Dr. Greger explains how fiber helps reduce cholesterol by increasing fecal bulk and intestinal flow and preventing excess nutrients from being back-absorbed. The main mechanism is given in the end: "The main mechanism currently attributed to insoluble fiber is its ability to increase fecal bulk and decrease transit time in the intestine. The only proposed mechanism concerning these observations pertained to disruption of bulk phase diffusion in the intestine, making movement of cholesterol to the enterocyte surface difficult."
