What is Fiber? Often called “roughage” or “bulk”, fiber is a plant-based nutrient that is a type of indigestible carbohydrate, that cannot be broken down into sugar molecules in the blood. This is crucial for people living with diabetes; fiber makes one feel fuller, but the carbohydrates it’s made up of will not affect blood sugar levels, with fiber passing through the digestive system relatively intact. Two Kinds of Fiber Fiber is commonly classified as soluble, which dissolves in water, or insoluble, which doesn’t dissolve. Soluble fiber: This type of fiber dissolves in water to form a gel