Milk and other dairy products play an important role in a healthy balanced diet, but for a portion of the population, these products can lead to very unpleasant stomach issues. Here’s a run-down of what causes lactose intolerance, and why.

Lactose intolerance is the inability to digest lactose, the sugar in milk. It’s different from a dairy allergy, which involves an immune response against milk and can result in far more dangerous symptoms. In people with lactose intolerance, their bodies do not make enough of the enzyme lactase, which is used to break down lactose. If it's not broken down, it doesn't absorb into the bloodstream, and instead moves to the large intestines or colon, Medical News Today reported. Here, bacteria in the gut can react with the lactose, causing unpleasant symptoms.

Lactose intolerance can cause anything from embarrassing gas to painful diarrhea when individuals eat dairy. These symptoms can vary, depending on the severity of an individual's intolerance and how much lactose they ingested.

Read: Lactose Intolerant? Be Thankful, It Could Save You From Certain Cancers

Certain dairy foods that have been fermented, such as yogurt and cheese, may not cause stomach problems. During the fermentation process, bacteria ingest lactose, which makes some products virtually lactose-free, Slate reported.

So, Where Did It Come From?

If you experience discomfort after eating dairy products, you’re in the majority! About 65 percent of the world’s population has decreased ability to digest the enzymes in milk after infancy. In fact, the ability to digest this enzyme is actually the result of a single genetic mutation.

As babies, we all produce lactase to allow us to break down the lactose in our mother’s milk. However, in most individuals, the body makes less lactose as they are weaned off milk and onto more solid foods. There is an evolutionary reason for this — kids need to stop drinking milk after a certain age so a mother has more milk and attention for future children, Slate reported. It’s a trait observed in nearly all mammalian species.

However, about 10,000 years ago, one simple genetic mutation allowed people to produce lactase forever, and respectively gave them the ability to drink milk throughout the rest of their lives without any unpleasant gut problems. This gene spread rapidly as a result, especially in regions that had a long history of depending on unfermented milk products as an important food source, the National Institute of Health reported. Today, about 80 percent of all Europeans have the ability to produce lactase throughout their entire lives, Slate reported.

Because of its genetic roots, primary lactose intolerance, the most common form of this condition, tends to run in certain populations. For example, genetic lactose intolerance is more common among individuals of African, Asian, or Hispanic ancestry. It is also found in some populations in Southern Europe, The Mayo Clinic reported.

Of course, not all lactose intolerance is genetically based. In the case of secondary lactose intolerance, this condition arises when the small intestines produce less lactase following illness or an injury. This form of lactose intolerance is often found in those with Crohn’s disease and in those who have undergone certain cancer treatments.

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