Of Man and Milk

Starving Africans throw away gifts of American powdered milk, complaining that it harbors evil spirits. Colombian Indians refuse to drink reconstituted milk and use it instead to paint their huts. On the Navajo reservation, many Indians discard Government-issue powdered milk rather than suffer diarrhea. All have a problem in common. A surprisingly large portion of the world's population cannot digest an important ingredient in milk: lactose.

Lactose supplies about 40% of the caloric value of most milks. It is most abundant in human milk, about 7% by volume; the average dairy cow's milk contains 5%. Although lactose is a rich and valuable nutrient, it cannot be metabolized directly by the human system; it must first be broken down by an enzyme called lactase from its original complex sugar form (disaccharide) to two simple sugars, glucose and galactose. Normal babies secrete lactase in abundance and thus have no difficulty in digesting mothers' milk, cows' milk or comparable substitutes.

Response to a Challenge. Why can virtually all infants and many adults digest lactose, while other adults cannot? One theory is that the ability to produce lactase, and thus to digest lactose, is the response to a challenge: if a person continues to drink milk after he has been weaned and through adulthood, he will always be able to digest it. But if he goes without milk for months or years, he loses that ability.

That theory is disputed by many investigators, most recently U.C.L.A. Anthropologist Robert McCracken. He believes that the ability to secrete lactase and digest lactose is determined in the genes. Virtually all normal mammals have a gene that turns on the supply before birth, maintains it at a high level until weaning, and then allows it to decline. When man first emerged as Homo sapiens, says McCracken, and for tens of thousands of years thereafter, he was a hunter and gatherer of food. He had no milk cattle. A baby was usually weaned by the age of two. Nature designed milk as a food for infants, not for adults. So at that stage in human development, because adults drank no milk, they needed no lactase. Nature's plan worked smoothly.

Then man decided to improve upon nature. He domesticated various species of cattle and invented dairying. Some pastoral peoples used the milk as a food supplement for youths and adults. Among these tribes, McCracken suggests, individuals whose lactase was not turned off after weaning benefited from milk's rich nutrients; they grew bigger and stronger than those who were deficient in lactase and weakened by milk diarrhea. Through natural selectivity, adults who continued to produce lactase eventually predominated.

As evidence of his theory, McCracken points to such herding cultures as the Bahima of Uganda, among whom lactase levels remain high throughout life, as they do in most civilized countries today where dairying is practiced and milk intolerance uncommon. Other herders (such as the neighboring Baganda and most of the Bantu in Africa), who keep cattle but do not drink the milk, have low lactase levels after childhood and a high incidence of intolerance to milk. In other parts of the world, the same pattern exists. More than 85% of Thai children over the age of five, for example, cannot tolerate milk.

Forest Ancestors. To bolster his argument, McCracken notes that the ancestral stock of American Negroes came from heavily forested areas of West Africa, where no milk-producing cattle were kept. The remaining indigenous populations of West Africa have low lactase levels and high intolerance--as have 75% of most U.S. Negro groups tested; the highest rate reported among most whites is only about 19%. High proportions of American Indians, in the U.S. and Central and South America, are also unable to digest milk.

An apparent exception to the high tolerance to milk among dairy cultures exists in Cyprus. The exception proves the rule, says McCracken. Cyprus produces lots of milk, but most of it is made into cheese, which--like milk that has been soured or fermented--contains only a trace of lactose. This suggests to McCracken that such low-lactose milk products should have a greater part in feeding programs around the world. That would enable millions of the hungry to circumvent nature's original law that milk is for infants only.

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