Lactose intolerance test: breath test, DNA or simply trying it out?

There are three routes for lactose intolerance: a hydrogen breath test, a DNA test on the lactase gene, and an elimination trial with reintroduction. None of the three measures the same thing, which is why they can give different answers. That is not a lab error, it follows from what each method looks at.

One distinction up front: lactose malabsorption means you absorb lactose poorly. Lactose intolerance means it also gives you symptoms. Those two do not always coincide.

How does lactose intolerance actually work?

Lactose is the sugar in milk, and you need the enzyme lactase to break it down. In a large share of the adult world population, lactase production declines after childhood. What is not broken down travels on to your colon, where bacteria ferment it and produce gas and fluid.

That is where the symptoms come from: bloating, rumbling, cramps and sometimes diarrhoea, usually within a few hours.

How much you tolerate varies widely. Many people with reduced lactase activity handle a portion of around 12 grams of lactose, roughly a glass of milk, without meaningful symptoms. The Dutch Nutrition Centre also notes that eating fully dairy free is rarely necessary.

Which lactose intolerance tests exist?

The hydrogen breath test, the genetic test and the elimination trial, each with its own angle. The breath test checks whether undigested lactose reaches your colon. The DNA test looks at predisposition. The elimination trial looks at your symptoms, which is ultimately the point.

That last row is there for a reason. Dairy is a fixture in the broad IgG panels, and a positive there mostly means you eat dairy (PMID 18489614).

How reliable is the hydrogen breath test?

Reasonably, when done properly, but it has known pitfalls. You take a lactose dose and blow at set intervals, where a rise in hydrogen concentration points to malabsorption. Some people, however, carry gut bacteria that produce little hydrogen, so the test reads negative while malabsorption is present.

A review in Gut discusses exactly that mix of usefulness and limitation, and argues for always reading the result alongside your symptoms (PMID 31427404).

Preparation weighs heavily. Smoking, exercising shortly before the test and certain foods the evening before can all disturb the measurement.

A breath test without a symptom record is therefore half an answer.

What does the DNA test say?

The genetic test looks at a variant near the lactase gene associated with continued lactase production into adulthood. Without that variant, a decline in lactase activity is likely. That explains predisposition, but it does not predict how much milk you handle today.

There is another reason to be careful with genetics alone. With temporary causes, for instance after a gut infection, your genotype still reads the same while your symptoms have only just started.

Take two athletes with exactly the same genotype. One drinks a milk-based shake after training without trouble, the other cannot finish half a litre. Genetically identical, practically not.

That is precisely why the elimination trial stays on this list.

Can you simply try it out?

Yes, and for many people that is the most practical first step. Cut lactose back for two weeks, log your symptoms daily, then reintroduce in steps: a small glass first, a larger portion later. If symptoms return cleanly at the larger portion, you have a usable answer.

The pitfall is expectation. Gut symptoms respond strongly to what you expect to happen, so record symptoms as you go rather than reconstructing them from memory afterwards.

The methodical approach is set out in more detail in food intolerance: which tests work.

If your symptoms do not clear once lactose is gone, look further. Gluten and wheat are the logical next candidates, see gluten intolerance or coeliac disease.

What does this mean if you train?

Dairy is an easy protein source for many athletes, so cutting it costs more than you would think. A litre of semi-skimmed milk supplies roughly 34 grams of protein, plus calcium and iodine. People who drop dairy without replacing it usually see it in their total protein intake.

There are plenty of middle routes. Lactose free milk carries the same protein, hard cheeses are naturally low in lactose, and many people tolerate yoghurt better than milk.

How much protein you actually need is covered in how much protein per day for muscle growth.

If you do have a panel drawn because symptoms have run long, ferritin and vitamin B12 are the values most often included in this context, for instance through 360 Health.

What I would do

Start with two weeks of logging and a staged reintroduction before ordering a test. If symptoms persist after that, or you lose weight without meaning to, discuss it with your GP, who can judge whether a breath test or other investigation is worthwhile.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Misselwitz B, Butter M, Verbeke K, Fox MR. Update on lactose malabsorption and intolerance: pathogenesis, diagnosis and clinical management. Gut. 2019;68(11):2080-2091. PMID 31427404.
• Stapel SO, Asero R, Ballmer-Weber BK, et al. Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report. Allergy. 2008;63(7):793-796. PMID 18489614.
• Costa RJS, Snipe RMJ, Kitic CM, Gibson PR. Systematic review: exercise-induced gastrointestinal syndrome. Aliment Pharmacol Ther. 2017. PMID 28589631.