If you regularly experience bloating, wind, cramps or diarrhoea after milk, cheese or ice cream, lactose intolerance is a likely explanation. It is extremely common worldwide — and, importantly, it is not a dairy allergy. Here is what causes it, how it is diagnosed, and how to manage it without giving up everything you enjoy.
What is lactose intolerance?
Lactose is the main sugar in milk and dairy products. It is a disaccharide, made of two smaller sugars — glucose and galactose — joined together.
To absorb it, your small intestine produces an enzyme called lactase, which splits lactose into its two component sugars. If you do not produce enough lactase, undigested lactose passes into the large intestine, where gut bacteria ferment it. That fermentation produces gas and draws water into the bowel — which is exactly why the symptoms are what they are.
Intolerance is not allergy
This distinction matters. Lactose intolerance is a digestive problem caused by an enzyme shortfall. Cow's milk protein allergy is an immune reaction, can cause rashes, swelling and breathing difficulty, and can be dangerous. Intolerance is uncomfortable; allergy can be serious. If dairy causes hives, facial swelling or breathing problems, seek medical advice urgently — that is not intolerance.
The three types
Primary lactase deficiency — by far the most common. Lactase production naturally declines after weaning. This is the biological default for most of humanity; continued lactase production into adulthood (lactase persistence) is actually the evolutionary exception, arising in populations with a long history of dairy farming.
Secondary lactose intolerance — temporary or ongoing loss of lactase due to damage to the gut lining, from gastroenteritis, coeliac disease, Crohn's disease, ulcerative colitis or some medications. This type often improves once the underlying condition is treated.
Congenital lactase deficiency — extremely rare, inherited in an autosomal recessive pattern, and apparent from the first feeds in a newborn.
Symptoms
- Bloating and abdominal distension
- Wind
- Stomach cramps and pain
- Diarrhoea
- Nausea, occasionally vomiting
- Rumbling or gurgling in the abdomen
Symptoms typically begin 30 minutes to 2 hours after eating dairy, and severity depends on how much lactose was consumed and how much lactase you still produce.
The genetics
Whether lactase production persists into adulthood is largely determined by variants in a regulatory region of the MCM6 gene, which controls the activity of the neighbouring LCT gene that codes for lactase itself. Prevalence varies enormously by ancestry — lactase non-persistence affects roughly 70–100% of people of East Asian descent, and a much smaller proportion of people of Northern European descent.
Getting a diagnosis
Do not simply cut out dairy long-term on suspicion — the same symptoms occur in coeliac disease, IBS and inflammatory bowel disease, and removing dairy before testing can obscure a diagnosis. Speak to your GP. Assessment may involve a hydrogen breath test, a supervised elimination and reintroduction, or genetic testing.
Living with it
Most people with lactose intolerance do not need to eliminate dairy completely. Many tolerate around 12 g of lactose (roughly a glass of milk) in one sitting, especially alongside other food.
- Naturally low-lactose dairy: hard aged cheeses such as cheddar and parmesan contain very little; live yoghurt and kefir are often well tolerated because their bacteria digest some lactose for you
- Lactose-free dairy: real milk with the lactose already broken down — same protein and calcium
- Plant alternatives: oat, soya, almond, coconut or rice. Choose fortified versions, as unfortified nut milks are low in calcium, iodine and protein
- Lactase supplements: taken before a dairy-containing meal, these can prevent symptoms
- Watch hidden lactose: it appears in bread, breakfast cereals, biscuits, processed meats, sauces and some medications
Protecting your calcium intake
If you reduce dairy significantly, make sure calcium comes from elsewhere — fortified plant milks, tinned sardines or salmon with bones, tofu set with calcium, leafy greens such as kale and pak choi, almonds and dried figs. UK adults need around 700 mg of calcium a day. Iodine and vitamin B12 also warrant attention if dairy was a main source.
Testing with Rightangled
Rightangled's DNA tests can look at MCM6/LCT variants associated with lactase persistence, and our blood tests can check for related deficiencies. Results are reviewed by our clinical team, which includes GPhC-registered independent prescribers, with medical oversight from our doctor, Dr Abdullah.
For symptom relief, see our IBS, bloating and abdominal pain range.
Verify us with the General Pharmaceutical Council (registration 9011933), see our LegitScript certification, and read reviews on Trustpilot.
This article is for general information and does not replace personalised medical advice. See your GP before cutting out a food group long-term.
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