Lactose intolerance happens when the small intestine makes too little lactase, the enzyme that digests the sugar in milk. Undigested lactose travels on to the large intestine, where it draws in water and is fermented by gut bacteria, and the result is bloating, wind, cramps and loose stools in the hours after dairy. Making less lactase in adulthood is the normal pattern for most of the world's population rather than a fault, and nearly everyone affected can still manage some lactose. This guide covers why lactase declines, how lactose intolerance differs from a milk allergy, how much lactose most people can handle, how it is investigated in the UK, and where other digestive enzyme shortfalls such as sucrase-isomaltase deficiency fit in.
What does lactase do?
Lactase sits on the lining of the small intestine and splits lactose, the sugar in milk, into two smaller sugars the body can absorb. When there is enough of the enzyme, milk is digested before it gets any further and causes no trouble.
When lactase is low, some of the lactose passes through undigested, and two things follow. The unabsorbed sugar pulls water into the bowel, which loosens stools. Further along, gut bacteria ferment it, producing gas that stretches the bowel and causes bloating, wind and cramping pain. A 2021 review describes this combination of water shift and fermentation as the whole mechanism behind the symptoms. It also explains their timing. Fermentation takes hours, so symptoms usually build gradually after a meal containing dairy rather than appearing at once, the delayed pattern typical of food intolerance symptoms in general.
How strongly someone reacts varies widely, even between two people with the same low lactase level. The amount of lactose eaten, the gut bacteria doing the fermenting, and how sensitive a person's gut is to stretching all shape the outcome.
Why do most adults make less lactase?
Almost every baby makes plenty of lactase, because milk is the entire diet in infancy. In most people worldwide, the gene that drives lactase production then winds down after early childhood. This is called lactase non-persistence, and it is not a disease or a deficiency in any meaningful sense. It is the original human pattern.
According to a 2026 review, around 70% of the world's adults are lactase non-persistent, with figures typically between 50% and 90% across Africa, Asia and South America. North-western Europe, including Britain, sits at the other end of the range. A genetic change that keeps lactase switched on into adulthood spread through populations with a long history of dairy farming, which is why lactose intolerance is less common here than in most of the world, as a 2019 review in the journal Gut sets out.
Low lactase does not automatically mean symptoms. Plenty of people who absorb lactose poorly feel nothing at everyday intakes. Lactose intolerance means the combination of the two, low lactase plus symptoms after lactose, and it is the symptoms that matter, not the enzyme level on its own.
Is it lactose intolerance or a milk allergy?
Milk can cause problems in two entirely different ways, and telling them apart matters. Lactose intolerance is a digestion problem with the sugar in milk. A milk allergy is an immune reaction to the proteins in milk, mainly casein and beta-lactoglobulin. The gut symptoms can look similar, and a 2024 review notes how often the two are mixed up, which leads to the wrong foods being restricted.
Allergy carries risks that intolerance never does. Call 999 if you or your child develops swelling of the lips, tongue or throat, difficulty breathing or wheezing, a widespread rash, or faintness after eating. These are signs of anaphylaxis and need emergency treatment, not a GP appointment.
Cows' milk allergy is most common in babies and young children, and that age group is clinical territory. If you think your baby reacts to milk, speak to your GP or health visitor. Diagnosis in infants belongs with them, never with any home test, and any change to how a baby is fed should only happen with their guidance.
When lactose intolerance follows another gut problem
Lactase lives on the surface of the gut lining, so anything that damages that lining can push its levels down. This is called secondary lactose intolerance, and it can follow a bout of gastroenteritis, or come with conditions such as coeliac disease and Crohn's disease. A 2022 review found that lactase levels are commonly reduced in untreated coeliac disease and that lactose can be behind some of the symptoms that linger in bowel conditions otherwise in remission.
The useful difference from the inherited form is that secondary lactose intolerance often improves. As the gut lining recovers, whether after an infection clears or once a condition is treated, lactase production can return, and dairy that caused trouble for a few months may become manageable again.
One caution sits alongside the coeliac point. Coeliac testing only works while gluten is still being eaten, so if your symptoms involve bread and pasta as well as dairy, do not cut gluten out before your GP has tested for coeliac disease. What happens if wheat still causes trouble after a negative coeliac test is a gluten intolerance question rather than a lactose one.
There is also a congenital form, where a baby is born making almost no lactase at all. It is very rare, it shows up in the first days of life, and it is managed by hospital specialists from the start.
How much lactose can you have?
Lactose intolerance is dose dependent, not all or nothing. A systematic review of 36 trials concluded that most people with lactose malabsorption can manage 12 to 15 grams of lactose in one sitting, which is roughly a glass of milk, especially when it comes with other food or is spread across the day.
The lactose content of dairy foods also varies enormously. Hard cheeses such as cheddar and parmesan contain almost none, because the lactose is removed with the whey and broken down during ageing. Butter carries very little. Live yoghurt is often better tolerated than milk, since its bacterial cultures digest part of the lactose themselves. Milk, soft cheeses, cream and ice cream sit at the higher end. Most people with lactose intolerance end up reducing and choosing rather than removing dairy altogether, and for most that is enough to control symptoms.
What about other enzyme deficiencies?
