Causes and diagnosis of lactose intolerance
Some people have digestive problems after drinking milk or eating dairy products. They only tolerate very small amounts of lactose because their bowel has trouble breaking it down.
Lactose is the main carbohydrate in milk produced by cows and other animals. Human breast milk also contains lactose. It is not present in vegetable products like soy milk. Lactose consists of two sugars: glucose and galactose. An enzyme in our small intestine called lactase quickly breaks down the lactose into its two parts. Only after the two sugars have been separated can they be absorbed by our bowel.
How does lactose intolerance develop?
Babies' digestive systems are designed to survive on breast milk alone. In order to digest the milk, babies produce a large amount of the enzyme lactase. When a child is weaned off breast milk, their digestive system gradually adapts to other foods. Their body then produces considerably less lactase, and can therefore only break down smaller amounts of lactose. This is referred to as primary lactase deficiency.
Due to genetic differences, some people's bodies produce much less lactase than others, so they don't tolerate milk as well as other people. The amount of lactase normally produced also varies between ethnic groups. For example, Northern European adults produce more lactase than adults from parts of Asia and Africa, where traditionally only very small amounts of dairy products are eaten.
Some people's bodies produce less lactase due to a bowel problem. This is called secondary lactase deficiency. It may be caused by things like gluten intolerance (celiac disease) or chronic inflammations in the membranes lining the small intestine, such as Crohn's disease.
What causes the symptoms of lactose intolerance?
When lactose enters the small intestine it is quickly broken down by the enzyme called lactase. Only then can the separate building blocks of the lactose (glucose and galactose) be absorbed by the small intestine. If that happens quickly and completely, only a small amount of lactose or no lactose at all enters the large intestine (colon).
The digestive system
But if large amounts of lactose do enter the colon, they are broken down and converted by bacteria. This increases the production of gases, and more liquid passes into the large intestine. The colon can absorb some, but not all, of these extra gases and liquids. This is what causes flatulence (bloating and “wind”) and diarrhea.
The longer the lactose stays in the large intestine, the more it is broken down, which can cause discomfort. Some people may not have much lactase, but their large intestine works more efficiently. Then larger amounts of lactose are eliminated faster and don't cause too much trouble. The amount and type of bacteria in the large intestine (called intestinal flora) might play a role here.
What is in the food and the amount of time it stays in the stomach or the small intestine also affects the symptoms. Food that has high amounts of fat in it can slow down your digestion. Then the lactase has more time to break down the lactose, and the symptoms are often less severe as a result.
What else could cause these symptoms?
Bloating, feeling full, pain and diarrhea are also symptoms of a condition called irritable bowel syndrome (IBS). IBS is very common too, especially amongst people over the age of 35. In irritable bowel syndrome the large intestine is sensitive and lactose may be less well tolerated.
These symptoms might also be caused by sensitivities to foods other than dairy products, like fructose. Emotional stress can play a role too.
How is lactose intolerance diagnosed?
Finding the cause of a food intolerance is not easy. There are various ways to diagnose lactose intolerance.
- Elimination diet: This involves avoiding anything that contains lactose. If your symptoms improve, it is taken as an initial sign that you have lactose intolerance. You also keep a log of what you ate and any symptoms you had. Then a tolerance test is done under medical supervision. This test checks whether the symptoms return after you drink a lactose solution.
- Hydrogen breath test: If the tolerance test results are inconclusive, the amount of hydrogen in your breath is measured before and after drinking a lactose solution. When lactose reaches the large intestine and is broken down by bacteria, hydrogen is produced as a byproduct. The hydrogen is released into the bloodstream through the walls of the intestine and ends up being breathed out once it reaches the lungs. So people with lactose intolerance generally have more hydrogen in their breath. The test solution for adults contains 50 mg of lactose in 0.4 liters of fluid and is taken on an empty stomach. The breath test is repeated every 15 to 30 minutes for at least two or three hours. But smoking, chewing gum, poor oral hygiene and other factors can influence the results of the test. That is why it is important to check whether the lactose solution also causes typical lactose intolerance symptoms like bloating and diarrhea. If not, then higher hydrogen levels are likely to be caused by something else.
- Lactose tolerance test: This test measures blood sugar levels after drinking a lactose solution. If your blood sugar doesn't rise significantly, it's a sign that you are lactose intolerant. This is because the body can't absorb the sugar building blocks if the lactose isn't broken down. The test solution for adults contains about 50 grams of lactose and is also taken on an empty stomach. Blood sugar levels are measured regularly over the next two hours. It is important to watch out for symptoms like diarrhea or cramps during this test as well.
Halpert A, Drossman DA. Irritable bowel syndrome. In: McDonald J, Burroughs AK, Feagan BG (Ed). Evidence-based Gastroenterology and Child Health. Oxford: Blackwell Publishing; 2004. Pp. 265-283.
Keller J, Franke A, Storr M, Wiedbrauck F, Schirra J. Klinisch relevante Atemtests in der gastroenterologischen Diagnostik – Empfehlungen der Deutschen Gesellschaft für Neurogastroenterologie und Motilität sowie der Deutschen Gesellschaft für Verdauungs- und Stoffwechselerkrankungen. Z Gastroenterol 2005; 43(9): 1071-1090.
Ledochowski M, Bair H, Fuchs D. Laktoseintoleranz. Ernährungsmed 2003; 5(1): 7-14.
Marklund B, Ahlstedt S, Nordström G. Food hypersensitivity and quality of life. Curr Opin Allergy Clin Immunol 2007; 7(3): 279-287.
Marklund B, Wilde-Larsson B, Ahlstedt S, Nordström G. Adolescents' experiences of being food-hypersensitive: a qualitative study. BMC Nurs 2007; 6: 8.
Marton A, Xue X, Szilagyi A. Meta-analysis: the diagnostic accuracy of lactose breath hydrogen or lactose tolerance tests for predicting the North European lactase polymorphism C/T-13910. Aliment Pharmacol Ther 2012; 35(4): 429-440.
Sahi T. Genetics and epidemiology of adult-type hypolactasia with emphasis on the situation in Europe. Scand J Nutr Näringsforskning 2001; 45(1): 161-162.
Shaukat A, Levitt MD, Taylor BC, MacDonald R, Shamliyan TA, Kane RL et al. Systematic review: effective management strategies for lactose intolerance. Ann Intern Med 2010; 152(12): 797-803.
Wilt TJ, Shaukat A, Shamliyan T, Taylor BC, MacDonald R, Tacklind J et al. Lactose intolerance and health. Evid Rep Technol Assess (Full Rep) 2010; (192): 1-410.
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