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

Biochemical Foundations of Human Vital Activity
Vitamins
Characteristics of Fat-Soluble Vitamins

Fat-soluble vitamin molecules contain long hydrocarbon chains, which is why they are insoluble in Water and dissolve only in non-polar Solvents such as fats, alcohols, and ethers. Consequently, the absorption of this group of Vitamins depends on the presence of fat and Bile in the gastrointestinal tract. For instance, in the absence of fat, only 10% of provitamin A is absorbed, whereas in its presence, this figure rises to about 60%. Fat-soluble vitamins can accumulate in the body alongside fats, leading to a much slower onset of avitaminosis during prolonged dietary deficiency. Excessive intake of these vitamins can cause hypervitaminosis, which may even be fatal. When vitamin supplements are taken, the recommended dosages must be strictly followed (Table 9).

Class="center">TABLE 9 Daily requirement of fat-soluble vitamins for an adult and their sources

Fat-soluble vitamins are resistant to Temperature and acids, but they are oxidized by atmospheric oxygen. The MECHANISM OF ACTION of fat-soluble vitamins is not yet fully understood because the specific Enzymes of which they are a component have not been identified.

Vitamin A

Biological action. Vitamin A (retinol) affects Vision as a component of the visual pigment, rhodopsin; it positively influences growth processes by enhancing METABOLISM/35.html">Protein Biosynthesis (anabolic effect), as well as germ Cell maturation and reproduction, and the state and Differentiation of the epithelium of mucous membranes in various Organs (Fig. 42). As an antioxidant, it prevents the escalation of Lipid Peroxidation in Cells, which typically occurs during physical activity and causes adverse Changes in the body.

Avitaminosis manifests as damage to the epithelial Cells of the Skin and mucous membranes of various organs (dryness, desquamation), including dryness of the cornea (xerophthalmia), which leads to blindness. Vitamin A and carotenes are used in the Treatment of Lung Cancer, psoriasis, and leukemia (Fig. 42).

Hypovitaminosis manifests as impaired visual acuity when moving from a well-lit area to a dimly lit one ("night blindness"). Vitamin A deficiency can be detected by The rate of dark adaptation of vision (not exceeding 6 s) or by using specialized adaptometers.

Hypervitaminosis leads to toxicosis accompanied by severe weight loss, nausea, Hemorrhage, Hair loss, and the loss of calcium salts from Bone tissue, resulting in frequent bone fractures or even death.

Chemical Structure. The vitamin A molecule contains double bonds which determine its redox and other properties:

The precursor for vitamin A synthesis in The Human Body is carotenes—provitamins A:

Fig. 42 Biological and therapeutic effects of retinol and carotenoids

Carotenes are found in orange-colored foods and were first isolated from carrots (from Latin *carota* — carrot).

The daily requirement for vitamin A is elevated in athletes whose sports involve high visual strain.

Vitamin D Group

Biological action. Vitamins of the D group (calciferols) regulate Calcium and phosphorus Metabolism in the Body, maintaining their constant Blood levels with the participation of Parathyroid hormone and Calcitonin; they enhance their Absorption in the Small Intestine and entry into the blood, as well as their release from bones and Kidneys (Fig. 43). Calciferols are also involved in regulating citric acid utilization, which is related to aerobic energy production and the functioning of the thyroid and Parathyroid glands, as well as the cardiovascular and immune systems. By regulating calcium metabolism, they influence Muscle contraction, Nerve Impulse transmission, and many other Ca²⁺-dependent processes.

Avitaminosis most commonly develops in infants under one year of age and is known as Rickets. In rickets, the supply of calcium and phosphorus to bones and skeletal Muscles is impaired. A decrease in their content leads to defective Bone Formation. Bones become soft, brittle, and deform under body weight. Children exhibit changes in Skull shape and delayed dental development. Skeletal muscles lose their contractile ability. The Development of rickets is observed when blood phosphorus levels drop from 0.05 to 0.03–0.02 g ∙ l^-1, which can be used for its detection.

Vitamin D is synthesized in the human body under The Influence of sunlight from provitamin D₃, which is why avitaminosis is rare in adults. Oil solutions of vitamin D are typically used in the treatment or Prevention of this deficiency. Recently, Ukrainian biochemists have created and successfully applied a vitamin D₃ preparation in the form of a protein powder called videin, which improves its absorption by children's bodies and does not cause allergies.

