Vitamins

Small-molecular-weight biologically active organic compounds necessary for the normal metabolism and vital functions of the body. They are not synthesized in human body cells, so they must be obtained through food. Humans need very little of them (daily norm – from a few micrograms to several milligrams, only vitamin C needs more – several tens of milligrams per day). Some vitamins (B group and vitamin K) are synthesized by intestinal bacteria, but the body does not get enough of them. The term “vitamins” was proposed by the Polish chemist M. Funk (1912), who identified an amino group in the structure of vitamin B₁ and wanted to emphasize the importance of this compound to life: vitamins – life amines. Vitamins, based on solubility, are divided into two groups: 1) fat-soluble vitamins: vitamin A (retinol, anti-xerophthalmic); vitamin D (calciferol, anti-rachitic); vitamin E (tocopherol, anti-sterile); vitamin K (phylloquinones, anti-hemorrhagic); 2) water-soluble vitamins: vitamin B₁ (thiamine, anti-neuritic); vitamin B₂ (riboflavin, growth vitamin); vitamin B₃ (pantothenic acid, anti-dermatitic); vitamin PP (B₅) (nicotinic acid or niacinamide, anti-pelagic); vitamin B₆ (pyridoxine, pyridoxal or pyridoxamine, anti-dermatitic); vitamin B₉ (folic acid, anti-anemic); vitamin B₁₂ (cobalamins, anti-anemic); vitamin H (biotin, anti-seborrheic); vitamin C (ascorbic acid, anti-scorbutic). All water-soluble vitamins, except C, contain nitrogen and are called B group vitamins. Vitamins are named in three ways: in Latin alphabet capital letters with numbers or without them (vitamin B₁, vitamin C), based on chemical structure, based on physiological action. For example, vitamin C (ascorbic acid, anti-scorbutic). The same vitamin’s specific activity can be characteristic of several compounds with similar chemical structures. Such compounds are called vitamers, for example, the activity of vitamin PP is characteristic of nicotinic acid and niacinamide. Some vitamins are obtained in the form of provitamins from food products. Water-soluble vitamins (with few exceptions) are obtained from food: bread, fruits, vegetables, meat, and dairy products. Small amounts of them (except for vitamin C) are produced by intestinal bacteria. The metabolism of all water-soluble vitamins in the body is similar. Vitamins present in food products are bound to various compounds (protein and non-protein origin). In the digestive tract, under the action of digestive enzymes, they are released and absorbed in the small intestine into the portal vein blood. Coenzymes are synthesized from them in tissues. When the concentration of water-soluble vitamins in the blood plasma is higher than the kidney threshold, they are excreted in the urine. Therefore, only very small reserves of water-soluble vitamins are formed in the body. They must be constantly replenished from food.

Fat-soluble vitamins (A, D, E, K) are found in both animal and plant-based food products. In addition, vitamin K is synthesized by intestinal bacteria. These vitamins are insoluble in water and are quite stable. They are absorbed in the small intestine, their absorption requires bile and fats. Diseases that occur when a person does not get enough vitamins are called avitaminoses. If there is insufficient intake of vitamins, hypovitaminoses occur, manifesting in various metabolic disorders. More common are not one, but several vitamin hypovitaminoses (polyhypovitaminoses). Their causes are exogenous and endogenous: lack of vitamins in food, increased body needs (growth, pregnancy, heavy physical work, etc.), impaired vitamin absorption in the small intestine, internal organ diseases (especially liver diseases, during which provitamins are not converted into vitamins, incorporation into enzyme composition is disrupted, etc.), improper food preparation (a large portion of the vitamins in food products is destroyed), use of drugs with antivitamin effects, alcoholism. Congenital avitaminoses are also known, the causes of which are genetic disorders related to vitamin transport, formation of coenzyme forms, or synthesis of coenzymes. Such avitaminoses are very severe and usually begin in childhood. They cannot be cured even with large doses of vitamins. When there is a lack of water-soluble vitamins, changes occur first in those tissues and organs where their activity is most pronounced: the nervous system, digestive tract, skin, and blood production are affected. When the body receives an excess of vitamins, hypervitaminoses occur. The most common overdoses are of fat-soluble vitamins A, D, and K. Large amounts of them are toxic to the body. Large doses of water-soluble vitamins are not dangerous for humans, except for nicotinic acid (PP), ascorbic acid (C), and pyridoxine (B₆). An excess of nicotinic acid (but not niacinamide) dilates the blood vessels of the skin (flushed face). Large doses of vitamin C can cause diarrhea, formation of oxalate stones in the kidneys, etc., large doses of vitamin B₆ (5 g per day) cause disturbances in sensory nerve function, and in rare cases – axonal degeneration. In the absence of fat-soluble vitamins, specific changes occur: vision, calcium and phosphorus metabolism, blood clotting, growth, etc., are affected.

Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė