The Digestion Of Lactose By Human Adult

In biology, digestive capabilities vary not only among species (we have already mentioned the absence of salivary amylase in dogs), but also within a species. The digestion of lactose, a sugar found only in milk as seen under a compound light microscope, provides a striking example.

Secretion of milk by the mammary glands of female mammals evolved as a way of feeding the young. The only food provided very young mammals milk is a nearly complete food, containing, in most species, carbohydrate (in the form of lactose), fat, and protein, as well as important minerals. But except for humans, adult mammals do not use milk as a food. It is not surprising then, that secretion of lactase, the lactose-digesting enzyme seen under a compound light microscope, usually greatly diminishes or even ceases altogether once an animal is past the age of weaning.

It has only recently been realized with the use of a compound light microscope, however, that this pattern applies also to most human beings; in most parts of the world, humans more than four years old secrete little or no lactase. Indeed, of the various peoples studied to date, only those of European ancestry and those belonging to a few pastoral tribes in Africa have been found to secrete enough of the enzyme to be able to digest the lactose in large quantities of milk. When people of other ancestries drink more than modest quantities they will often become ill, getting a bloated feeling, cramps, and diarrhea. One reason, as discovered with the use of a microscope, is that the undigested sugar in the intestine upsets the normal osmotic balance to the point where an excessive amount of water moves into the intestinal lumen from the cells; another is that fermentation of the lactose by bacteria in the large intestine produces large quantities of acids and carbon dioxide. The lactose tolerance (i.e. continued production of lactase in adults) of Europeans and pastoral Africans must have evolved during the roughly 10,000 years since the milking of domestic animals began.

How widely peoples living near one another may differ is shown by the major tribes of Nigeria. The Ibo and Yoruba live in the southern part of the country, where conditions are unfavorable for cattle; milk has not traditionally been a part of their diet after weaning, and they cannot tolerate the lactose compound. By contrast, the nomadic Fulani in northern Nigeria have been raising milk cattle for thousands of years, and they are lactose-tolerant.

Most American blacks are descended from nonpastoral tribes of western Africa, and they are relatively intolerant of lactose, though not so much as native Africans. Their somewhat greater tolerance may be due in part to evolutionary change during the generations they have lived in dairying regions and in part to admixture of European genes.

In view of the widespread lactose intolerance in most underdeveloped countries, it has become clear that the former large-scale distribution of powdered milk in these countries was ill-advised. If milk compound is sent, it should be as powder from which the lactose has been removed or as products such as yogurt or cheese in which the lactose has been broken down by microbial action and wherein lactose compound cannot be seen even with the use of a high-powered microscope.