The tiny intestine Lactose intolerance is the inability of a person to adequately digest the sugar (lactose) in milk. However, for those who are severely affected, surgery may be their only option to alleviate symptoms. In a majority of people, the enzyme lactase, which is produced by the cells that line the small intestine, breaks down lactose during digestion. This process is called lactase activity. In individuals where this does not happen, excess amounts of glucose and galactose enter the bloodstream instead. These substances cause inflammation and diarrhea, among other problems. Because of this, these people are considered lactose intolerant. Lactose intolerance is more common in certain populations. For example, it's estimated that between 5% and 20% of adults in Europe and North America are unable to produce enough lactase to digest milk efficiently. That means that they will experience symptoms whenever they drink milk or milk products. On the other hand, between 70% and 80% of African Americans, Asians, and Latinos appear to have reduced levels of lactase compared with Europeans; thus, they are also at risk of developing lactose intolerance if they consume dairy products regularly. Those who are lactose intolerant need to avoid products containing milk or milk products.