Lactose intolerance or milk allergy…

Like many people, you often have bloating, gas, diarrhea or constipation… The first thing you think is that it’s lactose. But is he really the one to blame?

Lactose is a double sugar found in milk and its by-products (e.g. ice cream, yogurt, cheese). Under the action of the enzyme called lactase, it splits into glucose and galactose, two sugars that the intestines are able to digest. People who are lactose intolerant do not produce enough, or any, lactase. Therefore, the lactose, not having been split, makes its way to the intestine and can cause either bloating, diarrhea, abdominal pain or cramps, vomiting or constipation.

Most people with lactose intolerance suffer from some of these symptoms, which often appear quickly after ingestion. The reaction can sometimes be delayed, however, until the next day.

Unfortunately, milk allergy can cause the same symptoms, but with an immune reaction, most often due to milk proteins (e.g. casein, alpha-lactalbumin, beta-lactoglobulin). A milk allergy or milk protein intolerance is much more serious than lactose intolerance. Because the symptoms are similar, it is sometimes difficult to differentiate them at first glance.

There are three tests to diagnose lactose intolerance.

The first (Breath Hydrogen Test) measures the level of hydrogen that is eliminated through the mouth when we breathe. The higher the hydrogen level, the greater our lactose intolerance due to the lack of lactase.

The second test consists of measuring the glucose in the blood following the ingestion of lactose. Since lactase converts lactose into glucose, a low blood glucose level following lactose ingestion is an indicator of lactose intolerance.

There is also a genetic test to detect PRIMARY lactose intolerance which is the most common intolerance.

Note that it is possible to be temporarily lactose intolerant due to :

  • severe gastroenteritis,
  • any disease that alters the integrity of the intestinal mucosa (e.g. celiac disease), chemotherapy or radiation therapy.
  • If the intestinal mucosa is destroyed, lactase production ceases. It is then necessary to wait a few weeks for this last one to be reconstituted to see the improvement of the absorption of lactose.

But how do you know if you are dealing with a lactose or dairy protein problem?

Here are a few tips to help guide you (see Figure 1).

Initially, it is advisable to remove dairy products from your diet for a few days. If your symptoms go away, we are probably on the right track. Second step, when the symptoms are gone, on an empty stomach, drink one or two glasses of regular milk. Observe the appearance of symptoms. If you don’t have any, there’s a good chance that dairy products are not the cause. If you have any, go a few more days without dairy products, then drink one or two glasses of lactose-free milk on an empty stomach this time. The absence of symptoms means probable lactose intolerance. The presence of symptoms suggests that you may have a problem with milk protein.

For those who are lactose intolerant, it is rare that you have a total absence of lactase, so you just have to find your own tolerance level. There are some good tips on this subject at

For those whose little home test suggests a problem with dairy proteins, it is important to consult your doctor because he is the only one who can make a diagnosis.

Lucie Hamel
Medical Technologist

Information and Research Center on Intolerances and Food Hygiene
Health Canada (