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Enduring misconceptions about the physical effects of fiber have led to misunderstandings about the health benefits attributable to insoluble and soluble fiber. In the small bowel, clinically meaningful health benefits (e.g. cholesterol lowering, improved glycemic control) are gel-dependent (‘visco-elastic’) effects, and highly correlated with the viscosity of soluble gel-forming fibers: high viscosity gel-forming fibers (e.g. psyllium, ß-glucan, raw guar gum) exhibit a significant effect on cholesterol lowering and improved glycemic control, while non-viscous soluble fibers (e.g. inulin, wheat dextrin) and insoluble fibers (e.g. wheat bran) do not provide these gel-dependent benefits. In the large bowel, there are two mechanisms that drive a laxative effect. Both mechanisms require that the fiber resist fermentation to remain intact and present throughout the large bowel, and increase stool water content, resulting in bulky/soft/easy-to-pass stools. The two mechanisms are the ‘plastic’ effect (insoluble fiber) and the ‘visco-elastic’ effect (gel-forming soluble fiber), both of which increase stool water content, albeit by different mechanisms: the ‘plastic’ effect mechanically irritates the gut wall to stimulate water/mucous secretion (e.g. wheat bran, but only if particles are large/coarse), while the ‘visco-elastic’ effect has high water-holding capacity that resists dehydration (e.g. psyllium). Fermentable soluble fibers (e.g. wheat dextrin), and finely ground insoluble fiber particles (e.g. fine wheat bran), do not provide a laxative effect and can actually have a constipating effect (harder stools). It is important to recognize which fiber types possess the requisite physical attributes to exert a beneficial physiologic effect. For most fiber-related beneficial physiologic effects, ‘fiber needs to gel to keep you well’.