Vitamins: Definition, Classification and Function

Vitamins: Definition, Classification and Function

Definition

Vitamins are organic molecules that are essential to an organism in small quantities for proper metabolic function. These essential micronutrients cannot be synthesized in the organism or in sufficient quantities to meet bodily needs, therefore must be obtained through consumption either from diet or synthetic sources. Hence vitamins are considered as essential nutrients.

Vitamins also differ from the other biological compounds in that relatively small quantities are needed to complete their functions. In general, these functions are of a catalytic or regulatory nature, facilitating or controlling vital chemical reactions in the body’s cells. If a vitamin is absent from the diet or is not properly absorbed by the body, a specific deficiency disease may develop.

Vitamins have diverse biochemical functions. Vitamin A acts as a regulator of cell and tissue growth and differentiation. The B complex vitamins function as enzyme cofactors (coenzymes) or the precursors for them. Vitamin D provides a hormone-like function, regulating mineral metabolism for bones and other organs. Vitamins C and E function as antioxidants.

Both deficiency and excess intake of a vitamin can potentially cause clinically significant illness, although excess intake of water-soluble vitamins is less likely to do so because unused by the body is primarily lost through urine.

In 1912, Casimir Funk coined the term “vitamine”.

All the vitamins were discovered between 1910 and 1948.

 Classification

Vitamins are generally classified as either fat soluble or water soluble based on their solubility.

Fat-soluble vitamins (vitamin A, vitamin D, vitamin E, and vitamin K) dissolve in fat and tend to accumulate in the body.

Water-soluble vitamins (vitamin C and the B-complex vitamins, such as vitamin B6, vitamin B12, and folate) must dissolve in water before they can be absorbed by the body and therefore cannot be stored. Any water-soluble vitamins unused by the body is primarily lost through urine.

The common and chemical names of vitamins of both groups, along with their main biological functions and deficiency symptoms, are listed in the table.

There are 13-vitamins other than choline listed by major health organisations:

Regulatory Role of Vitamin & their Sources

• The vitamins regulate metabolic reactions. Other dietary components known as micronutrients such as, fats, carbohydrates, proteins are utilized in the reactions regulated by the vitamins. Absence of a vitamin blocks one or more specific metabolic reactions in a cell and eventually may disrupt the metabolic balance within a cell and in the entire organism as well.
• All water-soluble vitamins except vitamin C, have a catalytic function, i.e., they act as coenzymes of enzymes that function in energy transfer or in the metabolism of fats, carbohydrates, and proteins. The metabolic importance of the water-soluble vitamins is reflected by their presence in most plant and animal tissues involved in metabolism.
• Some of the fat-soluble vitamins form part of the structure of biological membranes or assist in maintaining the integrity of membranes.
• Some fat-soluble vitamins also may function at the genetic level by controlling the synthesis of certain enzymes.
• Unlike the water-soluble ones, fat-soluble vitamins are necessary for specific functions in highly differentiated and specialized tissues; therefore, their distribution in nature tends to be more selective than that of the water-soluble vitamin

Sources: Both plants and animals are important natural vitamin sources for human beings. Vitamins are found in all living organisms; they are either synthesized in the organism or are acquired from the environment but not distributed equally throughout nature. For example, beta-carotene, which can be converted to vitamin A, is synthesized in plant tissues but not in animal tissues. On the other hand, vitamins A and D3 (cholecalciferol) occur only in animal tissues.

Food sources of vitamin D are limited, but it can be synthesized in the skin through ultraviolet radiation (from the Sun); therefore, with adequate exposure to sunlight, the dietary intake of vitamin D is of little significance. Certain vitamins (e.g., B vitamins, vitamin K) can be synthesized by microorganisms normally present in the intestines of some animals; however, the microorganisms usually do not supply the host animal with an adequate quantity of a vitamin.

All vitamins can be either synthesized or produced commercially from food sources and are available for human consumption in pharmaceutical preparations.

Effects of Commercial processing & Cooking- commercial processing of food like milling of grains frequently destroys or removes considerable amounts of vitamin, however, the vitamins are replaced by chemical methods in such cases. Loss of vitamins may also occur when food is cooked; for instance, heat destroys vitamin A, and water-soluble vitamins may be extracted from food to water and lost.

