A positive hydrogen breath test indicates what?

• A. Elevated breath methane indicates bacterial overgrowth causing bloating
• B. Malabsorption of lactose due to lactase deficiency
• C. Increased stool fat content
• D. Low breath hydrogen

Answer: (B)

A hydrogen breath test detects how well our small intestine absorbs certain sugars by looking for hydrogen produced when gut bacteria ferment unabsorbed carbohydrate. When lactase is deficient, lactose isn’t split and absorbed in the small intestine, so it travels to the colon where bacteria ferment it, generating hydrogen that then enters the bloodstream and is exhaled. A rise in breath hydrogen after a lactose challenge is, therefore, a sign of lactose malabsorption due to lactase deficiency. This helps explain symptoms like bloating and gas. In contrast, increased stool fat would point to fat malabsorption, and a low breath hydrogen would suggest little fermentation rather than lactose intolerance. (Methane can be produced by some gut microbes but is not the defining indicator for lactose malabsorption.)