Lactose Intolerance: How to Tell and What to Do About It

Lactose intolerance is that rare case where we call a "disorder" something most of humanity has. Roughly a third of the world's population can digest milk sugar as adults; the other two thirds are evolutionarily normal, if occasionally inconvenienced. Here is how to tell intolerance from allergy and from IBS, how to test for it, and why cutting out dairy entirely is nearly always overkill.

How it works#

Lactose is a disaccharide of glucose and galactose, the main carbohydrate in milk. To be absorbed it must be split by lactase, an enzyme produced by the brush border cells of the small intestine.

In every mammal, lactase activity falls after weaning — the enzyme is no longer needed. The same happens in humans, but around ten thousand years ago a mutation in a regulatory region of the MCM6 gene spread through populations that had taken up dairy farming, keeping lactase active for life. The condition is called lactase persistence.

Its prevalence varies sharply by region: around 90% in northern Europe, 60–70% in northern India, under 10% in East Asia and among indigenous American populations. The global average for adults with lactose malabsorption is about 68%.

Unsplit lactose behaves as a classic FODMAP: it draws water into the bowel lumen osmotically and ferments rapidly in the colon, producing hydrogen, carbon dioxide and short-chain fatty acids. Hence the whole symptom set — bloating, rumbling, cramps, loose stool, flatulence. They usually appear 30 minutes to two hours after eating.

What it is not#

Condition	Mechanism	Symptoms	Approach
Lactose intolerance	Lactase enzyme shortage	Bloating, diarrhoea, cramps; dose-dependent	Limit the dose, do not eliminate
Cow's milk protein allergy	Immune reaction (IgE or non-IgE)	Rash, swelling, vomiting, anaphylaxis in severe cases	Complete avoidance, allergist supervision
IBS	Visceral hypersensitivity, motility	Pain linked to defecation, altered stool	Multimodal management, possibly low-FODMAP
Coeliac disease	Autoimmune reaction to gluten	Diarrhoea, weight loss, anaemia, deficiencies	Lifelong gluten-free diet

Confusing intolerance with allergy is expensive in both directions. Someone with an allergy who believes they merely have intolerance risks a serious reaction. Someone with intolerance who believes they have an allergy spends years avoiding all dairy unnecessarily and loses calcium doing it.

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Testing#

Hydrogen breath test. The standard. After a lactose solution, hydrogen concentration in exhaled air is measured: if the lactose was not absorbed, bacteria ferment it and hydrogen rises. It is non-invasive but requires preparation and returns false negatives in people whose microbiota produces methane instead of hydrogen.

Genetic test. Identifies the variant in the MCM6 regulatory region. It shows predisposition to primary hypolactasia but not your current state and not secondary intolerance.

Elimination challenge. The most accessible method: remove lactose for two weeks, then reintroduce and observe. It works but is vulnerable to expectation effects, so keep a diary and check whether symptoms scale with dose.

One thing not to do is order an IgG "food intolerance panel". IgG antibodies to food proteins reflect exposure rather than intolerance, and allergy societies explicitly advise against these panels.

How much lactose is fine#

The key fact usually lost in the conversation: intolerance is dose-dependent. Systematic reviews find that most people with malabsorption tolerate up to 12 g of lactose in a single sitting without symptoms — roughly a glass of milk. Taken with food, or split across the day, the tolerated dose is higher still.

Food	Portion	Lactose	Usually tolerated
Milk	200 ml	9–10 g	Often yes, particularly with food
Kefir, yoghurt	200 ml	4–6 g	Generally yes: starter bacteria pre-digest some lactose
Cottage cheese	100 g	2–3 g	Yes
Hard cheese (parmesan, cheddar)	30 g	under 0.1 g	Almost always
Butter	10 g	under 0.1 g	Yes
Ice cream	100 g	5–6 g	Often yes
Condensed milk	50 g	6 g	Depends on the dose

Practical tactics that widen tolerance: drink milk with a meal rather than on an empty stomach; split portions; choose fermented products; and increase the dose gradually — the microbiota adapts and begins fermenting lactose more efficiently without pronounced symptoms. Lactase tablets taken before eating also work.

Читайте также: Fermented Foods: What Cultured and Pickled Foods Actually Do

The real risk is calcium, not milk#

Dairy supplies a substantial share of daily calcium, and in some diets most of it. Removing it entirely without replacement buys not only a quieter stomach but a risk of reduced bone mineral density across the following decades.

Substitution is possible, but do it deliberately:

• fortified plant drinks — check the label: unfortified almond milk holds a fraction of the calcium in cow's milk;
• tinned sardines and other fish with bones — very high bioavailability;
• tofu set with calcium salts — check the ingredient list;
• sesame and tahini, almonds — good sources, though portions are small;
• broccoli, kale, bok choy — calcium absorption here actually exceeds that from milk;
• spinach is a poor choice: plenty of calcium, but oxalates block its absorption.

Читайте также: Calcium: How Much You Need, Where to Get It, Who Needs Supplements

Secondary intolerance#

A distinct scenario: lactase was present and disappeared after damage to the small intestinal lining. Causes include gut infection (rotavirus and giardiasis especially), coeliac disease, Crohn's disease, radiotherapy and prolonged antibiotic courses.

The difference matters: secondary intolerance is reversible. Once the mucosa heals, lactase activity returns, usually over weeks or months. So if intolerance appeared abruptly after a gut infection, it is sensible to restrict lactose temporarily and retest tolerance in two or three months, rather than filing yourself as permanently intolerant.

Sudden onset in an adult is also a reason to rule out coeliac disease, particularly alongside weight loss, anaemia or iron deficiency.

The bottom line#

Lactose intolerance is not a disease but a state shared by most adults on the planet. Symptoms scale with dose, and the overwhelming majority handle up to 12 g of lactose per sitting, especially with food.

Distinguish it from cow's milk protein allergy, where the mechanism is immune and the approach is complete avoidance. And remember that the main risk of dropping dairy is not the absence of milk but a calcium shortfall: if you remove it, replace it deliberately.