Explain the mechanism of lactose intolerance testing or typical presentation.

• A. Breath hydrogen increases after ingestion of lactose due to bacterial fermentation of lactose in colon.
• B. Breath methane decreases after lactose ingestion.
• C. Increased urine ketones after lactose ingestion.
• D. Elevated blood glucose after lactose ingestion.

Answer: (A)

When lactose isn’t properly digested because of lactase deficiency, the lactose you drink reaches the colon unabsorbed. There, colonic bacteria ferment it, producing gases such as hydrogen (and some methane) and short-chain fatty acids. Hydrogen produced in the gut diffuses into the bloodstream and is exhaled in the breath, which is why the test for lactose intolerance hinges on a rise in breath hydrogen after a lactose load. A positive result is a clear increase in breath hydrogen within about 2 to 3 hours, reflecting lactose malabsorption.

In practice, this test is used because it directly reflects the fermentation process happening in the colon. Some people, however, produce methane instead of or in addition to hydrogen, so breath tests may measure both gases. Clinically, many people with lactose intolerance notice bloating, cramps, gas, and diarrhea after consuming dairy, and the severity depends on how much lactose is ingested and the individual's gut bacteria. The other options aren’t typical findings: breath methane doesn’t consistently decrease after lactose ingestion, urine ketones aren’t a marker of lactose malabsorption, and a significant rise in blood glucose isn’t a reliable or expected outcome in lactose intolerance.