Lactose Intolerance Symptoms: Signs, Tests and Treatment

Quick answer

Bloating, wind, cramps and diarrhoea after dairy; many still tolerate small amounts

Symptoms usually start 30 minutes to two hours after eating or drinking dairy and include bloating, abdominal cramps, rumbling, wind, diarrhoea and sometimes nausea. The amount of lactose matters: many people with lactose malabsorption tolerate about 12 grams of lactose (roughly a cup of milk) spread through the day, especially with meals. You can become lactose intolerant as an adult because lactase production naturally falls after childhood in most of the world’s population; it can also follow gut infections or coeliac disease. Diagnosis is based on symptoms and response to reducing lactose, sometimes with a hydrogen breath test. Treatment includes reducing lactose, choosing lactose-free products and taking lactase enzyme tablets.

Important. See a doctor for diarrhoea lasting more than a few weeks, blood in stools, weight loss, vomiting, or symptoms that start suddenly after a gut infection and do not settle. Hives, swelling or breathing difficulty after dairy suggest milk allergy, which is different and can be serious.

What is lactose intolerance?

Lactose is the natural sugar in milk. In the small intestine, the enzyme lactase splits it into glucose and galactose, which are absorbed. If lactase levels are low, lactose reaches the large intestine, where bacteria ferment it, producing gas and drawing water into the bowel. This is called lactose malabsorption. When it causes symptoms, it is called lactose intolerance. It is not an allergy and does not damage the gut.

Signs and symptoms

Bloating and a swollen tummy.
Abdominal cramps or pain.
Rumbling and gurgling noises.
Wind (flatulence).
Loose stools or diarrhoea.
Nausea, occasionally vomiting.
Symptoms typically 30 minutes to 2 hours after dairy, and worse with larger amounts.
For general bloating causes, see why am I so bloated.

Can you become lactose intolerant later in life?

Yes. Most people in the world produce less lactase after weaning, a pattern called lactase non-persistence. In populations of Northern European origin, lactase often persists into adulthood, while in East Asian, West African, Middle Eastern and many Native American populations, lactose malabsorption is very common. Symptoms may appear in the teens or adulthood. Lactose intolerance can also develop temporarily after a gut infection, during coeliac disease or Crohn’s disease, after bowel surgery, or with some medicines; this secondary type often improves once the cause is treated.

How to tell if you are lactose intolerant

Keep a food and symptom diary for two weeks.
Try cutting out lactose for two weeks and see whether symptoms improve.
Reintroduce a glass of milk and see whether symptoms return.
A doctor may arrange a hydrogen breath test, which measures gases produced when lactose is fermented.
Blood tests for coeliac disease may be advised, as it can cause similar symptoms.
Genetic tests show whether you have the lactase persistence gene but do not prove symptoms are due to lactose.

How much dairy can you have?

Many people tolerate about 12 g of lactose at once (about 250 ml of milk), especially with food.
Spreading dairy through the day helps.
Hard and aged cheeses such as cheddar, parmesan and emmental contain very little lactose.
Yoghurt with live cultures is often better tolerated than milk.
Butter contains very little lactose.
Lactose-free milk, yoghurt and cheese are widely available.

Treatment options

There is no cure for primary lactose intolerance, but symptoms are easy to manage. Reducing lactose to your tolerance level, choosing lactose-free or low-lactose products and taking lactase enzyme tablets or drops with dairy meals all help. A review in Gut highlighted that symptoms are often related to the amount of lactose and to gut sensitivity, so a balanced approach is usually better than complete avoidance. Those who avoid dairy completely should ensure enough calcium, vitamin D and protein from other sources, and may benefit from advice from a dietitian.

Lactose intolerance medication

Lactase enzyme supplements, taken just before eating dairy, help many people digest lactose. Their effectiveness varies between products and individuals. Probiotics have been studied with mixed results. For diarrhoea or cramps, standard over-the-counter remedies may help occasionally. Some medicines contain small amounts of lactose as a filler; this rarely causes symptoms, but a pharmacist can advise.

Lactose intolerance or milk allergy?

Milk allergy is an immune reaction to milk proteins, not the sugar. It is most common in babies and young children and can cause hives, swelling, vomiting, eczema flares and, rarely, anaphylaxis. Even tiny amounts of milk can trigger it. Lactose intolerance causes only digestive symptoms, depends on the amount, and is not dangerous. People with milk allergy must avoid milk protein entirely, including in lactose-free milk.

Three common situations

Bloating and diarrhoea after milky coffee: try lactose-free milk or a lactase tablet and see whether symptoms settle.
New dairy symptoms after a stomach bug: temporary lactose intolerance is common; reduce lactose for a few weeks and reintroduce.
Symptoms persist despite avoiding dairy: see a doctor to look for other causes such as IBS or coeliac disease.

Hidden sources of lactose

Bread, cakes and biscuits made with milk.
Processed meats and ready meals.
Creamy sauces, soups and mashed potatoes.
Milk chocolate and some sweets.
Whey powder and milk solids in protein bars and shakes.
Check ingredient lists for milk, whey, curds, milk powder and milk solids; small amounts rarely cause symptoms.

Key points to remember

Lactose intolerance causes bloating, wind, cramps and diarrhoea after dairy.
It is common and often develops in adulthood.
Most people can still eat some dairy, especially hard cheese and yoghurt.
Lactose-free products and lactase tablets help manage symptoms.

Questions about lactose intolerance

These answers are general information and do not replace an examination or individual medical advice.

Can lactose intolerance come on suddenly?

Primary lactose intolerance develops gradually, but secondary intolerance can start suddenly after a gut infection or illness.

Is lactose intolerance permanent?

Primary lactose intolerance is usually lifelong, while secondary intolerance often improves once the underlying cause is treated.

Can lactose intolerance cause constipation?

Diarrhoea is more typical, but some people experience constipation, possibly related to methane-producing gut bacteria.

Is lactose-free milk good for you?

Yes. It contains the same nutrients as regular milk, with the lactose already broken down.

Do lactase pills work?

They help many people when taken just before eating dairy, though the effect varies.

Sources

This guide is for general information and does not replace medical advice. How a problem is diagnosed and treated depends on your examination and medical history. If you think you have an emergency, contact your local emergency services.