You sink your teeth into a grilled cheese from your nearest comfort food joint. That first bite leaves you parched, so you chase the umami with a cold glass of milk. On your way home, you treat yourself to some ice cream, a Friday reward after a successful week.

Unfortunately, all the joy in the world couldn’t compensate for the cramps, bloating and bathroom torture you’ll endure thanks to this dairy joyride. 

Sounds familiar, right? Despite the ubiquity of dairy at grocery stores and restaurants, tens of millions of people in the U.S. are lactose intolerant. Let’s explore what, exactly, the term means, what causes lactose intolerance and how to avoid hurting your gut too much in a dairy-rich world.  

What Is Lactose Intolerance?

Simply put, lactose intolerance refers to a person’s inability to properly digest, or break down, lactose, a sugar found in dairy products like cheese and ice cream, in addition to many processed foods, like cereal. Those with the condition may experience certain symptoms — like bloating, gas and diarrhea — after eating foods containing lactose. 

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Lactose is a disaccharide, meaning it is composed of two types of simpler sugars, glucose and galactose, that are bound together. These form long chains, or polymers, in food. But the unique structure of these glucose-galactose chemical bonds necessitates a specialized tool to properly snip these molecular ribbons apart. 

Enter lactase, an enzyme designed to do just that. And virtually every human on Earth carries the genes to create it. The real hangup comes from our body’s ability (or inability) to produce sufficient enough amounts to process those cheesy cheat days. 

What Causes Lactose Intolerance?  

In gastrointestinal tracts with functioning lactase, the inflow of lactose is broken down in the small intestine into easily absorbed sugars, clearing the way to the colon. In those with lactose intolerance, the undisturbed lactose makes its way into the large intestine, where a motley assortment of gut microbes makes eager work of it.  

Unfortunately, instead of generating byproducts that benefit our bodies, for those with lactose intolerance, these gut bacteria produce gas and acidic compounds, leading to bloating, cramps, excessive flatulence, and diarrhea.   

Most of these bacteria are fermenters, capable of digesting sugars like lactose anaerobically, or without oxygen. But, unfortunately for your stomach, the key byproducts of this reaction include hydrogen gas and acidic chemicals. Around 4 in 5 people produce hydrogen during this anaerobic digestion process, and a breath test can be used to assess the degree to which their dairy consumption has stimulated their gut flora. 

How To Test for Lactose Intolerance   

However, Jeffrey Nathanson, a gastroenterologist at Comprehensive Gastrointestinal Health in Northbrook, cautions against using breath tests as a de facto diagnostic tool. “1 in 5 people are methane producers,” Nathanson says. “Their gut flora are generating methane as a byproduct of their anaerobic digestion, not hydrogen.” 

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Nathanson says that simply assessing your symptoms as you age provides a decent proxy for determining whether or not you’re lactase deficient. Most people who are lactose intolerant usually grow into the condition, and very few individuals are born without lactase.   

This high lactase production in our formative years is an evolutionary adaptation to help us consume a diet of breastmilk as babies.  

Can You Become Lactose Intolerant As You Get Older?    

Nonetheless, around young adulthood, lactase production tends to drop off, which is when symptoms of lactose intolerance typically arrive. As a result, dairy doses that were formerly tolerated now produce numerous unpleasant lactose intolerance symptoms. 

In general, the condition is so widespread that around 60 to 70 percent of the global population is deficient in their production of lactase enzymes.  

Are There Other Causes of Lactose Intolerance?  

Still, symptoms of lactose intolerance are not always caused by a deficiency in lactase. Sometimes trauma, radiations, or infections can disrupt the  small intestine’s pathways and compromise its function. Other times, broader conditions like irritable bowel syndrome or celiac disease can exacerbate inflammation in the gut, leading to similar symptoms as with lactose intolerance.  

Nathanson recommends talking to your doctor if your symptoms are more serious due to dairy. “If you’re getting some red flags, like you’re anemic, losing weight, vomiting, get in touch with someone.” he says. “Not everything can be pinned to lactose intolerance, so there might be some secondary cause.”  

Can You Eat Dairy Products if You’re Lactose Intolerant?  

But for the rest of us, not wholly incapacitated but certainly unpleasantly affected by milk and cheese, we’ll simply have to accept our fate. (Mostly.) Research suggests that the best treatment for lactose intolerance is simply avoiding lactose altogether.  

Still, not all dairy products are created equal. Milk might be off the table for some, at least in moderate doses, but Nathanson says that options like live-culture yogurt are more viable. The living probiotic culture of microbes in these products helps the sugar go down more easily, since the bacteria produce their own supplemental lactase.  

Read More: Why Do We Eat Eggs or Cereal for Breakfast and How Many Eggs Should We Eat in a Day?

That’s the same reason some people with lactose intolerance find unpasteurized milk easier to digest. Pasteurization heats the milk to kill off the resident bacteria, sterilizing it, but these microbes are believed to produce their own lactase enzymes that can facilitate digestion.  

Lactase supplements are another option, delivered to the body in capsule form. However, these capsules are sometimes not as effective as naturally elevated lactase production, since the release of the capsule’s cargo must be precisely timed to its entry in the small intestine. 

Nathanson says that there still is hope for people with lactose intolerance — specifically, that individuals can consume dairy in small amounts to build up their tolerance. “It is possible for the GI tract to naturally acclimate to lactose,” he says. “It won’t let you eat a full load, but it will raise the threshold, enough so that symptoms will be less intense.”  

Frequently Asked Questions About Lactose Intolerance 

How Long Do Lactose Intolerance Symptoms Last?  

Symptoms of lactose intolerance typically kick in within a couple hours after consuming food or dairy products that contain lactose. These effects are a result of lactase buildup in your lower intestine that’s not properly broken down. Since it can take up to 10 hours for food to travel to your large intestine, and another 24 to 36 hours afterwards to travel through your gut, symptoms may continue up to a day or two after eating lactose.  

Can Lactose Intolerance Cause Constipation?  

Lactose intolerance can cause digestive woes like abdominal pain, bloating, and diarrhea, as well as, less frequently, nausea and vomiting. And constipation is indeed a hallmark; it’s a symptom in approximately 30 percent of cases, according to a study published in Nutrients in 2022.  

What Does Lactose Intolerance Feel Like?  

For those with lactose intolerance, symptoms can manifest as uncomfortable bloating in the abdomen, nausea and stomach rumbling, and sharp gas pain ending in diarrhea. However, the severity of all these symptoms is dose dependent, meaning that the amount of lactose consumed, and the amount of lactase produced by the body, are important factors. 

Can People Cure Lactose Intolerance By Drinking Milk?  

Gastroenterologists used to recommend completely avoiding lactose-containing foods, but a growing body of research suggests that the digestive system can be conditioned to tolerate them. A literature review found that progressively higher doses of lactose can adapt the gut, and its corresponding microbiome, to better digest lactose, according to a 2015 meta-analysis.

Depending upon the individual, this can almost completely mitigate symptoms, so long as they consume increasing amounts of lactose in a controlled, steady manner. 

Read More: Is Milk Bad for You? Here’s What the Science Says

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