Lactose intolerance is the inability to digest significant amounts of lactose, which is the predominant sugar of milk. Fifty million American adults are lactose intolerant. Certain ethnic and racial populations are more widely affected than others. As many as 75 percent of all African-American, Jewish, Native American and Mexican-American adults, and 90 percent of Asian-American adults are lactose intolerant. The condition is least common among persons of Northern European descent.

Lactose...