Human Biological Adaptability: Nutritional Adaptation

We have seen that not all people can survive well at high altitude or under intense ultraviolet radiation. Similarly, there are major differences around the world in how effectively our bodies process particular foods. In addition, some people live well on daily diets that would be at a starvation level for others. Different human populations not only eat different foods, but their digestive systems often use them in somewhat different ways. For instance, the Inuit of Alaska, Northern Canada, and Greenland traditionally ate far more fat than most other populations and their gastrointestinal

systems apparently are more capable of breaking fats down for use by their bodies. This is an indication that adapting to local nutritional opportunities has led to the evolution of related genetic differences among the populations of the world.

Many Indians in the Southwestern United States have what have been described as "thrifty genes". Their bodies are unusually efficient at utilizing the calories in their food and, subsequently, need to consume less than other people of their size in order to maintain a stable weight. The Tohono O'Odham Indians of Southern Arizona are an example. Until well into the 20th century, these traditional subsistence farmers had diets that mostly consisted of beans, squash, and corn with little animal protein and fat. Late 19th century photographs usually showed them to have slender to medium body builds. That is no longer true. Now, obesity and associated type 2 diabetes are very common problems for most of them. A dramatic change in diet was responsible for this deterioration in health. Most of them abandoned their traditional diet in favor of the fat and protein rich foods of their Mexican and European American neighbors. Their bodies are responding to what for them is an excessive amount of calories by storing much of it as body fat. This fat is storage for a famine that never comes.

NOTE: "Tohono O'Odham" literally means "Desert People". They were formerly known as the Pima and Papago Inidans.

The best documented differences in nutritional adaptation relates to milk sugar, or lactose

, which is commonly found in uncooked dairy products. Most human adults have moderate to severe difficulty in digesting lactose. They experience bloating, stomach cramps, belching, flatulence, and even diarrhea when they drink milk. Not surprisingly, this commonly results in the exclusion of dairy products from their diet. This problem is most often due to an inability to produce sufficient amounts of the enzyme lactase

, which breaks down lactose in the small intestine to aid its absorption into the blood stream. Those who have this problem are said to be lactose intolerant due to their lactase deficiency.

The ability to produce lactase is genetically controlled. The gene that codes for it is on chromosome 2. The vast majority of babies throughout the world can digest their mother's milk. However, there is a decline in lactase production as people grow older. This decline usually begins by two years of age, which is shortly after the time when babies are weaned in most societies. For some people, the reduction in lactase production does not start to occur until they are around twenty. More rarely, lactase continues to be produced at sufficient levels to consume milk throughout life.

Lactose intolerance is at its highest frequency in some parts of Africa, East Asia, and among Native Americans (as shown in the table below). Northern Europeans generally have the lowest frequency of this dietary problem.

POPULATION	LACTOSE INTOLERANT ADULTS

U.S.
European Americans	2-19 %
Latinos (Hispanic Americans)	52 %
African Americans	70-77 %
Native Americans	95 %
Asian Americans	95-100 %
Mexico	83 %
Europe
Sweden	4 %
Switzerland	12 %
Spain	15 %
Finland	18 %
Estonia	28 %
England	32 %
Hungary	37 %
Greece	88 %
Jordan	79 %
Africa
Southern Sudan (cattle herders)	17 %
Ibo and Yoruba (Nigeria)	99 %
Asia
Japan	90 %
Thailand	99 %
Australia (Aborigines)	85 %

Source: Robert D. McCracken, "Lactase Deficiency: An Example of Dietary Evolution,"
Current Anthropology 12 (Oct.-Dec. 1971, pp. 479-517) and Norman Kretchner, "Lactose
and Lactase," Scientific American 277 (Oct. 1972, pp. 71-78)

Given this distribution pattern of lactose intolerance, it is not surprising that dairy products are popular among most Europeans but are rarely found in Asian, Native American, and most African cuisines (except among cattle herders in East Africa). In the majority of non-European populations, fresh milk is considered an unpleasant substance to be consumed only as a last resort. It is now clear that lactose tolerant Europeans are atypical for humanity and for the entire animal kingdom.

NOTE: In the Indian subcontinent and much of Central and Western Asia, dairy products are consumed frequently but usually only after bacteria (lactobacilli ) have broken down most of the lactose. When this has occurred, milk becomes yoghurt or kumis, both of which are relatively easily digested even by people who produce little lactase.

The common ability of people in Europe and some other areas of the world to continue producing lactase as adults is very likely a relatively recent evolutionary development. Prior to the domestication of cattle, sheep, goats, and horses after about 9000 years ago, milk was most likely only consumed by babies and very young children. That milk was human milk. Dairy products such as cow's milk, yoghurt, and cheese did not exist. When nutrient rich nonhuman milk became widely available in pastoralist societies, the rare genetic variations that allowed some adults to easily digest lactose were selected for and this trait became more common. In other words, natural selection shifted to favor lactose tolerant people, resulting in the progressive evolution of the gene pools of these populations. Support for this hypothesis was provided in 2007 by Joachim Burger and his team of researchers at the University of Mainz in Germany. Their analysis of DNA in bones from 10 Central and Eastern European human skeletons dated between 3,800 and 6,000 years ago showed that the allele that allows lactose tolerance in adulthood was not yet present despite the fact that these populations apparently had been raising milk producing farm animals for hundreds or even thousands of years. Sarah Tishkoff from the University of Maryland also reported in 2007 that the mutations among East Africans that keep the lactase gene permanently turned on are different from those of Europeans who share this trait. Her genetic studies among 43 East African ethnic groups also suggests that that 3 different mutations resulting in lactose tolerance in Africa arose 2,700-6,800 years ago.

Darwinian gastronomy exercise--why do people in some parts of the world eat spicy foods?
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