Beta-glucans are a type of fiber found in the cell walls of certain yeasts, bacteria, fungi, algae (like seaweed), along with other plants like oats, barley, wheat, rye, and mushrooms. Beta-glucans serve as a source of energy and structure in these various plant species.
Most grain-based beta-glucans are soluble fibers, which means they dissolve in water and are typically fermentable (i.e., prebiotic) in your digestive tract. Beta-glucans from bacteria, fungi, and yeast are typically insoluble in water.
More importantly, as a 2019 review in Molecules details, beta-glucans are a unique family of fibers that are biologically active, a characteristic that plays a role in their potent health benefits.