Carbohydrates are divided into four categories: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

There are three types of sugar conjugates: glycolipids, glycoproteins, and proteoglycans.

1. Monosaccharides Carbohydrates that cannot be hydrolyzed into simpler sugars are called monosaccharides.

1. Glucose: The common and most important monosaccharide, often exists in free form in fruits, honey and various plant liquids.

2. Fructose: Sucrose coexists in fruit juice and honey, especially in apples and tomatoes. Fructose is the sweetest among sugars, and its sweetness is 1.2-1.5 times that of sucrose

3. Galactose: It is a component of lactose in mammalian milk. It exists in the brain and nervous tissue in the form of D-galactoside, which can promote the synthesis of cerebrosides and polysaccharides and is an important material basis for the brain development of infants and young children. It is not widely distributed and contained in animals - snails, frog eggs, and cow lungs. In plants, it exists in the form of plant polysaccharides in various plant gums - agar, gum arabic, mesquite gum, etc.

4. Mannose: Very little content in food.

2. Disaccharide consists of two monosaccharide molecules

1. Sucrose: It exists in plants, and is abundant in sugar cane, beets and sweet fruits. It is the longest-used sweetener by humans.

2. Maltose: It is the hydrolyzate of starch. Commonly known as malt sugar, it is often used in food processing.

3. Lactose: It is the main sugar in milk. Cow milk contains 4% lactose, human milk contains 5.7%, and milk powder contains 25% lactose. Lactose is not easily soluble, and lactase in human digestive juices hydrolyzes lactose into corresponding monosaccharides. Not very sweet.

4. Trehalose: Found in fungi and bacteria, such as edible mushrooms.

3. Sugar alcohols are the products of reduction of monosaccharides. It is used in the food industry as a sweetener and humectant, and mannitol preparations are used in medicine.

IV. Oligosaccharides are composed of 3 to 9 monosaccharide molecules linked by glycosides. The molecular weight is not large, but it can stimulate taste buds and has a sweet taste. It is often used as a functional food additive and is mostly used in beverages. Its sweetness is 30-60% of sucrose, and it has good solubility, thermal stability and acid resistance. It cannot be broken down by human digestive enzymes, so it is not easily digested and decomposed in saliva and gastric juice, but some of it can be digested and utilized in the colon.

Several important functional oligosaccharides currently known are as follows:

Isomalto-oligosaccharides: Found in small amounts in some fermented foods such as soy sauce and wine.

Fructooligosaccharides:It is found in fruits and vegetables, with higher contents in onions, burdock, and asparagus.

Soy oligosaccharides: the general name for soluble sugars in soybeans. Found in soybeans, cottonseed, and sugar beets.

5. Starch polysaccharide (modern concept - carbohydrates generated from blood sugar)

1. Starch: a polysaccharide composed of D-glucose. In plant tissues, starch is divided into amylose and amylopectin.

Characteristics: Foods containing amylose are prone to aging and form indigestible resistant starch. Amylopectin easily absorbs water and then swells into a paste, with high digestibility. Stored in plant seeds and rhizomes. For example, corn and wheat contain 20-25% amylose and 75-80% amylopectin. Glutinous grains: Glutinous rice, waxy corn, and waxy sorghum contain more amylopectin.

2. Glycogen: Characteristics: Dissolved in water and quickly decomposed into glucose under the action of corresponding enzymes, quickly providing energy. It is abundant in shellfish molluscs.

3. Resistant starch: The general name for the undigested and absorbed starch and its degradation products remaining in the small intestine of healthy people. Features: Digested in the small intestine, fermented in the colon and completely absorbed. Widely found in some fruits, beans, cereal breakfasts and pasta products

Divided into three categories: (1) Physiologically unacceptable starches - whole grains and large starch granules

(2) Starch with special crystal structure - raw potatoes and bananas.

(3) Aged starch - cooked cold potatoes.

Non-starch polysaccharides: including cellulose, hemicellulose, pectin, gum, etc. Non-starch polysaccharides and lignin are also called dietary fiber

6. Polysaccharides bound to amino acids and lipids

1. Glycoprotein: a combination of protein and carbohydrate. It has protective and moisturizing effects on epidermal cells such as digestion and respiratory tract. It has a recognition effect on tumor cells and the immune system.

2. Glycolipids: divided into cerebrosides and gangliosides. As a component of cell membranes. It has an impact on cell-cell recognition, transmembrane information transmission, and adhesion.

Physiological functions of carbohydrates

1. Providing energy is the most economical and main source of energy. 1 gram of glucose is completely oxidized and decomposed in the body, releasing 4 kcal of energy.

2. Body components are important substances that constitute body tissues and participate in cell composition and various activities. It is a component of certain substances with physiological functions such as antigens, antibodies, enzymes, and hormones.

3. Save protein. All life activities of the body are based on energy. When the supply of carbohydrates is insufficient, glucose will be produced through gluconeogenesis and energy production from protein and fat.Meet energy needs. When food supplies a sufficient amount of available carbohydrates as energy, it can reduce the consumption of protein as energy, thereby saving protein.

4. Regulation of fat metabolism Incomplete fat oxidation can produce excessive ketone bodies, and adequate supply of carbohydrates can reduce the production of ketone bodies. Has antiketogenic effects.

5. Improve sensory quality and increase satiety

6. Detoxification: Glucuronic acid generated during carbohydrate metabolism is an important binding detoxifier, which can remove bacterial toxins, alcohol, arsenic and other toxic substances in the liver.

Seven. Enhance intestinal function. Non-starch polysaccharides - cellulose, pectin, resistant starch, and functional oligosaccharides can stimulate intestinal peristalsis and promote defecation.

Food sources and intake of carbohydrates

Carbohydrate content: cereals: 70-80%, roots and tubers: 15-25%, beans: 21%, fruits 10-20%, dried fruits 50-70%,

Supply:: Carbohydrates account for 55-70% of total energy

Foods that mainly provide carbohydrates:

Rice, noodles, cereals, beans, potatoes

Various snacks and sweets

Cold drinks, sweet drinks

Various candies, sugar, honey

Vegetables and fruits