Lactose malabsorption describes a physiological situation. It is the basis for lactose intolerance. The inability to digest lactose is a quantitative phenomenon related to the enzyme lactase and its amount and activity in the intestine. Lactose intolerance, then, is a clinical definition. It involves the concept that the individual is unable to tolerate physiologically the lactose present in milk and other dietary products because of an inability to digest the carbohydrate, due to insufficient activity of the lactase enzyme. Intolerance to lactose as a clinical entity has been recognized for some time. Early in this century, Abraham Jacoby hinted at the existence of lactose intolerance in speeches to the American Pediatric Society; later, in 1926, John Howland, in his presidential address to that same organization, was somewhat more explicit when he indicated that many of the infantile diarrheas were the result of a lack of “ferments” necessary for the digestion of carbohydrate (Flatz 1989).

More recently, interest of physicians and nutritionists in the digestion of lactose stimulated reports during the late 1950s. One by A. Holzel and colleagues (1959) reported on a severe diarrhea associated with the ingestion of lactose in two young siblings who, as a consequence, were “failing to thrive.” Another report, by P. Durand (1958), diagnosed two patients with lactose malabsorption and lactosuria. Since then, innumerable articles and reviews have appeared in the world’s literature (Scrimshaw and Murray 1988). Evaluations of that literature may be obtained by consulting G. Semenza and S. Auricchio (1989), G. Flatz (1989), and N. Kretchmer (1971).