It’s important to remember that lactose intolerance isn’t an allergy to milk – it's caused by an enzyme deficiency. People with a dairy allergy can react to even a tiny amount of dairy or milk, but most people with lactose intolerance can tolerate some foods in their diet containing lactose.
Lactose intolerance is caused by lack of an enzyme called lactase, usually present in the lining of the small intestine. Lactase breaks lactose down into two sugars (glucose and galactose) so it can be absorbed from your bowel into the bloodstream.
If there isn't enough lactase, lactose will remain in the digestive system, passing from the small intestine to the colon (large intestine), where bacteria break it down, producing fatty acids and gases, resulting in bloating, wind and diarrhoea.
There are different causes for lactase deficiency. Primary lactase deficiency is the most common worldwide and genetically inherited, and lactase production decreases over time, including during childhood. The amount of lactase you produce drops to between 10 and 30 per cent of the normal amount and this is permanent.
Secondary lactase deficiency is the most common cause of lactose intolerance in the UK and is a transitory problem caused by damage to the lining of the gut. Infections such as rotaviral diarrhoea in children and other types of infectious diarrhoea are common causes.
Conditions such as coeliac disease damage the lining of the gut, resulting in the body not being able to produce enough lactase, although this is reversible if coeliacs avoid gluten. Other conditions which can cause lactose intolerance include Crohn’s disease, a condition that causes inflammation of the digestive system.
In extremely rare cases, babies can be born with lactose intolerance where they produce little or no lactase or their small intestines aren’t fully developed.
Symptoms can start 30 minutes to two hours after you’ve eaten or drunk something with lactose in it, causing flatulence, bloating, stomach pain and/or cramps and rumbling, diarrhoea and nausea. Severity of symptoms will depend on how much lactose you have eaten or drunk and the amount of lactase your body is producing.
Your doctor may suggest you experiment with your diet over a two-week period by removing all milk and dairy products. If your symptoms get better in this time, it’s likely that you are lactose intolerant.
To confirm a diagnosis, you may have a hydrogen breath test, where the level of lactose is measured after drinking a lactose solution. If you aren’t digesting lactose, there will be more hydrogen on your breath.
In a lactose intolerance blood test, some blood will be taken to measure your blood (glucose) sugar after consuming lactose. If you are lactose intolerant, sugar levels will rise slowly or not at all.
Who gets it?
Lactose intolerance can develop at any age, although symptoms most commonly develop in people aged 20-40, but babies and children can develop it too. In the UK, primary lactose intolerance is more common in people of Asian or African-Caribbean origin.
There’s no cure, but many people can manage lactose intolerance by changing their diet. Most can still tolerate small amounts of cheese, yogurt and milk (200ml daily) if it’s drunk with a meal. Others with severe symptoms may have to avoid lactose altogether and replace with lactose-free alternatives.
Lactase supplements containing lactase are available as tablets or liquid form.
Dairy is an important source of many essential nutrients, including protein, calcium and vitamin D, and it’s important to manage your diet so you don’t become deficient in any of these.
Non-dairy foods that contain calcium are green-leafy vegetables, soya bean and tofu. Foods such as eggs, liver, salmon and tuna contain vitamin D. Check ingredients carefully and opt for calcium-enriched lactose-free products. Your GP may advise you take a calcium and vitamin D supplement.
For those with secondary lactase deficiency, it’s important to treat the underlying cause - for example, if you are diagnosed with coeliac disease, you must avoid gluten.