Lactase is the best known digestive enzyme, but it is not the only one that can run short. Sucrase-isomaltase breaks down sucrose, ordinary table sugar, and parts of starch. When it is deficient, sugary and starchy foods cause the same fermentation-driven bloating, wind and diarrhoea that lactose causes in lactose intolerance. The congenital form is rare and usually found in childhood, but a 2023 review reports that milder enzyme shortfalls in adults are more common than previously thought, and that some people diagnosed with IBS who do not improve on a low-FODMAP diet turn out to have one. In the UK this is specialist territory, reached through a GP referral when symptoms persist without an explanation.
Histamine intolerance is also linked to an enzyme, diamine oxidase, but it works differently enough to need its own explanation, and we cover it separately. Enzyme shortfalls sit alongside several other mechanisms that shape what a food intolerance is, from chemical sensitivities to fermentable carbohydrates.
How is lactose intolerance investigated in the UK?
Start with your GP if symptoms after dairy keep returning. The NHS approach to lactose intolerance usually begins with the simplest step, a short period without lactose to see whether symptoms improve, done with medical advice rather than as a permanent diet change. Reintroducing lactose afterwards matters as much as removing it, because improvement while off dairy and a return of symptoms on rechallenge is what actually points to lactose. If symptoms carry on unchanged without dairy, lactose is unlikely to be the cause, and both bloating after eating and diarrhoea after eating have several other explanations.
The main formal test is the hydrogen breath test. After a measured dose of lactose, the amount of hydrogen in your breath is tracked, because hydrogen is a product of bacterial fermentation, and a rise means lactose reached your gut bacteria undigested. It is accurate, non-invasive and widely used, though availability on the NHS varies by area. Current international practice treats a positive breath test together with typical symptoms as the basis for the diagnosis, not either one alone. Blood tests can be used too, and in severe or unclear cases a gastroscopy with a small tissue sample from the small intestine settles the question, although few people need one. Genetic tests for lactase persistence exist but are not part of routine NHS practice.
What can a food intolerance test show?
Our at-home food intolerance test measures IgG antibodies to around 230 foods and drinks, and for dairy that includes the individual milk proteins, alpha-lactalbumin, beta-lactoglobulin and casein. An elevated IgG result to milk proteins may indicate a food sensitivity to milk, meaning your immune system is producing antibodies against the milk proteins themselves.
That is a different thing from lactose intolerance, and the test's own report says plainly that it cannot diagnose lactose intolerance. No antibody test can, because lactose intolerance is about an enzyme and a sugar, and there is no immune reaction involved to measure. What the panel adds is the other half of the picture. If dairy reliably causes symptoms but a breath test is negative, or symptoms do not track the lactose content of foods, the question becomes whether your body is reacting to milk proteins rather than failing to digest milk sugar, and that is the question an IgG result speaks to. Our clinical team interprets results in exactly those terms, a sensitivity to milk proteins on one side, a lactase shortfall on the other, because the two point to different foods being the problem and different ways forward.
Before you cut out dairy
Dairy is the main source of calcium in the UK diet, along with iodine and vitamin B12, and removing it wholesale is not a neutral change. Long-term restriction without support can leave bone health short of the calcium it needs, which matters most for teenagers, women around the menopause and older adults.
Since most people with lactose intolerance tolerate a meaningful amount of lactose, reduction usually beats removal. Where dairy does need to come down, replacing what it provided matters, for example with calcium-fortified milk alternatives, and cutting out a whole food group is a step to take with guidance from your GP or a registered dietitian rather than alone. Lactase supplements taken with dairy help some people, and a pharmacist can advise whether they are worth trying. If a child's diet is involved, that guidance is not optional. Growing bodies have the least room for a nutritional gap.
When should you see a GP?
Symptoms that follow dairy and settle when you eat less of it fit the pattern of lactose intolerance. Some symptoms do not fit it and need a GP appointment whatever else is going on. Book one for blood in your stool, a change in bowel habit lasting more than three weeks, unexplained weight loss, bloating that is there most days for three weeks or more, difficulty swallowing, persistent vomiting, iron-deficiency anaemia, gut symptoms starting for the first time over the age of 50, or a family history of bowel or ovarian cancer.
In babies and young children, the warning signs are different and more urgent. Poor weight gain, blood in the stool, persistent vomiting, refusing feeds, unusual sleepiness or fewer wet nappies all need same-day advice from a GP or health visitor.
This guide is for information only and does not replace medical advice. Speak to your GP or pharmacist about your own symptoms.
Sources
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- Catanzaro R, Sciuto M, Marotta F (2021). Lactose intolerance: An update on its pathogenesis, diagnosis, and treatment. Nutrition Research. PubMed | DOI
- Usai-Satta P, Lai M, Oppia F (2022). Lactose Malabsorption and Presumed Related Disorders: A Review of Current Evidence. Nutrients. PubMed | DOI
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- Shaukat A, Levitt MD, Taylor BC, et al. (2010). Systematic review: effective management strategies for lactose intolerance. Annals of Internal Medicine. PubMed | DOI
- Darma A, Sumitro KR, Jo J, Sitorus N (2024). Lactose Intolerance versus Cow's Milk Allergy in Infants: A Clinical Dilemma. Nutrients. PubMed | DOI
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