Quick version
Lactose is a type of sugar found in dairy products and is broken down by the enzyme lactase in the small intestine. In lactose intolerance, lactase production is absent or reduced, which leads to lactose being broken down by bacteria in the large intestine instead, which can cause gas, bloating, diarrhea and stomach discomfort.
There are three types of lactose intolerance; congenital lactose intolerance, secondary lactose intolerance and primary lactose intolerance, which involves a hereditary reduction in lactase production after childhood, this is the most common form.
To find out if you are lactose intolerant, you can do a provocation test by excluding lactose from your diet and then reintroducing it to see if symptoms occur. Alternatively, a blood test can analyze a specific genetic variant in the lactase gene to determine if you have a hereditary predisposition to primary lactose intolerance. The test may be relevant for people with recurring stomach problems after dairy products, uncertain symptoms, or relatives with lactose intolerance.
What are lactose and lactase?
Lactose is a type of sugar that is naturally found in milk and other dairy products. It is a disaccharide that consists of two simpler sugars, glucose and galactose, which are joined together. In order for the body to absorb lactose, it must first be broken down in the small intestine by the enzyme lactase. Lactase is produced in the lining of the small intestine and breaks down lactose into glucose and galactose, which can then be absorbed by the body and used as energy.
What is lactose intolerance?
Lactose intolerance means that the body lacks the enzyme lactase, which in turn means that lactose cannot be completely broken down in the small intestine. Instead, the lactose is carried to the large intestine where it is broken down by bacteria – when this happens, it causes problems such as gas formation, bloating and diarrhea.
Common symptoms of lactose intolerance include:
- Gas and bloating
- Abdominal cramps and rumbling
- Nausea
- Diarrhea
- Abdominal distension
If you are lactose intolerant, symptoms usually appear within a few hours of eating dairy products and vary in severity depending on how much lactase you still produce.
What types of lactose intolerance are there?
There are three main types of lactose intolerance:
- Congenital lactose intolerance – This is a very rare genetic defect where lactase is completely absent at birth.
- Primary lactose intolerance – This is the most common form, caused by a hereditary decrease in lactase production after childhood.
- Secondary lactose intolerance – This is a temporary lactase deficiency that can occur when the lining of the small intestine is damaged, e.g. in celiac disease or intestinal infections.
How to find out if you are lactose intolerant
A common method that is often recommended is a provocation test where you completely exclude lactose from your diet for a number of weeks and then try drinking a larger amount of milk (more than 2 dl). If you have been symptom-free during the period without lactose and then develop symptoms when provoked, this indicates that you are lactose intolerant.
It is also possible to carry out a blood test that analyzes B-lactase gene genotype to identify the hereditary form of lactose intolerance. The test analyzes a specific genetic variant in the lactase gene that determines whether you retain or lose the ability to produce lactase in adulthood.
The result shows whether you have a gene variant that causes lactase production to decrease, which means you have an increased risk of lactose intolerance. If the test shows that you have the genetic predisposition for primary lactose intolerance, you can expect that lactose intolerance will persist throughout your life.
The test may be relevant for you who:
- Often get stomach problems after eating dairy products.
- Want to get confirmation whether you have hereditary lactose intolerance.
- Have unclear symptoms that resemble lactose intolerance and want to rule out other causes.
- Have relatives with lactose intolerance and want to know if you yourself carry the predisposition.
If you suspect that you are lactose intolerant, an analysis of B-lactase gene genotype can help you get a clear diagnosis and adapt your diet to your needs.