Lactose intolerance
Lactose is the sugar in milk. To absorb it, the small intestine has to produce an enzyme called lactase, which splits it into two simple sugars.
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Lactose is the sugar in milk. To absorb it, the small intestine has to produce an enzyme called lactase, which splits it into two simple sugars. If there is not enough lactase, the lactose reaches the large bowel untouched, bacteria there break it down, and the result is gas, bloating, rumbling and diarrhoea. That is lactose intolerance. One thing is worth grasping straight away: this is not a disease in the ordinary sense, and it is not an allergy. In most adults on the planet lactase declines with age, and that is a variant of normal rather than a fault. The question is usually not "can I have dairy or not" but how much you can manage at one sitting.
How it shows itself
Symptoms come on anywhere between half an hour and a few hours after a meal containing milk, and last until that portion has passed through the bowel:
- bloating and a feeling of fullness;
- loud rumbling and gurgling;
- excess wind;
- cramping pain around the navel and low in the abdomen;
- loose, frothy, acidic stools, sometimes with urgency shortly after eating;
- less often, nausea.
Two things on that list deserve highlighting. First, the symptoms are dose-dependent: half a glass of milk on porridge may pass unnoticed, while a large latte on an empty stomach will not. Second, how bad the symptoms are matches poorly with how much lactase is missing: people with identical test results tolerate milk very differently, because the make-up of the gut bacteria and the sensitivity of the bowel count for a great deal.
Older texts often attribute headaches, tiredness and joint pains to lactose intolerance. There is no convincing evidence for that link, and if those complaints exist their cause needs looking for separately rather than treating the matter as settled.
Why lactase runs short
The commonest reason is the adult type. Lactase is needed above all by a baby; after weaning, production gradually falls in most people, and by adolescence or adulthood only a small fraction of the original remains. Keeping lactase into adult life is a comparatively recent genetic trait, widespread in northern Europe and uncommon in East Asia, Africa and among the indigenous peoples of the Americas. That is why in some countries lactose intolerance is a curiosity while in others most adults have it. The decline is slow, over years, and people often notice it only in middle age: "I used to drink milk without a thought."
The second group of causes is damage to the lining of the small intestine, where the enzyme sits. This kind of intolerance appears abruptly, at any age, and is usually reversible:
- after a bowel infection — the commonest situation in children; the enzyme recovers over weeks, sometimes a couple of months;
- in coeliac disease, Crohn's disease and ulcerative colitis;
- in giardiasis and other parasitic infections;
- after bowel surgery and radiotherapy;
- with long-term use of certain medicines, and with alcohol dependence.
Children deserve a paragraph of their own. Congenital lactase deficiency, where the enzyme is absent from birth, is extremely rare, shows itself as severe diarrhoea from the first feeds and needs treatment from the first days of life. In premature babies the enzyme is temporarily immature and that resolves as they grow. A very common cause of alarm, on the other hand — green frothy stools and wind in a healthy baby who is gaining weight well — is almost never connected with lactose intolerance, and breastfeeding should not be stopped because of it.
This is not milk allergy
The two are constantly confused, and the difference matters enormously.
In lactose intolerance an enzyme is lacking. The immune system plays no part, the reaction always depends on the dose, and it is never life-threatening. The worst that an extra glass of milk will do is give you a few uncomfortable hours.
Cow's milk protein allergy is an immune reaction, and it is directed not at the sugar but at the protein. Trace amounts are enough, it is commoner in the first year of life, and it produces rash, hives, vomiting and blood in a baby's stools; in severe cases, swelling of the face and lips, wheezing, breathlessness and a drop in blood pressure. That reaction calls for an ambulance at once (in Spain, Italy, Portugal, Poland and Ukraine on the single European number 112). Lactose-free milk is no use whatever in protein allergy: the protein is still there.
So if dairy brings on a rash, swelling or difficulty breathing, this is not lactose intolerance, and it needs sorting out with a doctor rather than by trying lactose-free products.
What must not be blamed on lactose
The commonest mistake is to diagnose yourself, cut out dairy and let the matter rest, while another illness sits behind the same symptoms. Go to a doctor rather than to a diet if any of the following is present:
- blood or mucus in the stools, or black tarry stools;
- weight loss you were not aiming for;
- a persistent change from your usual bowel habit lasting weeks;
- pain or diarrhoea that wakes you at night;
- fever, pallor, weakness or signs of anaemia;
- symptoms appearing for the first time after the age of fifty;
- a family history of bowel cancer, coeliac disease or inflammatory bowel disease.
