Lactose Tolerance Test

Alternate Names

  • Hydrogen Breath Test

The enzyme lactase is necessary for the normal breakdown and use of lactose, the sugar found in all milk products. When lactase levels are decreased or absent, lactose cannot be broken down into galactose or glucose in the small intestine. This results in a build-up of lactose in the large intestine, which is then broken down by bacteria resulting in diarrhea, abdominal cramps, gas, and bloating. Low lactase levels are seen in some degree in all adults, but inherited lactase deficiency in babies can cause vomiting, diarrhea, and difficulty absorbing nutrients which leads to delayed normal growth. In adults, any conditions that cause bowel inflammation or difficulty absorbing nutrients can lead to lactose intolerance. There can also be the presence of large intestine bacteria that migrate up into the small intestine (where they normally do not reside), which causes also causes symptoms, since the lactose does not have had the chance to be broken down before being exposed to the bacteria.

You will be given a drink containing lactose; if lactase deficiency is present, blood glucose levels will not elevate. When bacteria digest the lactose in the large intestine, they release higher than normal amounts of hydrogen, which can be detected in exhaled breath. The breath test can also be used to test for other enzyme deficiencies; baseline levels are measured and measurements are then repeated at set intervals after you are given the drink containing lactose.


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