A soluble fiber found in the cell walls of oats and barley, celebrated for its cholesterol-lowering effect. In the gut it forms a viscous gel that traps bile acids, forcing the liver to draw on blood cholesterol to make more, which measurably reduces LDL levels; health claims for this effect are approved in multiple jurisdictions for foods providing sufficient daily amounts. The same viscosity slows glucose absorption, moderating blood sugar rises, and promotes satiety. Oats are the richest practical source, with porridge, oat bran, and oat-based cereals delivering meaningful doses. Beta-glucans from yeast and mushrooms have related but distinct immune-modulating properties studied separately. For heart health, few dietary components have as solid an evidence base as this humble oat fiber.