Fiber in the lower GI tract tends to hasten GI transit due to which mechanisms?

• A. Decreased stool weight
• B. Increased stool weight and water, plus increased bacterial mass from fermentation
• C. Reduced colonic motility
• D. Reduced water content

Answer: (B)

Fiber speeds lower GI transit mainly by increasing the bulk and the water content of the stool, and by adding bacterial mass through fermentation. Insoluble fiber raises stool weight and helps retain water in the colon, which stretches the colonic wall and stimulates peristaltic reflexes that move content along more quickly. Fermentation by colonic bacteria adds to stool mass with microbial biomass and produces short-chain fatty acids that further stimulate colonic motility and water movement into the lumen. This combination of larger, wetter stool and fermentative byproducts is what accelerates transit in the colon. In contrast, scenarios that reduce stool weight, reduce water content, or slow colonic motility would slow transit.