Role, Functions and Dietary sources of water-soluble vitamins

• Vitamin C: Important for immune function, collagen synthesis, and antioxidant protection. Vitamin C comes in two forms, the most common of which is known as ascorbic acid. The main dietary sources of vitamin C are fruits and vegetables. Cooked animal-sourced foods contain virtually no vitamin C, but low amounts can be found in raw liver, eggs, fish roe, meat and fish.
• Vitamin B1 (Thiamine): found in many forms including Thiamine pyrophosphate, Thiamine triphosphate, Thiamine mononitrate, Thiamine hydrochloride. Like the other B vitamins, thiamine acts as a coenzyme. Essential for energy metabolism and nerve function. The richest dietary sources of thiamine include liver, pork, seeds and whole-grain cereals.
• Vitamin B2 (Riboflavin): Riboflavin is the only water-soluble vitamin used as a food coloring. Plays a role in energy production and the metabolism of fats, drugs, and found in various foods, its richest sources include liver, meat, dairy products, eggs, leafy vegetables, almonds and legumes.
• Vitamin B3 (Niacin):is the only B vitamin your body can produce from another nutrient — the amino acid tryptophan. The most common forms are Nicotinic acid and Nicotinamide (niacinamide). Supports metabolism and is important for DNA repair and stress responses. Niacin is found in many different foods, such as liver, fish, poultry, eggs, dairy products, sunflower seeds and peanuts, to name few. Additionally, it is commonly added to processed food like flour and breakfast cereals.
• Vitamin B5 (Pantothenic Acid): Pantothenic acid plays a key role in a wide range of metabolic functions. It is required for the formation of coenzyme A, which is necessary for the synthesis of fatty acids, amino acids, steroid hormones, neurotransmitters and various other important compounds. Pantothenic acid is found in virtually all food. Other rich sources include yeast extract spread, shiitake mushrooms, caviar, kidneys, chicken, beef and egg yolks. Several plant foods are also good sources. In addition to those mentioned above, these include root vegetables, whole grains, tomatoes and broccoli.
• Vitamin B6 (Pyridoxine):Like the other B vitamins, vitamin B6 is a family of related compounds, such as Pyridoxine, Pyridoxamine, Pyridoxal. Involved in amino acid metabolism, neurotransmitter synthesis, and immune function. The richest dietary sources are liver, salmon, sunflower seeds and pistachio nuts, to name a few.
• Vitamin B7 (Biotin): Biotin is either found in its free form or bound to proteins. When proteins that contain biotin are digested, they release a compound called biocytin. The digestive enzyme biotinidase then breaks biocytin into free biotin and lysine, an amino acid. Same as all B vitamins, biotin functions as a coenzyme. It is required for the function of five carboxylases, enzymes involved in several fundamental metabolic processes. For instance, biotin serves an essential role in fatty acid synthesis, glucose formation and amino acid metabolism. It is found in a wide range of foods. Good sources include organ meats, egg yolk, meat, legumes, cauliflower, mushrooms and nuts.
• Vitamin B9: Vitamin B9 comes in several different forms, including Folate, Folic acid and L-methylfolate. Vitamin B9 acts as a coenzyme and is essential for cell growth, DNA formation and amino acid metabolism. It is very important during periods of rapid cell division and growth, such as in infancy and pregnancy. Additionally, it is required for the formation of red and white blood cells, so deficiency may lead to anemia. Important for DNA synthesis and repair, and crucial during periods of rapid growth such as pregnancy. It is found in both plants and animals. Rich sources include liver, legumes and leafy greens.
• Vitamin B12 (Cobalamin): Vitamin B12 is the only vitamin that contains a metallic element, namely cobalt. For this reason, it is often referred to as cobalamin. There are four main types of vitamin B12 — cyanocobalamin, hydroxocobalamin, adenosyl cobalamin and methyl cobalamin. Crucial for red blood cell formation, neurological function, and DNA synthesis. It is found in virtually all animal-sourced foods but is absent from plant foods.

Role, Functions and Dietary sources of Fat-soluble vitamins

• Vitamin A: essential for maintaining the light-sensing cells in your eyes and for the formation of tear fluid, necessary for cell growth in the body, vital for hair growth, maintains fertility and vital for fetal development. Vitamin A is only found in animal-sourced foods. The main natural food sources are liver, fish liver oil, butter. Sufficient amounts can also be derived from provitamin A carotenoids, like beta-carotene, which are found in vegetables, such as such as carrots, kale, and spinach.
• Vitamin D: Vitamin D regulates the circulating levels of calcium and phosphorus, which are the most important minerals for bone growth and maintenance. It also regulates and strengthens your immune system function. Dietary vitamin D can be classified as vitamin D2 (ergocalciferol), which is found in mushrooms and some plants, and vitamin D3 (cholecalciferol), which is found in animal-derived foods, such as eggs and fish oil and produced when our skin is exposed to sunlight.
• Vitamin E: Vitamin E is a group of related compounds divided into tocopherols and tocotrienols. Alpha-tocopherol is the most common type. Vitamin E’s key role is to serve as an antioxidant, protecting cells against free radicals and oxidative damage. The richest dietary sources of vitamin E include certain vegetable oils, seeds, and nuts.
• Vitamin K: Vitamin K plays an essential role in blood clotting. In fact, the “K” stands for “coagulation,” the Danish word for coagulation, which means clotting. But vitamin K has other functions as well, including supporting bone health and helping prevent the calcification of blood vessels, potentially reducing the risk of heart disease. The main dietary forms are vitamin K1 (phylloquinone), found in plant foods, and vitamin K2 (menaquinone), found in animal-derived foods and fermented soy products.

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