Lactose intolerance is the impaired ability to digest lactose (the naturally occurring sugar in milk). The enzyme lactase is needed to digest lactose, and a few children and many adults do not produce sufficient lactase to digest the milk sugar. The condition is rare in infants.

Only one-third of the population worldwide retains the ability to digest lactose into adulthood. Most adults of Asian, African, Middle Eastern, and Native American descent are lactose intolerant. In addition, half of Hispanics and about 20% of Caucasians do not produce sufficient lactase as adults.¹

A simple test for lactose intolerance is to drink at least two 8-ounce glasses of milk on an empty stomach and note any gastrointestinal symptoms that develop in the next four hours. The test should then be repeated using several ounces of cheese (which does not contain much lactose). If symptoms result from milk but not cheese, then the person probably has lactose intolerance. If symptoms occur with both milk and cheese, the person may be allergic to dairy products (very rarely can lactose intolerance be so severe that even eating cheese will cause symptoms). In addition to gastrointestinal problems, one study has reported a correlation in women between lactose intolerance and a higher risk of depression and PMS.² However, this study is only preliminary and does not establish a cause-and-effect relationship.