New understanding of dietary fiber

Speaking of dietary fiber or cellulose, we seem to be very familiar with it, but also seem to be unsure. In most people's cognition, dietary fiber is a mixture of a group of plants, including various plant cell wall materials such as cellulose, hemicellulose, pectin and lignin, as well as intracellular polysaccharides such as gum and plant mucus. Mammalian digestive enzymes cannot hydrolyze these substances, so people cannot digest and absorb dietary cellulose smoothly. But in a broad sense, dietary fiber also includes other indigestible components in food, such as wax, cutin, resistant starch, Maillard reaction products, oligosaccharides and indigestible cell wall proteins that also come from plant foods, as well as chitosan from animal foods. In order to give us a more scientific understanding, researchers have made a more precise and systematic definition and classification of dietary fiber. First of all, the total fiber in food refers to the sum of dietary fiber and functional fiber, among which dietary fiber includes inherent and original indigestible carbohydrates and lignin in plants, and functional fiber includes isolated and extracted indigestible carbohydrates that can produce beneficial physiological effects on the human body. To put it simply, dietary fiber comes from nature, while functional fiber needs to be obtained artificially.

The physiological functions of dietary fiber on the human body mainly depend on its physical and chemical properties. In the past, people judged its health effects based on its solubility, so dietary fiber was divided into soluble dietary fiber and insoluble dietary fiber, but this distinction or classification is not accurate. Recent studies have shown that the viscosity of fiber and its sensitivity to fermentation may better reflect its physiological effects. Studies have shown that viscosity is the most consistent characteristic with the cholesterol-lowering effect of fiber. Most viscous separated fibers can reduce human blood cholesterol concentration, such as pectin, psyllium, guar gum, carob gum and modified fibers (such as carboxymethyl cellulose); if the viscosity of the fiber decreases, their ability to lower cholesterol will also decrease. Conversely, studies have also found that non-viscous separated fibers or fiber precursors, such as cellulose, lignin, corn bran and wheat bran, rarely change plasma cholesterol levels. Therefore, people who need to lower plasma cholesterol can pay attention to diets rich in viscous polysaccharides such as oat bran, barley, and legumes. Similarly, viscous dietary fiber is also beneficial for blood sugar control. The fermentation properties of fiber have little effect on plasma cholesterol, but can affect stool volume. In general, fiber sources that are difficult to ferment increase stool weight the most, such as wheat bran; fiber sources that are more easily fermented can moderately increase stool output, such as fruits and vegetables; while beans and pectin only slightly increase stool volume. The increase in stool weight mainly depends on the number of intestinal flora in the stool or the amount of unfermented fiber. In the intestines, wheat bran is difficult to ferment, so it effectively increases the undigested residue, thereby increasing the weight of the stool; while the fiber in fruits and vegetables and oat bran can be fermented in large quantities, thereby increasing the number of flora in the stool. Therefore, people with defecation problems can try different fermentation properties of fiber to improve symptoms. The viscosity and fermentation properties of common fibers are shown in the table below.

Viscosity of some fibers and their sensitivity to glycolysis