Describe the impact of fiber on the upper and lower GI tract.

• A. Upper GI transit is faster, and fiber in the lower GI slows transit.
• B. Fiber has no effect on upper GI and speeds lower GI.
• C. Upper GI transit is slower (especially with soluble fibers); lower GI transit hastens due to increased stool weight, water, and bacterial mass.
• D. Upper GI transit speeds up with soluble fiber, lower GI slows due to fiber fermentation.

Answer: (C)

Fiber affects different parts of the GI tract in distinct ways. In the upper GI tract, soluble fiber increases luminal viscosity and forms gels, which slows gastric emptying and small-intestinal transit, delaying how quickly contents move through that region. In the lower GI tract, fiber is fermented by colonic bacteria and adds bulk to stool while retaining water. This increased stool weight and moisture, along with microbial mass, stimulates colonic peristalsis and speeds transit through the colon. So describing a slower upper-GI transit with soluble fiber and a hastened lower-GI transit due to stool bulk, water, and bacterial mass captures these region-specific effects.