Polysaccharides are complex carbohydrates formed from long chains of simple sugars. Starch and fiber are different types of polysaccharide, with an important difference. Humans have enzymes that allow it to digest starch, but fiber is indigestible and passes through the digestive system to the gut microbiota , which can digest it.
Starch is produced by plants and stored for energy in their roots and seeds (e.g. in potatoes, wheat, rice, maize, cassava). Starches are made up of amylose and amylopectin, which are chains of simple sugars linked together with so-called alpha bonds that the enzymes in the human body can easily break down. Once broken down into glucose, it can be quickly absorbed into the bloodstream and thus results in higher insulin response.
Fiber is the part of plant-derived food that the human digestive system cannot digest, due to a lack of the necessary enzymes. Fiber is found in all plant foods. There are several components of fiber including cellulose, resistant starch, resistant dextrins, inulin, lignins, chitins, and pectins. Cellulose is a structural component of plants, a very abundant source of carbohydrates. Fiber is linked by beta bonds which the body can’t break down and much of it passes through the small intestine undigested. Fiber affects how other nutrients are absorbed in the gastrointestinal tract. Fiber can reduce cholesterol by binding bile (especially oats). Importantly, fiber slows the absorption of sugar into the bloodstream preventing insulin spikes. Fiber is processed by gut bacteria through a process called fermentation and turned into short chain fatty acids , not glucose.
Soluble and insoluble fiber Extra Soluble and insoluble fiber Nutrition science distinguishes between soluble and insoluble fiber. Fiber-rich foods typically contain both types. The distinction is simple: Soluble fiber dissolves in water; insoluble fiber does not. Soluble fiber absorbs water as it is digested and forms a gel that slows the absorption of glucose. It may provide a longer sense of satiety or fullness. Soluble fiber is the prebiotic that is fermented by gut bacteria . Insoluble fiber is thought to be important for digestive function. It is unaffected by digestion, but adds bulk and absorbs water to stool making it softer and easing defecation.