Lactose intolerance is the inability to digest the main sugar found in milk and other dairy products. This is caused by a deficiency of lactase, the enzyme responsible for metabolizing lactose in the small intestines, according to the National Institutes of Health (NIH).
The prevalence of lactose intolerance in adults varies from less than 5 percent to almost 100 percent among different populations, according to research published in the Scandinavian Journal of Gastroenterology. The lowest prevalence is in northwestern Europe, around the North Sea, and the highest prevalence is in Asians and American Indians. About 30 million American adults are somewhat lactose intolerant by the age of 20, according to the NIH.
People can acquire lactose intolerance at any point in their life, and some people develop it over time, said Dr. Sophie Balzora, a gastroenterologist at NYU Langone Medical Center in New York City.