What makes some people lactose intolerant as adults?

The ability to digest the sugar in milk, called lactose, usually declines after infancy because the body produces less lactase, the enzyme that breaks it down. Most people with lactose intolerance have low lactase activity.

The genes that influence lactase activity are inherited, and this is why some ethnic groups have higher rates of lactose intolerance. For example, it's estimated that 75-90% of people of Asian descent have some degree of lactose intolerance.

Other factors, such as certain illnesses and injuries to the small intestine, can also contribute to lactose intolerance.

The only way to be certain if you have lactose intolerance is to have a hydrogen breath test.