Between 30 and 50 million Americans ar...

Between 30 and 50 million Americans are lactose intolerant, meaning they are deficient in the enzyme lactase. The majority of lactose-intolerant people can consume small amounts of lactose, or lactose in certain foods, but may experience symptoms of intestinal upset if they consume too much. [Photograph by Leonard Lessin. Peter Arnold, Inc. Reproduced by permission.]