Lactose Intolerance: How to Know If You're Affected and How to Adjust Your Diet

What exactly is lactose intolerance?

Lactose is the sugar naturally found in milk. To digest it, your small intestine produces an enzyme called lactase (or β-galactosidase), which breaks lactose down into two simple sugars — glucose and galactose — that are easily absorbed into the bloodstream.

When this enzyme is absent or insufficient, undigested lactose travels on to the colon. There, gut bacteria ferment it, producing gases (hydrogen, methane) and acids that irritate the gut lining and draw in water. The result: bloating, cramps, flatulence, and diarrhoea — the classic signs of intolerance.

This process is typically triggered once lactase activity drops by at least 50% below normal levels.

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Why does lactase decline?

There are several forms of lactose intolerance:

• Primary (the most common): lactase production naturally decreases after early childhood, from around 2 to 5 years of age, according to a programme encoded in the LCT gene on chromosome 2. Some adults retain this ability thanks to a genetic mutation historically linked to cattle farming — known as "lactase persistence", which is particularly common in Northern Europe.
• Secondary (acquired): temporary or lasting intestinal damage — from gastroenteritis, Crohn's disease, or coeliac disease — can harm the cells that produce lactase. In these cases, treating the underlying cause is often enough to restore tolerance.
• Congenital (very rare): a complete absence of lactase from birth, diagnosed in infancy.

Globally, more than 60 to 70% of adults show some degree of lactose malabsorption. Prevalence is very high in East Asia and sub-Saharan Africa, and considerably lower in Northern Europe due to genetic adaptation. If both your parents are lactose intolerant, your own risk is significantly higher.

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How do you know if you're affected?

Symptoms to look out for

Symptoms typically appear 30 minutes to 2 hours after consuming dairy products:

• Bloating and wind
• Abdominal pain or cramps
• Diarrhoea or loose stools
• Occasional nausea

These symptoms closely resemble those of irritable bowel syndrome (IBS), which can sometimes complicate diagnosis.

Available tests

• Hydrogen breath test: after consuming a measured dose of lactose, the amount of hydrogen exhaled is measured — a by-product of bacterial fermentation. This is the diagnostic gold standard.
• Genetic test: identifies variants of the LCT gene associated with lactase non-persistence, useful for confirming a hereditary primary form.
• Elimination and reintroduction test: remove dairy products for 2 to 4 weeks, observe whether symptoms disappear, then reintroduce to confirm the link.

⚠️ Be careful not to confuse lactose intolerance (digestive, enzymatic) with cow's milk protein allergy (immunological, rarer in adults). If in doubt, consult a gastroenterologist.

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How to adapt your diet without cutting everything out

Good news: a total and permanent elimination of dairy is not always necessary. Lactose also plays a prebiotic role, encouraging the growth of bifidobacteria in the colon.

High-lactose foods to watch

• Cow's, goat's, and buffalo milk (approximately 4 to 5 g per 100 ml)
• Fresh and soft cheeses: ricotta, mozzarella (> 10 g per 100 g)
• Unfermented yoghurts, some dairy-based desserts
• Processed foods: shop-bought bread, biscuits, cured meats, milk chocolate — often enriched with whey

Better-tolerated options

• Aged cheeses (cheddar, Parmesan, Comté): the ageing process breaks down lactose, reducing it to below 0.5 g per 100 g
• Fermented yoghurts: lactic acid bacteria produce their own lactase, aiding digestion
• Butter and cream: contain only trace amounts of lactose, generally well tolerated

Practical day-to-day strategies

• Spread your intake: small amounts throughout the day are better tolerated than one large portion in one go
• Pair dairy with a meal: slowing gastric emptying gives digestion more time
• Use exogenous lactase (enzyme supplements available from pharmacies) before a meal containing lactose
• Choose lactose-free milk: the lactose is pre-hydrolysed by the addition of lactase — slightly sweeter in taste but much better tolerated
• Explore plant-based alternatives (almond or oat drinks) whilst checking for any problematic additives

Don't overlook calcium

Reducing dairy without compensation can affect your intake of calcium and vitamin D. Think tinned sardines with bones, leafy green vegetables (kale, broccoli), nuts and seeds — and if necessary, a supervised supplement.

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In summary

Lactose intolerance is a widespread biological reality, but it does not necessarily mean eliminating every trace of dairy from your diet. Understanding your own level of tolerance, trialling lower-lactose foods, and adopting a few simple habits is often enough to achieve lasting digestive comfort — without unnecessary frustration.