1. Introduction to the topic
2. Water-soluble vitamins
3. Fat-soluble vitamins
5. Trace elements
Definition and classification of vitamins
Vitamins represent a heterogeneous group of organic substances that the body, with a few exceptions, is unable to synthesize and thus must be obtained through diet. Different organisms differ in their capacity to synthesize various vitamins. That is why, what counts as a vitamin for humans must not be essential for other animal species. Apart from diet, other important source of some types of vitamins (e.g. K or biotin) are bacteria colonizing our large intestine.
Identical vitamins often occur in the form of multiple compounds known as vitamers, which differ from each other in structure (for example in having different substituents or functional groups) or function (e.g. vitamers of vitamin A – retinol, retinal and retinoic acid).
In general, vitamins can be divided according to the polarity of their molecule (affecting the solubility in water) into two groups:
1) Water soluble vitamins
These vitamins have hydrophilic character, but apart from it, from the chemical point of view, they do resemble each other only very slightly. Their absorption is easier compared to the fat-soluble vitamins and the do not require any special blood transport molecule. When taken in excess, they can easily be excreted through urine without any risk of overdose. This group includes B group vitamins and vitamin C.
2) Fat-soluble vitamins
Altogether, fat-soluble vitamins are derivatives of isoprene and have a lipophilic character. Their absorption requires intact absorption of lipids and their transfer in blood takes place via lipoproteins (like other lipids) or via specific transport proteins, e.g. vitamin D binding protein or retinol binding protein. Lipophilic character enables their storage in adipose tissue (or more generally in all tissues rich in fat), where they may accumulate. On one hand it may lead to their toxicity when taken in large quantities, but on the other, the adipose tissue may act as their storage, and release them when necessary. This group involves vitamins A, E, D and K.
Some vitamins enter our bodies in the form of precursor molecules called provitamins. Provitamins do not show any biological activity, but within the body they are converted into the active molecules of vitamins. Examples include a pigment β-carotene, the provitamin of vitamin A.
Function and pathology of vitamins
Vitamins are usually required only in small amounts (in order of micro- or milligrams), but they play an irreplaceable function within the body. Many vitamins act as enzyme cofactors and participate in the enzyme-catalyzed reactions of metabolic pathways. Some vitamins are antioxidants and protect cellular structures against the oxidative stress.
Vitamin deficiency, which can occur for various reasons (inadequate intake of vitamins in diet, impaired intestinal absorption or metabolism of provitamins), may lead to hypovitaminosis or, in extreme cases, avitaminosis. Clinical manifestations differ (depending on the extent of deficiency or the type of the missing vitamin) – e.g. beriberi disease (thiamine deficiency) or scurvy (vitamin C deficiency). Pathological conditions may rarely develop due to an excess intake of some vitamins as well. They concern mainly fat-soluble vitamins, most often vitamin A and D and are termed hypervitaminosis.
Water soluble vitamins
Vitamin B1 (thiamine)
The structure of vitamin B1 contains substituted thiazole nuclei and pyrimidine. Biologically active form is called thiamine pyrophosphate (TPP, thiamine diphosphate) and its formation involves a special transferase located in brain and liver tissue.
Thiamine diphosphate is a cofactor of reactions that involve a transfer of an active aldehyde residue. Such reactions are for example an oxidative decarboxylation of α-keto acids, where it participates in forming a multienzyme complexes (e.g. pyruvate dehydrogenase complex). Second group of reactions with the transfer of aldehyde residue, where thiamine plays an important part, the so-called transketolation reaction, occurs for example in the pentose cycle.
Thiamine is found in large quantities in the outer layers of the coating covering cereal grains, in yeast (which generally contain vitamins of B group), legumes, pork meat or milk. On the other hand, white bread (until it is not fortified) or peeled polished rise are low in its content. The recommended daily dose of thiamine is around 1.1 mg.
When eating thiamine deficient diet (for example containing processed cereal grains with the coating removed) a disease called beriberi develops. Beriberi is characterized by an impaired saccharide and amino acids metabolism and its symptoms include peripheral myopathies, fatigue and anorexia, later joined by edema, cardiovascular, neurological and muscle disorders. In the past, beriberi occurred extensively in Southeast and East Asia, where de-husked rice had served as the main food source.
Chronic alcoholics can develop a neurological condition known as Wernicke’s encephalopathy, after years of alcohol abuse, also caused by the vitamin deficiency.
Vitamin B2 (riboflavin)
Chemical structure of riboflavin (from Latin flavus – yellow) contains alcohol called ribitol connected to a heterocyclic core.
Our bodies phosphorylate and transform riboflavin to one of its active forms – flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD). They both form prosthetic groups of a set of oxidoreductases called flavoproteins. One of the most well known enzymes are part of the respiratory chain – NADH- dehydrogenase or succinate dehydrogenase.
Riboflavin is present in yeast, liver, kidneys, eggs or milk. The recommended daily intake is around 1.4mg.
Riboflavin deficiency fortunately does not cause significant problems. Usually only uncharacteristic symptoms, typical for deficiency of other vitamins of B group as well, occur – e.g. inflammations in the oral cavity (lips, tongue, corners of the mouth), skin changes or delayed wounds healing.
Vitamin B3 (niacin)
Vitamin B3 is a collective term for two compounds: nicotinic acid and nicotinamide. Niacin used to be termed vitamin PP (pellagra- preventive).
Biologically active forms of niacin are nicotinamide adenine dinucleotide (NAD+) and its phosphorylated derivative – nicotinamide adenine dinucleotide phosphate (NADP+).
Both are ubiquitous, acting as cytosolic and mitochondrial coenzymes of oxidation-reduction enzymes. NAD+ is generally a cofactor of oxidoreductases in oxidative pathways (for example Krebs cycle), NADPH is a part of dehydrogenases or reductases participating in so-called reductive syntheses – occurring for example in the fatty-acid metabolism or pentose cycle.
Acting through special G-protein coupled receptors, expressed mainly in the adipose tissue (but present in liver tissue or immune cells as well), the nicotinic acid inhibits lipolysis and release of free fatty acids from the adipose tissue. This process reduces their availability in the synthesis of lipoproteins in liver and thus the plasmatic levels of VLDL (and consequently LDL and total cholesterol) decrease. This effect has not been observed in nicotinamide.)
Good sources of niacin are liver, fish (or other meat), yeast or bran. