Vitamins are essential substances for life that we can not synthesize and that we have to ingest irremediably through the diet so they are essential nutrients . The exception is vitamin D, a substance that is not always essential in the diet, since we can synthesize it in the skin from cholesterol and the action of solar radiation.
Classifying vitamins is very complicated. They are chemically very different and each has multiple functions . There are antioxidant vitamins, such as vitamin E and vitamin C, others act as coenzymes and prosthetic groups or precursors of these, such as the group of B vitamins , and even some act as hormones, such as vitamin D that regulates mineral metabolism or Vitamin A that is involved in the regulation of cell growth and differentiation. Some can even display several functions at once.
The most commonly used classification criterion is the ability to solubility in water and fat , a very general characteristic that does not report on its activity but does give an idea about which vitamins can contain a certain type of food and general metabolic routes: factors that Influence their absorption, excretion pathways or patterns of storage and distribution in the body.
Based on this criterion, vitamins are classified into two major groups:
- Water soluble vitamins : they are water – soluble vitamins and aqueous, non – fat. They are vitamin C and 8 vitamins of group B.
- Fat-soluble vitamins : substances soluble in lipids and fatty media, not in aqueous media. They are vitamins A, D, E and K.
Water soluble vitamins
The group of water soluble vitamins is formed by vitamin C and the eight B vitamins . All of them are soluble in water and all act as enzymatic cofactors , either as coenzymes or as prosthetic groups , in addition to having other effects.
Being soluble in water, they will dissolve in the aqueous part of food. They are absorbed directly into the circulatory torrent in the enterocytes through various membrane mechanisms. The excess is excreted through the urine and does not accumulate in the body , except for vitamin B12 that is stored in liver stores, which is why they should be consumed more frequently and regularly than fat-soluble vitamins.
The vitamin C is ascorbic acid , particularly L – enantiomer, and is an essential substance for many types of organisms, but almost all the be synthesized, the human species can not. In solution it is in the form of ascorbate, a potent antioxidantthat acts as a substrate for the enzyme ascorbate-peroxidase . It also acts as a coenzyme of several vital enzymes that catalyze collagen hydroxylation, carnitine synthesis, norepinephrine synthesis, synthesis of various hormones, and tyrosine metabolism.
The B vitamins are a chemically diverse group, although all nitrogen in its molecule, involved in the regulation of cell metabolism :
- Vitamin B1 (thiamine) : coenzyme in the catabolism of sugars and amino acids.
- Vitamin B2 (riboflavin) : precursor of the FAD(flavin adenine dinucleotide) and FMN (flavin mononucleotide) cofactors required for many flavoproteins and for the activation of other vitamins.
- Vitamin B3 (niaciona) : precursor of the cofactors NAD and NADP, necessary in many metabolic reactions.
- Vitamin B5 (pantothenic acid) : precursor of coenzyme A, necessary to metabolize many molecules.
- Vitamin B6 (pyridoxine) : coenzyme in many metabolic reactions.
- Vitamin B7 (biotin) : coenzyme of carboxylases, enzymes necessary in the synthesis of fatty acids and in gluconeogenesis.
- Vitamin B9 (folic acid) : it is a cofactor of several metabolic reactions and participates in the formation, repair and methylation of DNA, especially important in the stages of greater development, especially childhood and embryonic development.
- Vitamin B12 (cobalamin) : coenzyme necessary for the metabolism of fatty acids, amino acids and DNA synthesis.
Fat soluble vitamins
Fat-soluble vitamins are present in foods that contain a relatively large fat fraction. They are absorbed in the small intestine through the chylomicrons together to the triglycerides, phospholipids, cholesterol and other fatty substances, so they need bile salts and are absorbed into the lymphatic circulation before passing into the blood. Vitamins are liposoluble vitamins A, D, E and K . All of these are stored in the liver and accumulate in adipose tissue .
Due to this accumulation, unlike water-soluble vitamins, an adequate intake is possible without necessarily consuming them daily and also makes it possible to reach toxic levels more easily (hypervitaminosis), especially if nutritional supplements and food are consumed Fortified.
Many times nutritional supplements and food additives can be found with water-soluble forms of these vitamins that are then metabolized in the body to the active form, but should not be confused with water-soluble vitamins.
- Vitamin A (retinol, retinal, carotenoids) : is part of rhodopsin, a pigment present in the rods of the retina that is necessary for vision, especially in low light conditions. It participates in the growth and differentiation of the eye, respiratory system and gastrointestinal tract. It is necessary in osteogenesis and improves the immune response.
- Vitamin D (calciferol) : is a precursor of calcitriol, considered a hormone since it can be synthesized completely endogenously from cholesterol and the action of the UV ration of the Sun. It increases the absorption of calcium and phosphorus in the intestine, inhibits the Formation of osteoclasts ( bone resorption cells ) and reduces the secretion of parathyroid hormone (PTH), its most important role is the regulation of bone remodeling, calcium homeostasis and general metabolism, but also involved in muscle function And in immune function.
- Vitamin E (alpha-tocopherol) : acts as an antioxidant with a prominent role in the synthesis of heme, an essential part of hemoglobin and oxygen transport, and in the prevention of lipid peroxidation .
- Vitamin K (phytonadione) : required in blood coagulation processes and in the formation of red blood cells.
|Fat soluble vitaminsA, D, E and K||Water soluble vitaminsC and group B|
|Soluble in fatty medium, not in aqueous medium||Soluble in aqueous medium, not in fatty medium|
|They are absorbed together with the lipids. Requires bile salts||Absorption is simpler|
|They are not excreted||They are excreted in urine|
|They are stored in the liver and accumulate in fatty tissues||Not stored or accumulated (except B12)|
|Deficiency manifests itself when reserves are exhausted||Deficiency manifests itself faster in the presence of insufficient intakes|
|Relatively high doses separated over time may prevent deficiency||Require more regular contribution|
|Excessive hypervitaminosis is more likely||Hypervitaminosis is unlikely; The excess is excreted|