerance than northern Europe, though agriculture developed earlier in the south than in the north. Moreover, the Fertile Crescent, where agriculture is said to have originated with the domestication of several plant and animal species, remains highly lactose intolerant.
Beja-Pereira A et al. (2003) Gene-culture coevolution between cattle milk protein genes and human lactase genes. Nat Genet 35: 311−313.
Ingram CJ, Mulcare CA, Itan Y, Thomas MG, Swallow DM (2009) Lactose digestion and the evolutionary genetics of lactase persistence. Hum Genet. 124(6): 579-91.
Keller, A.; A. Graefen; M. Ball; M. Matzas; V. Boisguerin; F. Maixner; P. Leidinger; C. Backes; R. Khairat; M. Forster; B. Stade; A. Franke; J. Mayer; J. Spangler; S. McLaughlin; M Shah; C. Lee; T. Harkins; Al. Sartori; A. Moreno-Estrada; B. Henn; M. Sikora; O. Semino; J. Chiaroni; S. Rootsi; N. Myres; V. Cabrera; P. Underhill; C. Bustamante; E. Egarter Vigl; M. Samadelli; G. Cipollini; J. Haas; H. Katus; B. O’Connor; M. Carlson; B. Meder; N. Blin; E. Meese; C. Pusch; & A. Zink (2012) New insights into the Tyrolean Iceman’s origin and phenotype as inferred by whole-genome sequencing. Nature Communications. DOI: 10.1038/ncomms1701