Hypovitaminosis leads to disruptions in phosphorus-calcium metabolism across all organs and tissues, primarily reducing the influx of Ca²⁺ into the blood from the intestines. It can also develop in adults who lack adequate sunlight exposure. Consequently, calcium and phosphorus leach from the bones into the blood, resulting in bone softening (Osteoporosis), dental decay, and altered Muscle Function (hypotonia).

Hypervitaminosis is accompanied by increased intestinal absorption of calcium and phosphorus into the blood, their deposition in bone growth plates (which inhibits growth in children) and in many other tissues—particularly Arteries and kidneys—thereby impairing their function.

Chemical structure. Calciferols (over 10 analogs) are unsaturated cyclic alcohols closely related to sterols. Vitamin D₃ is synthesized in the human body within the subcutaneous adipose tissue from provitamin 7-dehydrocholesterol under the action of ultraviolet rays. In the liver, it is converted into its active form and transported to the kidneys and intestines, where it regulates the release of calcium into Blood Plasma (see Fig. 43):

Fig. 43. Vitamin D metabolism in the body and its role in regulating blood calcium levels

A regular intake of vitamin D is particularly important for weightlifters and strength athletes, whose skeletal systems undergo tremendous stress.

Vitamin E Group

Biological effects. Vitamin E encompasses several tocopherols that vary in chemical structure and activity (from the Greek *tokos* meaning offspring, and *phero* meaning to bear). Tocopherols prevent Infertility and ensure normal reproductive processes, which is why they were named the reproduction vitamins. Vitamin E is one of the most potent antioxidants; that is, it protects cell Membrane Lipids and Fatty acids from excessive lipid peroxidation, thereby preserving their biological Functions. Due to its antioxidant activity, vitamin E prevents Fatty liver disease and promotes the synthesis of Hormones vital to the body. It influences redox processes in the body that proceed with energy release. Tocopherols maintain blood vessel elasticity, reduce blood clotting, and enhance Protein Synthesis in skeletal muscles, thereby exhibiting an anabolic effect.

Aviatminosis (vitamin deficiency) manifests as metabolic disturbances in skeletal muscles: The amount of the contractile protein Myosin decreases, while the amount of Collagen in Connective Tissue increases. This impairs muscle contractility and diminishes muscle energetics due to a reduction in Glycogen, creatine phosphate, and ATP levels.

Hypovitaminosis is accompanied by a decrease in Blood Plasma Proteins and muscle dystrophy.

Chemical structure. The most important member of the tocopherol group is $\alpha$-tocopherol. Its structure consists of a trimethylhydroquinone cyclic compound and a phytol alcohol:

Other forms of vitamin E differ from $\alpha$-tocopherol in the number and position of methyl groups in the quinoid ring, as well as in lower biological activity. Currently, eight naturally occurring compounds with vitamin E biological activity are known.

The daily requirement for tocopherols increases with high intake of Unsaturated fatty acids, intense physical exertion, and particularly under hypoxic conditions such as high-altitude climbing. This requirement decreases when the body is adequately supplied with the trace element selenium.

Vitamin E is used to treat and prevent atherosclerosis, coronary artery disease, Hypertension, vascular thrombosis, and reproductive disorders. In sports practice, vitamin E is widely utilized due to its broad spectrum of biological activities to maintain high physical performance and endurance.

Vitamin K group

Biological effects. Vitamin K group (phylloquinones) are components of enzymes that regulate blood clotting by promoting The conversion of fibrinogen into fibrin, which forms the blood clot. As a component of the Respiratory Chain (ubiquinone or coenzyme Q), vitamin K participates in oxidation-reduction reactions and influences aerobic energy production processes.

Aviatminosis is associated with impaired prothrombin formation in the liver. This slows down blood clotting and is accompanied by bleeding, as well as subcutaneous, intramuscular, and gastrointestinal hemorrhages. One of the causes of vitamin K deficiency can be impaired intestinal absorption due to liver disease or other digestive disorders, or massive blood loss.

Hypovitaminosis is extremely rare, as the intestinal microflora typically produces vitamin K in sufficient quantities.

Hypervitaminosis manifests as enhanced Blood Coagulation and thrombus formation.

Chemical structure. Vitamin K vitamins (K₁ and K₂) are derivatives of the cyclic ketone naphthoquinone and differ from each other in the length of their side chain and the number of double bonds within it. Vitamin K₁ exhibits the highest biological activity:

In addition to vitamins K₁ and K₂, there are naphthoquinone derivatives that exhibit vitamin properties while being highly water-soluble. One such preparation (vikasol, a vitamin K₁ substitute) was synthesized by the Ukrainian biochemist O.V. Palladin in 1943 and is widely used in clinical practice for massive blood loss.