Lactulose reduces ammonia by which mechanism?

• A. Lowering colonic pH to convert ammonia to ammonium
• B. Increasing colonic pH
• C. Enhancing ammonia absorption
• D. Inhibiting lactulose metabolism

Answer: (A)

Lactulose lowers ammonia by acidifying the colon. When it reaches the colon, resident bacteria break it down into short-chain organic acids, which drop the colonic pH. In this acidic environment, ammonia (NH3) picks up a proton to become ammonium (NH4+). NH4+ is charged and poorly absorbed, so it stays in the lumen and is excreted in the stool, decreasing the amount of ammonia that enters the bloodstream. This helps reduce neurotoxic effects in hepatic encephalopathy. If colonic pH were raised, more ammonia would remain as NH3 and could be absorbed, making the situation worse. Options proposing increased ammonia absorption or inhibition of lactulose metabolism don’t produce the desired effect.