People who develop symptoms after drinking milk can be divided into two groups. Some may have an intolerance to the milk sugar lactose that is present in milk, while others may have an allergy to cow’s milk protein.

In lactose intolerance, there is a deficiency of the enzyme lactase, which is produced in the small intestine. Lactose is made up of the molecules glucose and galactose, and lactase breaks the bonds between these molecules; this is how lactose is digested in the body. In the Netherlands, the lactase enzyme is absent in 5 percent of adults of Western origin. When lactose is not broken down in the intestine, symptoms such as diarrhoea and nausea can occur.

A milk allergy occurs when the protein casein is not tolerated. In the human body, beta-casein type A2 is produced. This protein has a protective function and is present in breast milk. Milk from certain types of cows contains type A1 casein, and the human immune system may see this as a foreign substance – even though it closely resembles type 2, which does not trigger an immune response. The immune system reacts excessively, which can cause an allergic reaction. Cow’s milk protein allergy occurs worldwide in 2.8 percent of infants.

With this test, we focus on determining or ruling out whether there is a congenital lactose intolerance or whether it is a lactase deficiency caused by damage to the intestine. A cow’s milk allergy can often be detected with a blood or skin test. If you suspect a cow’s milk allergy, consult your GP.

Background to lactose intolerance

What makes the approach to lactose intolerance confusing is that, from an evolutionary perspective, a decrease in lactase production in the small intestine is normal. Humans – like every other mammal – stop drinking milk at a certain age. As a result, the body no longer needs to produce the enzyme lactase to break down lactose. Populations that began consuming animal milk retained the enzyme and the body adapted genetically. The advent of dairy farming, around 10,000 years ago, led to this gene variant being passed on and to people continuing to produce lactase at an older age. Because consumption of dairy products by older children and adults is considered normal in Western countries, a lack of lactase production is regarded as an abnormality. Whereas around 10 to 20 percent of the population in Europe does not tolerate milk sugar, in Southeast Asia this applies to almost 100 percent of the population.

Primary or secondary lactose intolerance

It is therefore debatable what should be regarded as ‘normal’ with respect to the level of lactase activity in humans. Thousands of years ago, decreasing lactase activity at a young age was not a disease but genetically the normal course of events. A genetic change (mutation) in the lactase gene created different variants of the gene, and some people were able to digest lactose at a later age as well. This was considered beneficial, and people who developed symptoms after consuming dairy products later in life were consequently described as lactose intolerant.

When lactase activity is absent or decreases at a young age (lactase non-persistent), this is called primary lactose intolerance. It is congenital, and that person will never tolerate lactose and must follow a lactose-free diet. People who have the gene variant that keeps lactase activity high later in life (lactase persistent) can still develop symptoms after consuming milk. This is called secondary lactose intolerance, and the intestinal wall is then often damaged by underlying diseases. If symptoms disappear after a lactose-free diet, it is likely that the person has secondary lactose intolerance. If the symptoms do not disappear after a lactose-free diet, lactose intolerance is ruled out and other tests (for example for coeliac disease) should be carried out.

The test

By collecting cells from the inside of the cheek and isolating DNA from these cells, the real-time PCR method can be used to determine whether someone has congenital lactose intolerance or whether the person is still able to produce lactase but has symptoms because of a damaged intestinal wall. This test is called genotyping. Compared with the H2 breath test, its advantage is that it is much less time-consuming and sample collection is very simple. It can also clarify whether the condition is primary or secondary lactose intolerance, allowing a treatment plan to be drawn up in a much more targeted way.