True or false: You can become lactose intolerant as an adult

I'm sorry to break it to you, but...yes.

Remind me...what is lactose intolerance?

Lactose intolerance, of course, is when your small intestine doesn’t make enough of the enzyme lactase to break down the sugar in dairy, a.k.a. lactose.

Because your body can’t digest and absorb those sugars, it has to eliminate them somehow, and that can bring on the fun symptoms of nausea, bloating, cramps, diarrhea, and gas.

When can you become lactose intolerant?

Lactose intolerance usually begins in kids around age 5; and about 30 million American adults are lactose intolerant by age 20, according to the National Institutes of Health (NIH).

For most, lactose intolerance is genetic. “All babies can digest and absorb lactose as their first food. As we get older, for many of us that potential diminishes and parts of the intestine that secrete lactase stop working,” says Robin Foroutan, R.D., a spokesperson for the Academy of Nutrition and Dietetics and a health coach at The Morrison Center who specializes in food sensitivities.

Though lactose intolerance most often starts by your teens or early twenties, “it’s pretty common for people to become lactose intolerant as adults,” says Foroutan. In fact, some research suggests as many as 75 percent of the world’s population loses the ability to digest milk at some point.

What causes lactose intolerance in adults?

Foroutan says there are a lot of triggers that can mess up your body's ability to digest lactose, such as:

Food poisoning
Stomach bug
Small-intestine diseases (like Crohn's)
Extreme stress

“Lactase enzymes live on the brush border, or the tippy-top of edge of the small intestine. If there’s irritation to the digestive tract, that section of the brush border is the first to be affected, so those enzymes are often the first to go,” explains Foroutan.

Other people find they simply “age out” of their ability to digest lactose. “No one really knows exactly why, but it’s common enough,” says Foroutan. “If you think about it from an evolutionary perspective, for an infant, milk is the only food source. But as you get older, your evolutionary requirement to absorb nutrients from milk is not as important.” (Your tastebuds may not agree.)

It could also very well be that you were always lactose intolerant, you just never realized it. “Adults tend to be better at making connections between what they eat and how they feel,” says Foroutan.

How do I know I’m lactose intolerant?

Foroutan says that when lactose doesn’t break down and those sugars hang out in digestive tract, your body tries to flush out that sugar by bringing water into the colon-leading to gas, loose stools, and cramping. Most symptoms pop up within two hours of eating...so, your body lets you know pretty quickly that it's not happy.

However, Foroutan says that people sometimes assume they're lactose intolerant when they're actually sensitive to casein or to whey-the protein in milk and the watery substance that remains after milk is turned into curds. For example, if you can’t stomach hard cheeses, which don’t contain lactose, it may point to a casein sensitivity. You might also be sensitive to any combination of lactose, casein, and whey.

The best way to figure out your sensitivities and limits is to work with a nutritionist to eliminate dairy, then slowly reintroduce it to your diet. After avoiding all dairy for two to three weeks, you’ll begin eating certain products one at a time-usually butter, then hard cheeses, then yogurt or kefir. “That way you can really note what foods your digestive tract is comfortable with and your body tolerates well,” says Foroutan.

If you find that you are still intolerant to a certain dairy product, Foroutan says you can eliminate it for a few months until your digestive tract calms down enough, then try it again. If you still have symptoms when you re-introduce the food, try eliminating and re-introducing it one more time. “After three times of eliminating and reintroducing, if you still have symptoms, you’re better off avoiding it,” she says.

Can I prevent lactose intolerance?

As long as you’re not genetically predisposed to lactose intolerance, you should be able to digest lactose your whole life (barring illness or injury to your digestive tract), says Foroutan. And it's an important part of your diet: The USDA and Academy of Nutrition and Dietetics recommend aiming for three servings of dairy per day.

Cut dairy from your diet for an extended period of time, however, and you might wind up with sensitivities when you do eat it again. “If you don’t use it, you lose it,” says Foroutan. Even then, you can teach your body to tolerate dairy again. “You just have to go little by little,” she adds-following the same kind of re-introduction plan that you would for an elimination diet.

If, however, you are genetically intolerant to lactose, you probably won’t ever be 100 percent okay eating dairy products. “When you eat or drink anything that your body doesn’t digest well, it creates a lot of inflammation in the digestive tract, which in turn increases inflammation in your body. This is why it’s important to avoid the foods that you know trigger any kind of unpleasant reaction,” says Foroutan.

There’s no reliable test to tell whether you’re genetically intolerant to lactose, although lactose intolerance in close family members is a relatively reliable signal.

What should I do if I’m diagnosed with lactose intolerance?

Most people who are diagnosed with lactose intolerance can still enjoy up to eight ounces of dairy per day without symptoms, according to AND. Foroutan says most people do better with fermented dairy products, like yogurt and kefir, than liquid milk. Foroutan suggests experimenting with different types of milk: You may find that cow’s milk roils your tummy but not sheep’s or goat’s milk, or vice versa. Otherwise, popping a lactase pill may help keep symptoms under control.

If all dairy is off limits, never fear: Lactose-free milk and yogurt are required by the FDA to contain zero lactose. You can usually get all the calcium you need from other calcium-rich foods, like dark leafy greens, tahini, tofu, and sardines. And most of us-unless we live in a sunny southern state-need to take a vitamin D supplement to meet our needs whether or not we eat dairy, says Foroutan.