Biochemical Foundations of Human Vital Activity - Volkov, N.I., Nesen, E.N. 2000

Biochemical Foundations of Human Vital Activity
Biochemistry of Carbohydrates
Chemical Composition and Biological Role of Carbohydrates

The critical role of CARBOHYDRATES in providing energy for Muscle activity was recognized as early as the first half of the 20th century. Carbohydrates serve as the primary energy substrates for ATP resynthesis during high-intensity and prolonged physical exertion. Physical performance and the onset of fatigue depend directly on their storage levels in skeletal Muscles and the Liver. Carbohydrates hold a prominent place in athletes' diets, as they must be continuously replenished. Therefore, it is essential to examine the Structure AND Functions of individual carbohydrate classes, their intercellular METABOLISM, and their impact on the energetics of muscular activity.

Carbohydrates are a Class of Organic compounds composed of carbon (C), hydrogen (H), and oxygen (O) atoms in a 1:2:1 ratio. The general formula for carbohydrates is С_nН_2nО_n or (СН₂O)_n, where n = 3—9 carbon atoms. According to the International Classification, carbohydrates are referred to as glycides, although this term is rarely used. Certain carbohydrates may also contain other chemical elements, such as nitrogen, sulfur, and phosphorus.

Regarding their chemical structure, carbohydrates are polyhydroxy aldehydes (aldoses) or polyhydroxy ketones (ketoses). Aldoses contain a single aldehyde functional group

at the first carbon atom and several hydroxyl groups (-OH) attached to the remaining carbon atoms. Ketoses contain a single keto group at the second carbon atom along with hydroxyl groups. Glucose is a classic example of an aldose, while fructose is an example of a ketose.

Carbohydrate content in The Human Body is relatively low, accounting for up to 2–3% of total body mass. They are stored as Glycogen in the liver (5 to 10% of total organ mass), skeletal muscles (1–3%), and The Heart (up to 0.5%). Glycogen reserves in an adult with a body mass of 70 kg average about 500 g. In addition to glycogen, the body contains free glucose, the concentration of which in the Blood is relatively small—around 5 g. Carbohydrates store approximately 2000 kcal of energy, enabling the body to perform high-intensity physical work for 30 minutes to 1 hour, and low-intensity work for several hours (up to 12).

In the human body, carbohydrates are synthesized only in negligible amounts via Gluconeogenesis; therefore, the vast majority must be obtained through diet. Carbohydrates are found predominantly in plant-based foods, as their primary synthesis occurs in green plants during Photosynthesis:

In certain plants, such as cereal grains, carbohydrate content reaches up to 80% of their dry mass. The daily human requirement for carbohydrates is 300–400 g, whereas for athletes, it ranges from 400 to 700 g.

In the human body, carbohydrates perform the following biological functions: energetic, structural (plastic), nutritive, specific, protective, and regulatory.

✵ Energetic. During carbohydrate breakdown, the released energy is either dissipated as heat or captured within ATP molecules. Carbohydrates account for about 50–60% of the body's daily Energy Expenditure, rising to up to 70% during endurance exercise. The oxidation of 1 g of carbohydrates yields 17 kJ (4.1 kcal) of energy. Free glucose or stored glycogen serves as the primary energy source in the body.

✵ Structural (Plastic). Carbohydrates (ribose, deoxyribose) are utilized in the synthesis of ATP, ADP, other NUCLEOTIDES, and Nucleic Acids. They are also constituents of certain Enzymes. Individual carbohydrates serve as Structural components of Cell membranes. Glucose derivatives (glucuronic acid, glucosamine, etc.) are integrated into Polysaccharides and complex Proteins found in Cartilage and other Tissues.

✵ Nutrient storage. Carbohydrates are stored (reserved) in skeletal muscles, the liver, and other tissues as glycogen. Glycogen reserves depend on body mass, the functional state of the body, and dietary habits. Physical activity significantly depletes glycogen stores, which are subsequently replenished during the post-exercise recovery period primarily through food intake. Systematic physical training leads to increased glycogen storage, thereby enhancing the body's energetic capacity.

✵ Specific. Certain carbohydrates contribute to blood group Specificity, act as anticoagulants, function as receptors for various Hormones or pharmacological agents, and exhibit antitumor activity.

✵ Protective. Complex carbohydrates are Components of the immune system; mucopolysaccharides are present in mucous secretions that coat the surfaces of the nasal passages, Bronchi, digestive tract, and urogenital tract, protecting against bacterial and viral invasion as well as mechanical damage.

✵ Regulatory. Dietary fiber is not digested in the intestine, yet it stimulates intestinal peristalsis and digestive tract enzymes, thereby improving Digestion and the absorption of nutrients.