Lactose is a disaccharide composed of linked molecules of the simple sugars glucose and galactose. Dietary lactose is obtained almost exclusively from milk. Infants and young children digest lactose with an enzyme, lactase, which splits the molecule into the two readily absorbable simple sugars. The majority of adults, however, have lost this ability and are lactose malabsorbers. Those malabsorbers who display clinical symptoms after milk consumption are described as lactose intolerant.

Lactose is a major constituent of the milk of all mammals except sea lions (Kretchmer 1993). Human milk contains the highest lactose concentration, about 7 percent; lactose levels in commonly milked animals, such as camels, goats, sheep, and cows, run between 4 and 5 percent. Adult animals, like most humans, lose the ability to digest lactose. This suggests that adult loss of lactase is a normal mammalian trait and that adult ability to split lactose is an “abnormal” evolutionary innovation.

Definition and History

Lactose malabsorption and intolerance must be distinguished clinically from allergy to milk proteins, which is a rare but serious genetic problem in infants. This essay focuses on primary adult onset lactase deficiency, but two other forms of the syndrome must be noted. Lactase deficiency may be secondary due to damage to the small intestine from heavy parasitic infections (especially the protozoan Giardia lamblia); to other severe intestinal infections; to AIDS; and to ionizing radiation, some drugs, and gastric surgery (Castiglia 1994;Tamm 1994). Total inability to synthesize lactase is another rare genetic disorder that was obviously lethal until modern times. If diagnosed promptly, such cases can now be managed with soy-based infant formulas.