Causes and diagnosis of lactose intolerance

Babies’ digestive systems are designed to survive only on breast milk. In order to digest the milk, they produce large amounts of lactase. When a child is weaned off breast milk or baby formula, their digestive system gradually adapts to digest and process other foods. When this happens, some people no longer produce enough of the enzyme lactase to break down the lactose in the foods and drinks that they consume. As a result, they don't tolerate lactose-containing products as well as other people do. This is referred to as primary lactase deficiency.

In some people, it is a bowel disease that is affecting the production of lactase. This is called secondary (acquired) lactase deficiency. It may be caused by things like celiac disease (gluten intolerance) or chronic inflammations in the membranes lining the small intestine, like in Crohn’s disease.

When lactose enters the small intestine, it is broken down by an enzyme called lactase. Only then can the separate building blocks of the lactose (glucose and galactose) be absorbed by the small intestine. If there's enough lactase in the small intestine, only a small amount of lactose will reach the large intestine (colon).

If bigger amounts of lactose do enter the colon, they are broken down and converted by bacteria. This fermentation process increases the production of gases like carbon dioxide (CO2), methane and hydrogen, as well as other byproducts like liquids and fatty acids. This is what causes flatulence (bloating and “wind”) and diarrhea.

The severity of the symptoms depends on both the amount of available lactase and other factors: Some people may produce hardly any lactase, but have a large intestine that works quickly. They can handle fairly large amounts of lactose quite well because the lactose doesn’t stay in their colon for very long. The longer it stays in the colon, the more likely it is to cause problems. The amount and type of bacteria in the colon (called intestinal flora) might play a role too.

Another factor is the type of food a person eats and what they drink, because that determines how long lactose stays in the stomach or small intestine. Food that has a lot of fat in it can slow down your digestion. This gives the small intestine more time to produce enough lactase to break down the lactose, often resulting in less severe symptoms.

Bloating, feeling full, pain and diarrhea are also symptoms of irritable bowel syndrome (IBS). IBS is very common as well, especially in people over the age of 35 years. In irritable bowel syndrome the colon is very sensitive and lactose may be less well tolerated. Emotional stress can play a role too.

Digestion problems might also be caused by sensitivities to foods other than dairy products, like fructose.

Finding the cause of a food intolerance is not easy. Your family doctor will refer you to a gastroenterologist to get a diagnosis.

There are various ways to diagnose lactose intolerance.

• Hydrogen breath test: This involves measuring the amount of hydrogen in your breath before and after drinking a lactose solution. When lactose reaches the colon and is broken down by bacteria, hydrogen is made as a byproduct. The hydrogen is released into the bloodstream through the walls of the bowel and ends up being breathed out once it reaches the lungs. So people with lactose intolerance generally have more hydrogen in their breath. To do this test, you drink a lactose test solution on an empty stomach. After that, the level of hydrogen in your breath is measured every 15 to 30 minutes. In total, the hydrogen breath test takes at least two or three hours. Smoking, chewing gum, poor oral hygiene and other factors can influence the results of the test.
• Lactose tolerance test: A lactose tolerance test measures your blood sugar levels before and several times after drinking a lactose solution. If your blood sugar doesn’t rise significantly, it’s a sign that you’re lactose intolerant. The lactose is not broken down if you have lactase deficiency. That means that no sugar building blocks are produced either, which the body would otherwise absorb into the bloodstream. This test is also done on an empty stomach. Blood sugar levels are measured regularly over the next two hours.
• Elimination diet: This test involves avoiding anything that contains lactose for roughly four weeks. If your symptoms improve, it is a sign that you could have lactose intolerance. You also keep a log of what you ate and any symptoms you had. After four weeks, you are then given a lactose tolerance test. This test checks whether the symptoms return after you drink a lactose solution.

The hydrogen breath test and lactose tolerance test measure whether your body can break down and absorb the lactose properly. They don’t provide any information about the cause of your symptoms. So it’s important to look out for symptoms like diarrhea and cramps as well during the test. You can only be certain that you’re lactose-intolerant if the lactose solution also causes typical symptoms.