The Everyday Dairy-Free Cookbook : Free Cookbook

Lactose is the primary sugar found in milk. Digestion of lactose requires the enzyme lactase, which breaks lactose into simple sugars. When the intestine produces little or no lactase, milk sugar is not digested. New-born babies require high intestinal lactase levels for survival. Later in life though about two-thirds of all people lose the ability to procuce lactase. Most of the people who keep producing it throughout adulthood are those of European ancestry but in other ethnic groups – Mexican, Jewish, African, Asian and Native American – 75 to 100 percent of adults are lactose intolerant. Primary lactose intolerance may begin at any time but usually develops in early adolescence and continues through life.