Nutrition and Therapeutic Dietetics

Author: Zhang Leqi / et al.
Editor-in-Chief: Li Weidong
Publisher:
Publish Date: 1999-08-01
Features:
Excerpt: Microbial fermentation utilization. For example, ginsengol and verbasose in legume foods, and cyclodextrin and fructooligosaccharide applied in the food and pharmaceutical industries. (IV) Polysaccharides
Polysaccharides (polysaccharides) are large and complex molecules composed of many monosaccharide units, with a high molecular weight. Their properties differ significantly from those of simple sugars like monosaccharides and disaccharides. Polysaccharides are tasteless, non-crystalline, and insoluble in water. Starch, dextrin, glycogen, and dietary fiber are representative examples, which have been emphasized in recent years.
1. Starch: A form of stored energy in plants, consisting of chains of hundreds to thousands of glucose units. It is classified as amylose (linear chains) or amylopectin (branched chains), with glycosidic bonds in the form of α-1,4 and α-1,6 linkages. Starch is insoluble in water; the outer layer of raw starch provides protection. However, after heating and absorbing water, the outer layer breaks, causing it to become viscous—a process called gelatinization.
2. Dextrin: A partial hydrolysis product of starch, with a molecular size about one-fifth that of starch. It is produced during the process of converting starch into syrup.
3. Glycogen: A polysaccharide stored in the liver and muscles of animals, also known as animal starch. The human body can rapidly break down glycogen to balance blood sugar levels. The liver converts excess glucose into glycogen for storage, lowering blood sugar. If blood sugar is too low, hormones can promote glycogen breakdown into glucose, which is then transported to the bloodstream to raise blood sugar. However, glycogen in muscles cannot regulate blood sugar levels and is only converted into glucose when muscle cells need energy for oxidation. Generally, the total amount of glycogen stored in the human body is about 350g. The glycogen stored in the liver can be converted into glucose to provide energy for any cell in the body. However, it only supplies energy for 3–4 hours. If the body needs more energy, it may have to rely on other heat sources.
4. Dietary Fiber: Polysaccharides and lignin from plants that cannot be digested by human digestive enzymes are dietary fiber. Since the small intestine cannot absorb them, they do not provide energy or nutritional benefits. Due to the increasing refinement of dietary habits, diets high in calories, fat, and cholesterol are severely lacking in dietary fiber, leading to a rise in the incidence of constipation, diverticulitis, colorectal cancer, rectal cancer, hypertension, and diabetes. Therefore, dietary fiber intake has gained increasing attention in recent years. Dietary fiber can be roughly divided into two categories (Table 2-1): water-soluble and non-water-soluble.
- Water-soluble includes pectin, plant gum, and alginic acid, with food sources such as fruits, legumes, whole grains, vegetables, and seaweeds.
- Non-water-soluble includes cellulose, hemicellulose, and lignin, with primary food sources being whole grains and fruits and vegetables. Dietary fiber is very important for modern dietary life. If the body does not have special conditions that cannot tolerate dietary fiber, such as gastrointestinal ulcers, it is recommended that people in China adopt a high dietary fiber intake in their daily diet, as dietary fiber has significant physiological functions:
(1) Preventing constipation: Dietary fiber has water-holding capacity, which can soften feces.
(2) Preventing hemorrhoids and diverticulitis: Dietary fiber can reduce intestinal pressure and absorb water, stimulating intestinal peristalsis to facilitate defecation.
(3) Preventing colorectal cancer: Improving the distribution of intestinal flora, reducing the occurrence of harmful bacteria using bile acid and cholesterol for secondary decomposition to produce toxic carcinogens, and shortening the time waste stays in the intestine.
(4) Reducing cardiovascular diseases: Can bind to cholesterol, bile acid, etc., to prevent deterioration of blood circulation.
(5) Stabilizing diabetes: Water-soluble dietary fiber can slow down the rate of blood sugar rise and reduce the need for insulin.
(6) Weight loss: Dietary fiber is bulky, low in calories, and provides a feeling of fullness, which can reduce calorie intake and help control weight.
(7) Detoxification: Can adsorb toxic substances, reducing the production of toxic substances.
To increase dietary fiber intake, consume more whole-grain staple foods rich in dietary fiber in daily meals—brown rice, whole-grain rice (black rice, wheat, Job's tears, mung beans, etc.), whole-wheat bread, and bran buns; various vegetables, sprouts, seaweeds, wild greens, and fresh fruits; while reducing the intake of refined processed foods to the minimum.
### II. Functions of Carbohydrates
1. Providing Energy: 1 gram of sugar produces 4 kcal of energy when oxidized. For the human body, carbohydrates are the primary source of energy. If the diet provides sufficient carbohydrates, the body will prioritize them as an energy source, using proteins for tissue repair and storing fats for future needs. If the diet lacks energy, the body will prioritize energy supply and use fats and proteins in tissues to make up for the deficiency.
2. Constituting Tissues: About 100g of glycogen is stored in the liver, while the remaining 200–250g is distributed in muscle tissues. Important physiological compounds and blood and body fluids also contain carbohydrates, such as DNA, RNA, the carbon skeleton of non-essential amino acids, and glycosides.
3. Saving Proteins: Only when the diet provides sufficient carbohydrates can proteins be fully used for tissue repair and construction without energy shortages.
4. Regulating Fat Metabolism: Fat needs to be oxidized normally with an appropriate amount of carbohydrates. Otherwise, if fat is not fully oxidized, it may produce large amounts of ketone bodies, leading to acidosis, dehydration, and electrolyte imbalance.
5. Assisting the Digestive System: Lactose can promote the growth of beneficial bacteria in the intestines, leading to the synthesis of vitamin B complex and enhanced calcium absorption. Dietary fiber also has effects on intestinal health, preventing constipation and colorectal cancer.