Lactose intolerance: symptoms, causes, diagnosis and dairy

Lactose is a sugar found naturally in milk and milk products. Lactose intolerance is when you get symptoms, such as tummy pain, after eating or drinking something that contains it, as the UK's National Health Service (NHS) puts it.

It is very common worldwide. The US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) reports expert estimates that about 68% of the world's population has lactose malabsorption, meaning the small intestine cannot digest all the lactose it receives. In Africa and Asia, most people do. The US National Library of Medicine's MedlinePlus Genetics notes that it is very common in people of Arab descent, among others.

Lactose intolerance does not always mean giving up dairy altogether. Below: why it happens, how it differs from a milk allergy, how it is diagnosed, how much dairy many people can still have, and how to keep your calcium up.

What happens to lactose in the gut?

Lactose is normally broken down by an enzyme called lactase, which is made by cells lining the small intestine. That is your body's own enzyme, and it is what lets you digest milk.

When the small intestine makes too little lactase, NIDDK explains, some of the lactose is not digested. It passes into the large intestine, where bacteria break it down and create fluid and gas. In some people, this extra fluid and gas causes symptoms.

Not everyone who digests lactose poorly has symptoms. NIDDK calls the inability to digest all the lactose "lactose malabsorption", and only people who get symptoms are described as lactose intolerant.

Why does it happen?

NIDDK and the NHS describe several causes:

• Lactase non-persistence. This is the most common cause. After infancy, the small intestine makes less lactase, and levels fall further with age. Symptoms may not start until later childhood, the teenage years or early adulthood. MedlinePlus Genetics adds that most people with lactase non-persistence still make some lactase, and can include varying amounts of lactose in their diet without symptoms.
• Damage to the small intestine. Infections, coeliac disease, Crohn's disease and other conditions that injure the small intestine can lead to lactose intolerance. This is called secondary lactose intolerance, and if the underlying cause is dealt with, you may be able to tolerate lactose again. Our guide to gluten and coeliac disease explains one of these conditions.
• Premature birth. In babies born too early, the small intestine may not make enough lactase for a short time after birth.
• Congenital lactase deficiency. In this rare condition, the small intestine makes little or no lactase from birth.

The NHS adds that lactose intolerance can start at any age, and can also follow bowel surgery or an injury to the bowel.

What are the symptoms?

Symptoms can start a few minutes, or a few hours, after having food or drink that contains lactose, the NHS says. Common ones include:

• tummy pain or discomfort;
• bloating;
• farting;
• tummy rumbling;
• diarrhoea or constipation;
• feeling sick, or being sick.

Some people also have symptoms outside the gut, such as headaches, tiredness, or joint or muscle pain. NIDDK notes that symptoms may be mild or severe, depending on how much lactose you have. Our guide to gas and bloating covers those two symptoms in more detail.

Is it the same as a milk allergy?

No. NIDDK describes them as different conditions with different causes. Lactose intolerance is a problem digesting lactose, the natural sugar in milk. A milk allergy is the immune system reacting to one or more of the proteins in milk and milk products.

A milk allergy most often appears in the first year of life, while lactose intolerance usually appears later. Lactose intolerance can be uncomfortable, but a serious allergic reaction to milk can be life threatening. The NHS lists difficulty breathing, itchy skin, and swelling of the lips, face and eyes among the signs of a food allergy.

How is lactose intolerance diagnosed?

Your doctor will ask about your symptoms, your family and medical history, and your eating habits. They may ask you to stop having milk and milk products for a while to see whether your symptoms go away.

Tests can include, according to NIDDK and the NHS:

• a hydrogen breath test, in which you drink a liquid containing a known amount of lactose and then breathe into a container every 30 minutes over a few hours, to measure how well you digest it;
• blood tests;
• a gastroscopy, if symptoms are severe and do not get better, to take a small sample from the small intestine.

Several other conditions can cause similar symptoms, NIDDK adds, including irritable bowel syndrome, coeliac disease and inflammatory bowel disease. That is why the NHS advises not to cut foods out of your diet, or your child's, without the advice of a GP or a dietitian.

How much dairy can you still have?

NIDDK says most people with lactose intolerance can have some lactose without getting symptoms, and that different people can handle different amounts. Research suggests that many people could have 12 grams of lactose, the amount in about 1 cup of milk, without symptoms or with only mild ones.

NIDDK suggests these ways to find what suits you:

• Drink small amounts of milk at a time, alongside other food.
• Add milk and milk products back a little at a time, and see how you feel.
• Try yoghurt and hard cheeses, which are lower in lactose than other milk products. MedlinePlus Genetics explains that the fermentation used to make them breaks down much of the lactose in milk.
• Try lactose-free and lactose-reduced milk and milk products, which NIDDK describes as just as healthy as regular ones.

Your doctor, pharmacist or a registered dietitian can advise on other options.

Check labels. Lactose is in milk from cows, goats and sheep, and in butter, cheese, cream, yoghurt and ice cream. The NHS adds that many processed foods contain it, including some cereals, baked goods such as bread, biscuits and cakes, sauces and salad dressings, and diet and protein shakes. NIDDK lists words on ingredient lists that mean lactose is present: milk, lactose, whey, curds, milk by-products, dry milk solids and non-fat dry milk powder. Small amounts of lactose are also found in some medicines, so ask your doctor if you react to even small amounts.

How can you get enough calcium?

Milk and milk products are some of the main sources of calcium and vitamin D. NIDDK warns that avoiding them completely can leave you short, and that a lack of calcium can make bones weak and more likely to break (osteoporosis).

The NHS says adults need 700mg of calcium a day, and that you should be able to get it from a varied diet. Good sources besides dairy include:

• green leafy vegetables such as curly kale, and okra (but not spinach, as the body cannot digest all of its calcium);
• fish where you eat the bones, such as sardines;
• soya drinks with added calcium, and bread made with fortified flour;
• almonds, dried beans and tofu, which NIDDK also lists, along with oranges and broccoli.

For vitamin D, NIDDK mentions eggs and some kinds of fish such as salmon, and notes that sunlight helps your body make it. Ask your doctor or a dietitian whether you are getting the nutrients you need.

When to see a doctor

The NHS advises seeing a GP if you have symptoms of lactose intolerance and:

• your symptoms keep coming back and often happen after eating;
• your poo has changed in a way that is not usual for you, such as looser poo, pooing more often, or constipation, for 3 weeks;
• you have had blood in your poo for 3 weeks;
• you have had tummy bloating and discomfort for 3 weeks;
• you have been losing weight without trying.

MedlinePlus advises contacting a doctor if a baby or toddler under 2 or 3 years old has symptoms of lactose intolerance, or if a child is growing slowly or not gaining weight.

Call your local emergency number if your lips, mouth, throat or tongue suddenly swell; you are breathing very fast or struggling to breathe; your throat feels tight or you are struggling to swallow; your skin, tongue or lips turn blue, grey or pale; you suddenly become very confused, drowsy or dizzy; or someone faints and cannot be woken. The NHS warns that these can be signs of a serious allergic reaction (anaphylaxis), which needs treatment in hospital immediately.