Positive lactose hydrogen breath test 2023-01-16T09:09:11Z Elevated hydrogen gas level in exhaled breath following oral administration of lactose. In individuals with lactose intolerance, undigested lactose in the colon will be metabolized by colonic bacteria, releasing hydrogen gas. The enzyme lactase phlorizin hydrolase, located at the intestinal brush border, is necessary for the hydrolysis of lactose, the main sugar in milk. Due to the genetically programmed decrease in intestinal lactase activity that occurs post-weaning (lactase non-persistence), a large proportion of the human population loses, in adult age, the possibility to digest and absorb lactose. Positive hydrogen breath test