Why Are Some People Lactose Intolerant? The Fascinating Science Behind Digesting Milk
Key takeaways:
Lactose intolerance is when the body has an issue digesting lactose, a sugar found in milk and other dairy products.
People with lactose intolerance may experience gas, bloating, and diarrhea when they consume milk or other dairy products.
Lactose intolerance doesn’t mean you have to give up dairy entirely. You can choose lactose-free foods or use lactase enzymes.
If you get bloated or gassy when you drink milk or eat ice cream, there is a chance you may have lactose intolerance. But you wouldn’t be alone. Lactose intolerance affects up to 70% of the world's adult population. It’s more common in places where milk or dairy farming has historically not been prevalent.
The good news is that having lactose intolerance doesn’t mean your dairy-eating days are over.
What causes lactose intolerance?
Lactose intolerance happens when you don’t have enough of an enzyme called lactase. This enzyme helps break down lactose, which is a sugar found in milk. If lactose passes through the intestines when no enzyme is present, it can cause gas, bloating, and diarrhea.
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There are three different kinds of lactose intolerance:
Congenital lactase deficiency: This is diagnosed at birth and is quite rare.
Primary lactose intolerance: This is the most common type of lactose intolerance and is often diagnosed in childhood.
Secondary lactase deficiency: This happens due to an infection or disease, such as celiac disease or Crohn’s disease. It can occur at any age and tends to be short-lived rather than permanent.
Lactose intolerance can develop at any age, but it is most commonly seen in children. Human breast milk contains lactose, and babies begin life with a natural ability to digest it. Infants produce lots of lactase enzymes, but this decreases as they grow. Once babies start to replace breast milk and cow’s milk with a variety of other foods, they make less lactase.
You are more likely to be lactose intolerant if you have a parent who is. But most people with lactose intolerance can consume some amount of lactose without having symptoms.
History and evolution of lactose intolerance
Researchers say that humans started drinking milk as far back as 10,000 years ago. They predict that lactose intolerance likely started more than 4,000 years ago due to a genetic mutation in the LCT gene. This gene provides the body with instructions on how to make lactase enzymes.
About 5,000 years ago, Northern Europeans experienced a genetic change that made milk easier to digest. (By recent estimates, 95% of Northern Europeans can now digest milk.) Scientists think that this early ability to tolerant lactase may have developed as a result of:
Famine
Drought
Harmful pathogens
Poor hygiene
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It's likely that those who were able to tolerate lactose were able to survive these situations and pass on the ability to digest milk to the next generation.
Which groups of people suffer from lactose intolerance?
Though it’s not completely clear why, lactose intolerance seems to affect people who live in parts of Asia, Africa, and South America the most:
More than 50% of people in South America, Africa, and Asia have lactose intolerance.
People of East Asian descent are the most likely to have lactose intolerance as adults, with 70% to 100% of this population affected.
Lactose intolerance is very common in people of West African, Arab, Greek, and Italian descent.
Some scientists think that people whose ancestors raised cattle or consumed milk for thousands of years may tolerate lactose better. But not all historically milk-drinking populations are tolerant to lactose.
Which foods contain lactose?
In general, milk and products made from milk contain lactose. These dairy products include:
Milk
Cream
Ice cream
Fresh cheese
Cottage cheese
Cream cheese
Whey
Milk powder
Whipped cream
Some yogurts
Foods made with these ingredients — like certain salad dressings, bread, and baked goods — also contain lactose.
Are there foods that contain lactose that lactose intolerant people can eat?
People with lactose intolerance can digest different amounts of lactose. Some people can consume up to 1 cup of milk, which contains 12 grams of lactose, with mild symptoms or no symptoms. Other people react to smaller amounts of lactose.
Fermented dairy products contain less lactose. This is because the lactose is broken down in the fermenting process. Fresh cheese, such as ricotta, is high in lactose. Fermented cheeses — such as Parmesan, cheddar, and Swiss — are low in lactose. Fermented yogurt and kefir are often low in lactose.
When to talk to a doctor about lactose intolerance
If you get gas and bloating after having milk products, talk to your healthcare provider. They can find out if you have lactose intolerance with one of these two tests:
Hydrogen breath test: This test requires you to drink milk and then breathe into a machine that measures hydrogen levels. If you breathe out too much hydrogen, you aren't fully digesting lactose.
Lactose tolerance test: This involves having the amount of glucose in your bloodstream measured after you drink milk. Normally, the body breaks down lactose into smaller sugars (glucose and galactose). If your glucose level doesn’t rise after you drink milk, it means your body isn’t properly digesting lactose.
Can lactose intolerance go away?
Lactose intolerance isn’t something that can be cured, but it can be managed. Here are some tips to help prevent the symptoms of lactose intolerance when you consume dairy:
Assess your tolerance level. You may be able to consume a small amount of lactose with no symptoms.
Use lactase enzymes. You can consider taking these enzyme supplements before you consume high-lactose foods.
Try naturally low-lactose dairy products. These include fermented cheese and yogurt.
Choose lactose-free dairy products. Generally, these are milk products that have had lactase enzymes added to them. If you’re not able to handle foods that contain lactose, these products are an option.
Some people with lactose intolerance find that avoiding dairy altogether is a better option for them. In this case, make sure you replace dairy foods with plant-based options that provide calcium, protein, and vitamin D. A dietitian or nutritionist can help you plan a balanced diet that works for you.
The bottom line
While lactose intolerance has been around for thousands of years, it’s still unclear why it affects certain populations more than others. What we do know is that, even if you can’t digest lactose, it doesn’t necessarily mean you need to be dairy-free. If you are lactose intolerant, talk with a healthcare provider about trying lactase enzymes or choosing low-lactose or lactose-free foods to ease your symptoms.
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References
Dairy Australia. (2021). What is the lactose content of different dairy products?
Di Costanzo, M., et al. (2018). Lactose intolerance: Common misunderstandings. Annals of Nutrition and Metabolism.
Evershed, R. (2022). Dairying, diseases and the evolution of lactase persistence in Europe. Nature.
GI Society. (n.d.). Lactose intolerance.
Mayo Clinic. (n.d.). Lactose intolerance.
MedlinePlus. (2010). Lactose intolerance.
National Institute of Diabetes and Digestive and Kidney Diseases. (2018). Definition & facts for lactose intolerance.
National Institute of Diabetes and Digestive and Kidney Diseases. (2018). Eating, diet, & nutrition for lactose intolerance.
ScienceDirect. (n.d.). Lactose-free dairy products.
Segurel, L., et al. (2020). Why and when was lactase persistence selected for? Insights from Central Asian herders and ancient DNA. PLOS Biology.