On coeliac disease specifically: cutting out dairy before being tested for it is pointless, and cutting out gluten is actively harmful, because on a gluten-free diet the coeliac tests turn negative and the diagnosis can no longer be made. The diet is changed after the investigation, not before it.
One more point: a substantial share of people convinced they are lactose intolerant in fact have irritable bowel syndrome. In studies where milk and lactose-free milk were given blind, many reacted to both alike. That does not mean the symptoms are imagined; it means that cutting out dairy is not enough and something else needs treating.
How it is confirmed
The usual starting point is the simple one: remove lactose-containing foods for two to four weeks, then bring them back and see whether the symptoms return. The step back is more important than the first one — without it, it is easy to give up dairy for years to no purpose.
Of the formal tests, the hydrogen breath test is the one most used: after a measured dose of lactose, hydrogen in the breath is measured at regular intervals. If the lactose has not been absorbed and has reached the bacteria, hydrogen rises. The test is straightforward but needs preparation: certain foods are restricted the day before, no smoking is allowed, and antibiotics must not have been taken for some weeks beforehand.
A genetic test shows whether someone is predisposed to keeping lactase into adult life. It answers the question about the adult type, but says nothing about secondary intolerance following an infection or arising from another illness.
Blood tests and gastroscopy with a biopsy of the small bowel are not there to confirm the intolerance itself, but to look for its cause — above all coeliac disease and inflammatory bowel disease.
How much lactose you can actually have
Giving up dairy altogether is rarely necessary. Most people with the adult type manage roughly a glass of milk a day without trouble, provided it is not drunk all at once or on an empty stomach. Simple measures work:
- split the daily amount into small portions and take dairy with other food rather than on its own;
- choose the products in which almost no lactose is left: hard matured cheeses such as parmesan and mature cheddar, and butter;
- favour yoghurt and kefir with live cultures — their bacteria break down part of the lactose themselves, so they are tolerated noticeably better than milk;
- use lactose-free milk and soft cheese, in which the manufacturer has already added the enzyme;
- take a lactase preparation before a meal containing milk; the amount is worked out individually;
- read the ingredients of prepared foods: lactose is added to bread and baked goods, sausages, sauces, instant soups and protein shakes.
Lactose is also present as a filler in many tablets, but in fractions of a gram, which almost never makes any difference to how you feel; it is no reason to give up a prescribed medicine.
If the intolerance is secondary, the diet is temporary: once the lining has recovered from the infection or the underlying illness has been treated, dairy usually returns to the diet in the previous amount. There is also an effect in the other direction — regularly taking small amounts of lactose improves tolerance over time as the gut bacteria adapt, whereas cutting it out completely makes going back harder.
Not losing out on calcium
This is what almost always gets forgotten, and it is the main practical risk. Dairy is the mainstay source of calcium, and vitamin D, iodine, vitamin B12 and riboflavin go with it. A long-term exclusion without replacement is a route to falling bone density, particularly in growing children, in teenagers, in pregnancy and in women after the menopause.
Replacing them without milk is quite possible. There is calcium in sardines and other small fish eaten with the bones, in sesame and almonds, in tofu set with calcium salts, in broccoli and leafy cabbages, in beans, and in mineral water with a high calcium content. Plant drinks — oat, soya, almond — are only suitable if they are calcium-fortified, and they need shaking before use, because the calcium settles to the bottom of the carton. Ordinary unfortified plant drinks contain hardly any calcium, and rice drink is not suitable for young children.
If dairy has had to go completely after all, calcium and vitamin D supplements are worth discussing with a doctor. Dairy is not withdrawn from a child at all without medical advice: a growing body has far less margin on calcium than an adult one.
Online consultation
At an online appointment the doctor works from your description of the symptoms to separate lactose intolerance from what resembles it: milk protein allergy, coeliac disease, irritable bowel syndrome and the aftermath of a bowel infection. You will be told which investigations make sense in your case and in what order, and how to carry out the trial exclusion and the essential reintroduction properly so that the result means something. Your real tolerance threshold and the ways of eating dairy without consequences are gone through separately, along with how to cover calcium and vitamin D if some foods have to go. The doctor will also spell out the signs where diet will not help and a face-to-face examination is needed.
This material is for information only and does not replace medical advice.
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Discuss your symptoms and possible next steps with a doctor online.
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Discuss your symptoms and possible next steps for Lactose intolerance with a doctor online.
