The enzyme lactase is synthesised in the small intestine and breaks down lactose (a disaccharide) into two monosaccharides, which can then be absorbed into the body. If this enzyme (lactase) is not present, or if not enough of it is produced, this is called lactose intolerance or lactase deficiency. Lactose, also known as milk sugar, will reach the sections of the intestines further down without having been digested, and can therefore not be absorbed via the small intestine.
In the following sections of the intestines there are intestinal bacteria that break down the lactose. This, however, does not happen with the aid of the enzyme lactase, but through the enzyme beta-galactosidase (also known as β-galactosidase) instead. In this case they will produce hydrogen, short-chain fatty acids and CO2. With the help of hydrogen (which, by the way, does not cause any lactose intolerance symptoms) it is possible to diagnose lactose intolerance by means of a painless breath test. But this depends on the kind of intestinal bacteria present. There are bacteria which produce methane. In order to do this the bacteria will also use the hydrogen that is produced during synthesis. This will result in severe flatulence for those people affected, but a proof of lactose intolerance is not possible by using the hydrogen breath test because the hydrogen is no longer being exhaled via the lungs as it has been compounded with the methane.
The CO2 causes bloating and the short-chain fatty acids (acetic acid, butyric acid also known as butanoic acid,...) will change the osmotic properties in the intestine and lead to fluids entering the gut lumen. This causes runny diarrhoea. In some rare cases, when methane is produced, the result will be excessive flatulence.
There are patients, however, who can experience these symptoms for other reasons than food intolerance. For this reason it is absolutely essential to have a test done by a medical professional, if a person believes they may be suffering from food intolerance. In an experiment conducted in 1995, 30 people were tested who believed that they suffered from lactose intolerance and reported severe symptoms after drinking milk. 9 of these patients were not lactose intolerant and were therefore able to digest lactose without any consequences at all.
Before you consult your medic it is very useful to keep a food diary; (you can find a print-out blank form for such a food diary in the download section)
Allergies to milk (casein allergy, lactalbumine allergy, cow’s milk allergy) are not be confused with lactose intolerance! They are not food intolerances, but true allergies to certain ingredients in milk!
Suarez F.L., Savaiano D.A., Levitt M.D: A Comparison of Symptoms after the Consumption of Milk or Lactose-Hydrolyzed Milk by people with Self-Reported Severe Lactose Intolerance, N Engl J Med. 1995 Jul 6;333(1):1-4.
Medow et al.: Lactose Intolerance. New England Journal of Medicine 333 (1995): 1358-1359
Ledochowski M., Bair H., Fuchs D.: Laktoseintoleranz. J Ernährungsmed. 1/2003 (2003): 7-14