Have you ever eaten too much dairy? Maybe a milkshake, some ice cream, and yogurt as a late night snack?

You may have noticed that afterward, your stomach started grumbling. Maybe you had stomach cramps or were overly gassy. Or maybe you had to urgently run to the restroom.

If this sounds familiar, I have some bad news for you...

You may be lactose intolerant.

What is lactose intolerance?

To answer this question, we first need to think about what milk is made of. Milk contains vitamins, minerals, proteins, and sugars. People who are lactose intolerant have problems digesting (breaking down) the sugars inside milk. The particular sugar is called lactose.

How common is lactose intolerance?

According to the National Institute of Health, about 68 percent of the world's population is lactose intolerant. While most infants can digest lactose at birth, many people develop a reduced ability to digest lactose as they get older. Certain parts of the world have increased incidence (more people have lactose intolerance) than in other parts of the world.

What causes lactose intolerance?

We know that lactose intolerance is when your body cannot break down the sugar lactose. The culprit is an enzyme found in the small intestines known as lactase.

Infants depend on milk for most of their nutrients. As we get older, we are able to eat more things, so we don't rely on milk as much to get those nutrients. As a result, our bodies stop making as many lactase enzymes and instead use those resources to make different enzymes. Less lactase in our bodies means less proteins breaking down lactose... which means more problems with digestion!

Why is lactose intolerance more common in certain parts of the world than in other parts?

Lactose intolerance is a genetically inherited condition. This means that if your parents are lactose intolerant, you will be lactose intolerant, too!

Generally, people of Caucasian descent (people whose ancestors are from northern Europe) are less likely to have lactose intolerance than people of African American, Hispanic, or Asian descent. This is because they have a variant (different version) of the MCM6 gene (section of DNA) that contains the instructions for making the lactase enzyme.

Individuals with this gene variant can continue to produce high levels of the lactase enzyme even as adults. As a result, they are able to digest milk easier.

Individuals without this gene variant cannot continue to produce high levels of the lactase enzyme into adulthood and eventually become lactose intolerant.