Lactose intolerance

What lactose intolerance is

Lactose is the sugar in milk. To be absorbed it has to be split into glucose and galactose first, and the enzyme that does the splitting, lactase, sits on the surface of the small bowel lining. Lactase activity peaks at birth, because an infant lives on milk, and in most people it declines through the first two decades of life.

When there is not enough lactase, lactose travels on into the colon undigested. The bacteria there ferment it, producing hydrogen, carbon dioxide, methane and short chain fatty acids. The sugar also holds water in the bowel by osmosis. That combination of gas and fluid is what produces the bloating, wind, rumbling, cramping and loose motions people recognise.

The important point is that the first part of this process is not abnormal. Reduced lactase in adulthood is the ordinary human pattern worldwide, and symptoms are a separate question from whether the enzyme is there.

Four words that get mixed up

These terms are used interchangeably in everyday conversation and they do not mean the same thing. Separating them is most of the work of a consultation about dairy.

• Lactase non-persistence. The gradual fall in lactase through childhood and adolescence. This is the usual human phenotype worldwide, not a disorder.
• Lactose malabsorption. Lactose passing through into the large bowel instead of being absorbed. The consequence of low lactase, or of fast transit or bacterial overgrowth.
• Lactose intolerance. Symptoms after lactose in someone who malabsorbs it. This is the only one of the four that is a clinical problem.
• Self-reported lactose intolerance. The belief that dairy is the trigger, without testing. Common, frequently wrong, and the reason many people restrict more than they need to.

Why it is not a milk allergy

Lactose intolerance and cow's milk protein allergy get conflated constantly, including on restaurant menus and in the way people describe themselves. They are different conditions with different risks.

Lactose intolerance is a problem of digesting a sugar. The immune system is not involved, the bowel is not damaged by it, and the symptoms are confined to the gut. Cow's milk protein allergy is an immune reaction to the protein in milk. It can cause skin reactions, vomiting, wheeze and in some people anaphylaxis, and it is managed by strict avoidance under allergy supervision.

What it feels like

Symptoms usually come on within a few hours of the lactose, and they are dose related. Small amounts may pass unnoticed while a large milky coffee on an empty stomach does not.

• Bloating and a visibly distended abdomen
• Wind, often more than the amount eaten seems to justify
• Audible rumbling and gurgling
• Cramping central or lower abdominal pain
• Loose or watery motions, sometimes urgent
• Nausea, less often

What lactose intolerance does not cause is weight loss, blood in the motions, fever, anaemia, waking at night to open the bowels, or symptoms that keep going when lactose is genuinely out of the diet. Any of those shifts the question to something else and is worth investigating rather than managing as a dairy problem.

Why lactase falls, and why it is normal

In most of the world, adults have low lactase. The minority who keep high lactase into adult life do so because of a specific genetic change. In people of northern European descent it is a single alteration on chromosome 2, upstream of the lactase gene, which keeps the gene switched on after weaning. Separate changes in the same region arose independently in parts of the Middle East, Ethiopia and Sudan, and in Kenya, Tanzania and South Africa: different mutations arriving at the same outcome, in populations that herded animals.

The genetic evidence for this being advantageous is strong. The lactase persistence allele spread through European and North African populations alongside animal domestication over thousands of years, under one of the heaviest selection pressures recorded for any human gene.

In practice that history matters in a city like Sydney. Lactase persistence is common in people of northern European ancestry and uncommon in those of East Asian ancestry, with a wide spread across South Asian, Middle Eastern, African, Mediterranean and Aboriginal and Torres Strait Islander backgrounds. Ancestry shifts the likelihood, but it does not make the diagnosis, and it says nothing at all about whether a given person will have symptoms.

When something else has caused it

Lactase sits right at the tip of the villi, the finger-like projections of the small bowel lining, which makes it one of the first things lost when that lining is injured. Lactose malabsorption that appears in someone who could previously handle dairy is called secondary, and it is a signpost rather than a diagnosis in itself.

Infection

Gastroenteritis is the commonest cause, and usually the most temporary. In a study of young children with rotavirus infection, lactose malabsorption was present in 60 per cent, against 49 per cent of children with diarrhoea from other causes. Most of this recovers over weeks.

Coeliac disease

Untreated coeliac disease flattens the small bowel lining, and a lactose breath test at the time of diagnosis is often positive. The encouraging part is that many people regain the ability to digest lactose after six to twelve months on a strict gluten-free diet. Permanently writing off dairy on the strength of a test done at coeliac diagnosis is a mistake worth avoiding.

Crohn's disease

In pooled studies, inflammatory bowel disease carried an odds ratio of 1.6 for lactose malabsorption, and the association was strongest in Crohn's disease involving the small bowel, where the relevant lining is directly affected.

Other causes

Rapid transit through the small bowel, small intestinal bacterial overgrowth, and conditions such as systemic sclerosis that alter small bowel function can all produce the same result. So can treatments that damage the gut lining.

What gets ruled out first

Bloating, wind and loose motions after food are the shared language of a long list of conditions. Lactose is one entry on it, and it is a comfortable one to land on because the treatment feels obvious. The conditions below present the same way and need excluding before dairy takes the blame.

• Coeliac disease. Blood tests first, taken while still eating gluten, and biopsies at gastroscopy if those are positive.
• Inflammatory bowel disease. Suggested by blood in the motions, nocturnal symptoms, weight loss, anaemia or a raised faecal calprotectin.
• Microscopic colitis. Watery diarrhoea with a normal-looking colon, diagnosed only on biopsies, and easily mistaken for food intolerance.
• Bile acid diarrhoea. Common, treatable, and routinely missed, particularly after gallbladder surgery.
• Irritable bowel syndrome. Frequently the real answer, and frequently co-existing with genuine lactose malabsorption.
• Thyroid disease, pancreatic insufficiency and medication effects. Cheap to check and worth checking.

The chronic diarrhoea page sets out that first round of tests in full and what each one answers.

How the diagnosis is made

Five tests of lactose digestion exist. They answer different questions, and only one of them measures symptoms at the same time as digestion, which is why it is the one generally used.

Hydrogen breath test

A measured dose of lactose is swallowed and breath hydrogen is sampled over the next three hours. Hydrogen is not produced by human enzymes, so its appearance means the sugar has reached bacteria. A dose of 20 to 25 g works best in practice: smaller doses miss cases, and the 40 to 50 g doses used in research provoke symptoms even in people who manage ordinary amounts of dairy without trouble. A starting hydrogen level below 20 parts per million is needed for the test to be readable, and a rise of 20 parts per million or more within three hours indicates malabsorption. Samples at baseline, 90, 120 and 180 minutes are enough. Symptoms are recorded through the test, and it is the combination of the two that matters.

The test has two limitations worth knowing. Some people carry a gut flora that produces methane rather than hydrogen, which gives a falsely negative result, and measuring methane alongside hydrogen partly covers that. Bacterial overgrowth and rapid transit can produce a falsely positive one.

Genetic test

A blood or cheek swab sample for the lactase persistence variant. It is reliable in people of European descent, where almost all lactase persistence comes from the one change. It is not recommended for clinical use in people of African or Asian heritage, because several different variants are involved and a standard test will miss them. It also cannot detect secondary lactose malabsorption at all, which is a significant gap given how often that is the situation.

Lactase activity on a biopsy

Measured directly on small bowel samples. It picks up both primary and secondary malabsorption, but a gastroscopy is not justified for this question alone. It is worth adding when endoscopy is being done for another reason.

Lactose tolerance test and the gaxilose test

The tolerance test follows blood glucose after a 50 g lactose dose, which means repeated blood samples for information a breath test gives more comfortably. The gaxilose test measures a marker in urine or blood after a lactase-specific substrate, and its accuracy has varied between studies. It is not in routine use here.

Your care pathway

Tests not worth paying for

Food intolerance is a well-served market. IgG food antibody panels, hair mineral analysis, applied kinesiology, electrodermal testing and bioresonance are all sold for this purpose and none of them is a valid test for lactose intolerance or any other food intolerance. An IgG antibody to a food is a normal finding in people who eat that food.

The harm is not only the cost. These panels typically return long lists of foods to avoid, and people act on them. The spread of unvalidated testing is the single largest reason food intolerance ends up badly managed, and undoing a diet built on one takes longer than the diagnosis would have.

How much lactose is actually a problem

Cow's milk contains about 5 g of lactose per 100 mL, so a 250 mL glass carries roughly 12 g. Most people with lactose intolerance manage that much without symptoms, and more when it comes with other food. Reviews of the evidence put a workable daily allowance at around 12 to 15 g, spread through the day rather than taken at once.

Dairy foods differ enormously, and not in the way people expect. Cheese is low because the lactose goes into the whey during making and the rest is consumed as the cheese matures. Yoghurt is lower than milk because its bacteria have already done part of the job.

Representative values. Brands vary, and the lactose in yoghurt keeps falling while it sits in the fridge.

Lactose powder is also used widely as a food additive, for texture and flavour, in sausages, gravies, some margarines, bread, sauces, crisps, soups and prepared meals. The amounts in any one serve are small, which is why people on a reduced-lactose diet rarely need to chase them, but they explain the occasional unexpected reaction. Some tablets and capsules contain lactose as a filler too, which matters only for the small number of people who are very sensitive.

