Why are some people lactose intolerant: Reasons and symptoms explained

If you have ever wondered why a friend can drink a full glass of milk without blinking while you feel bloated, gassy or uncomfortable after just a few sips, you are definitely not alone. Lactose intolerance is extremely common, and millions of people experience digestive discomfort after consuming milk or dairy products. This has nothing to do with poor digestion or lifestyle habits. It is largely linked to how your body handles lactose, the natural sugar found in milk. When your system struggles to break it down, symptoms begin. Understanding why this happens helps you manage your diet wisely and avoid unnecessary discomfort.

A peer-reviewed global analysis published in The Lancet Gastroenterology & Hepatology found that around 68 per cent of the world’s population has lactose malabsorption, meaning their bodies do not efficiently digest lactose.


The researchers noted that lactose intolerance is strongly influenced by genetics, ethnicity, age and gut health. This means some people are born more likely to become lactose intolerant, while others develop it gradually over time due to changes in enzyme levels or gut conditions.

Reasons for lactose intolerance in people explained

Why some people are lactose intolerant due to genetics

One of the main reasons why some people are lactose intolerant is genetic variation. Humans naturally produce lactase (the enzyme needed to digest lactose) during infancy. However, for most populations worldwide, lactase levels drop sharply after childhood. This decline is called lactase non-persistence. Only people with a specific mutation on the LCT gene continue producing lactase into adulthood. Those who lack this mutation are far more likely to experience symptoms when they consume dairy.

Genetics also explains why lactose intolerance is more common in certain groups. It affects around 65 to 95 per cent of adults in East Asia, Africa, South America, the Middle East, and South Asia, but only around 5 to 20 per cent of Northern Europeans due to historical adaptation to dairy consumption.

Lactose intolerance due to age-related enzyme decline

Even individuals who digested dairy comfortably in childhood can become lactose intolerant as they age. Lactase production naturally decreases over time, which can lead to bloating, diarrhoea or gas after eating dairy products. This change often happens slowly, so people may not immediately realise dairy is the culprit. Age-related lactose intolerance is completely normal and not a sign of poor health. Symptoms may fluctuate daily and often feel worse after larger meals or during stressful periods when digestive function becomes slower.

People who are lactose intolerant because of gut health issues

Your gut plays a significant role in lactose digestion. Conditions that damage the small intestine can temporarily or permanently reduce lactase production. These include coeliac disease, Crohn’s disease, severe gastroenteritis, parasitic infections and long-term antibiotic use. This form is known as secondary lactose intolerance. Once the underlying condition is treated and the gut heals, lactose tolerance often improves.

Dietary habits that influence lactose intolerance

Dietary habits influence how well your body handles lactose. People who regularly consume small amounts of dairy sometimes tolerate lactose better because the digestive system adapts. On the other hand, complete avoidance of dairy for long periods may reduce tolerance even further. This does not work for every individual, but exposure can make a difference for some.

Fermented dairy products such as yoghurt or kefir contain probiotics and partially broken-down lactose, making them easier for many lactose-intolerant people to digest. Hard cheeses also contain very little lactose and may be tolerated better.

Symptoms of lactose intolerance

• People with lactose intolerance often report bloating, cramps, nausea, excessive gas, diarrhoea or rumbling stomach after consuming dairy.
• The severity depends on how much lactase the body lacks and how much lactose is eaten.
• Some individuals can tolerate small quantities, such as milk in tea, while others react strongly to even small amounts.

Lactose intolerance is extremely common and completely normal. Genetics, age, diet, gut health and environmental factors all play a role in how well your body digests lactose. The good news is that it is manageable. Many people live comfortably by choosing lactose-free milk, eating smaller dairy portions, taking lactase supplements or choosing fermented dairy products. Understanding why some people are lactose intolerant helps you make informed decisions and enjoy food without discomfort.


Disclaimer: This article is for general informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider regarding any medical condition or lifestyle change.


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