Why Lactose Causes Problems for So Many People Around the World

Lactose: a harmless sugar… but only for some

A glass of milk in the morning, an ice cream after dinner — for many people, these simple pleasures lead to digestive discomfort. Bloating, cramps, diarrhoea: the symptoms are very real, and they affect a surprisingly large proportion of the world's population. Around 65 to 75% of adults have a reduced ability to digest lactose after childhood. So what exactly is happening in our gut?

What is lactose, and why does it cause problems?

Lactose is a disaccharide — a sugar made up of two molecules: glucose and galactose. It is found primarily in milk and dairy products. To be absorbed by the body, it must be broken down into its two components by an intestinal enzyme called lactase, produced in the small intestine.

In people who are intolerant, this enzyme is present in insufficient quantities — a condition known as hypolactasia. Undigested lactose then travels onwards to the colon, where the gut microbiota steps in. It ferments this sugar, producing gases (hydrogen, methane) and lactic acid, whilst also drawing in water through osmosis. The result: abdominal bloating, flatulence, pain, nausea, and sometimes diarrhoea.

An intolerance that is often natural, not a disease

It is important to understand that primary lactose intolerance — the most common form — is not a disease. It is an entirely normal physiological development. After weaning, the bodies of most humans gradually reduce lactase production. This regulation is encoded in our genes, at the level of the LCT gene.

Historically, only populations that have kept dairy animals for thousands of years — particularly in Northern Europe — developed a genetic mutation allowing lactase production to continue into adulthood. This phenomenon is known as lactase persistence, and it is far from universal.

There is also a secondary form of intolerance, caused by damage to the intestinal lining following an infection, coeliac disease, or chemotherapy. In such cases, the underlying cause must be treated first.

Prevalence varies greatly depending on background

The figures clearly illustrate this genetic and cultural dimension:

• East Asia (Japan, China): 70 to 100% of the population affected
• Africa and Latin America: 70 to 90%
• United States: approximately 52% (with disparities across ethnic backgrounds)
• France: approximately 37%
• Northern Europe (Germany, United Kingdom): 15 to 20%

These figures show that lactose intolerance is the global norm, not the exception. Yet with the worldwide spread of dietary habits and the rise of dairy consumption in regions that have traditionally had little exposure to milk, symptoms are increasing among populations that were never genetically prepared for it.

Which foods should you watch out for first?

Not all dairy products contain the same amount of lactose. The most problematic include:

• Milk (around 12 g of lactose per glass, often the critical threshold)
• Unfermented yoghurts and dairy desserts
• Fresh cheeses (ricotta, cottage cheese, mozzarella)
• Ice cream and frozen desserts
• Certain processed foods containing whey or milk powder (industrial breads, sauces, breakfast cereals)

Aged cheeses, on the other hand (comté, parmesan, mature cheddar), naturally contain very little lactose and are often well tolerated.

Managing intolerance in everyday life

The good news is that complete avoidance of dairy products is not always necessary. Current medical guidance recommends a gradual, personalised approach:

• Identify your tolerance threshold: many people can handle up to 12 g of lactose per day without significant symptoms
• Consume dairy products as part of a solid meal, which slows transit and reduces symptoms
• Use lactase supplements before meals containing lactose
• Incorporate probiotics (particularly strains of Bifidobacterium), which help ferment lactose more efficiently in the colon
• Opt for lactose-free products or calcium-enriched plant-based alternatives (oat drinks, almond drinks, etc.)

If in doubt, the hydrogen breath test is the gold-standard examination for confirming a diagnosis. A genetic test or an elimination-reintroduction protocol may also be considered with the help of a healthcare professional.

What your microbiota has to say

The gut microbiota plays a central role in how the body handles lactose. Certain intestinal bacteria are able to ferment lactose more or less gently, influencing the intensity of symptoms. This is why two people with the same level of lactase activity can experience intolerance very differently. Looking after your microbiota — through a varied diet rich in fibre and fermented foods — can therefore make a real difference.

Lactose intolerance is a benign, extremely common, and very manageable condition. Understanding how it works is already a significant step towards a more comfortable digestive life.