How it is managed

The aim is to control symptoms without narrowing the diet more than necessary. Unlike coeliac disease or a food allergy, this condition does not call for strict avoidance.

Find the amount that suits you

Reduce lactose until symptoms settle, then build back up to establish where the limit sits. Spreading dairy across the day and taking it with other food both raise the threshold, because they slow the delivery of lactose to the colon.

Choose the forms that carry less

Hard cheese, yoghurt, butter, cream and lactose-free dairy all let people keep dairy in the diet. Lactose-free milk is ordinary milk with lactase added, so the nutrition is unchanged and only the sugar is pre-split.

Lactase supplements

Lactase tablets and drops are sold in pharmacies without a prescription, taken with the dairy or added to milk beforehand. They do reduce symptoms and improve measured lactose digestion, but the effect is modest: around an 18 per cent reduction in symptoms overall in the published work. They suit occasions rather than every day.

Do not expect too much from probiotics

Probiotics producing lactase have been studied and a systematic review found an overall positive effect, though not consistently better than lactase supplements and in poor quality studies. There is more promising evidence for prebiotics, which work by shifting the colonic bacteria rather than replacing the enzyme. In one randomised trial, 30 per cent of people given a galacto-oligosaccharide prebiotic considered themselves lactose tolerant after a month, against 6 per cent on placebo.

When lactose is not the whole story

Lactose belongs to a family of poorly absorbed carbohydrates known as FODMAPs, and people with a sensitive bowel often react to several. A low FODMAP diet improves symptoms in 50 to 80 per cent of people with irritable bowel syndrome. It is demanding, it is meant to be temporary, and it is best run with a dietitian. Worth noting: in a large study neither the clinical picture nor the breath test result predicted who would respond to it.

Calcium and bone health

Dairy is a convenient source of calcium, protein and vitamin D, and the usual consequence of cutting it out without thinking about it is a diet short of calcium. The Australian recommended daily intake is 1,000 mg for adults, rising to 1,300 mg for women from 51 and men from 71.

There is some genetic evidence behind the concern. Studies using the lactase persistence variant as a natural experiment found vitamin D levels around 2.3 times lower in people with the non-persistence genotype, and a meta-analysis found a lower fracture risk in those able to digest lactose, with an odds ratio of 0.81. A European study with a similar design did not confirm the fracture finding, so this is a reason for attention rather than alarm.

• Hard cheese and yoghurt, both low in lactose and high in calcium
• Lactose-free milk, which has the same calcium as ordinary milk
• Calcium-fortified soy, oat or almond drinks, checking the label since not all are fortified
• Tinned salmon and sardines eaten with the bones
• Tofu set with calcium, tahini, almonds and leafy green vegetables
• Vitamin D, which comes mostly from sunlight rather than food and is worth checking if dairy intake has been low for years

When cutting out dairy is the wrong answer

This part gets less attention than it deserves. Lactose intolerance is not a trivial condition, and the anxiety that food will cause pain and bloating is itself part of the problem. Studies have found that people who believe they are lactose intolerant but turn out not to be report a quality of life as poor as those who genuinely have it.

That worry tends to generalise. People restrict dairy, then legumes, then dried fruit, then wheat, and the diet narrows without the symptoms improving. At the severe end this behaviour is recognised in its own right as avoidant restrictive food intake disorder, with weight loss and nutritional consequences.

The way out is usually not a longer list of exclusions. It is a clear answer on what the bowel is actually doing, which is the reason to test rather than guess, and then treatment aimed at the sensitivity rather than at one more food.

Getting the most out of the appointment

• A two-week food and symptom record with amounts, not just food names
• What has already been tried, for how long, and whether it helped
• Any previous coeliac blood tests, with the dates and whether gluten was being eaten at the time
• Previous breath tests, stool tests or endoscopies, with the reports if you have them
• A current medication list, supplements included
• Family history of coeliac disease, inflammatory bowel disease or bowel cancer
• Whether dairy has ever caused a rash, swelling, wheeze or vomiting, which points at allergy rather than intolerance

Where lactose intolerance is assessed in Sydney

A/Prof Viraj Kariyawasam is a consultant gastroenterologist and hepatologist whose practice is focused on inflammatory bowel disease and the investigation of long-standing bowel symptoms. He sees people with suspected food intolerance, bloating and chronic diarrhoea for consultation at Norwest, Rouse Hill, Pennant Hills and Macquarie Park.

Hydrogen breath testing and coeliac blood tests are arranged through pathology providers. Gastroscopy and colonoscopy, where they are needed to answer a separate question, are performed at Norwest Private Hospital, Macquarie University Hospital and Pennant Hills Day Surgery. A referral from your GP is needed for a consultation.