Anthropology P380: Prehistoric Diet and Nutrition
Lecture 2: Biological constraints on human nutrition
Lactose Tolerance is an interesting, complex example, of the interplay between genetics, culture and human diet... all the more interesting because it illustrates how cultural biases can help guide disastrous health policies.
Remember the recent "Milk is good for everybody" "Milk does a body good" advertising slogan? Milk has long had an image in the U.S. as the "perfect food" something thats good for your kids... A couple years ago supermodel Christie Brinkley was pushing milk as a "natural food"! and Madison avenue & Dairy Council has pushed its benefits for adults (e.g. avoid osteoporosis in old age... etc), and for many years tons of powdered milk were a prominent part of food aid...
Robert Lowie, an anthropologist working in the first half of the century, was fascinated by cultural differences in milk-drinking -- was particularly astonished at the AVERSION that many Chinese, Korean, and other S.E. Asian peoples had to drinking milk. Lowie attributed this to cultural caprice -- an example of arbitrary / symbolic behavior.
"A cool, inviting glass of wholesome milk"
vs
"An ugly-looking, foul-smelling, glandular secretion" that no normal person would think of swallowing...
Marvin Harris describes how resentful American officials became during the Kennedy Administration when the U.S. sent tons of powdered milk to Brazil as "Food for Peace" and the Brazilians complained that it made them sick...
Digesting Lactose: What's up? The culprit is lactose a complex sugar (di-saccharide) found only in mammalian milk.(Though not in Pacific Pinnipeds -- the seals and walruses, which lack lactose in milk and lactase in intestine). It cannot diffuse from mammary gland into bloodstream. To digest lactose, you need to break it in two: glucose and galactose = monosaccharides which can be absorbed by cells in the lining of small intestine, and then into bloodstream. This requires that mammals produce a special enzyme, called "lactase" which cleaves the sugar molecules in two.
Mammals ingest lactose only during nursing... and activity of lactase enzymes drops off sharply after weaning -- production of enzyme shuts off -- why produce an enzyme you don't need?
What happens if an adult mammal drinks milk? Depends upon individual... but called "lactose intolerance" -- prompting a range of gastro-intestinal distress (although intolerant individuals who drink milk frequently can sometimes learn to tolerate it -- there seems to be plasticity in genetic control of enzyme production).
Yet some human adults maintain lactase activity into adulthood -- descended from populations in regions of the world where dairying has had a long history = culture-history argument first proposed by Simoons. (Danes, Finns, Hungarians, Mongols, northern Indian groups, and Tutsi & Fulani pastoralists).
This is a good example of a physiological trait being culturally selected -- it appears to be inherited as an autosomal dominant genetic trait -- and shows that such traits can become widespread in a relatively short period of time (dairying less than 10,000 years old). Lactose tolerance is often cited as an example of "evolution in action" and the coevolution of human biology and culture (e.g. a biological adaptation evolving in a "cultural" environment.)
There are selective advantages to digesters in some cultural circumstances. As you suggested in class, there could be many possible reasons why two populations in the same region could have very different frequencies of lactose digesters: different dependence on raw milk products, compared to cheese and yogurt (where fermentation "pre digests" the lactose) is one great example.
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Copyright 1998 Jeanne Sept
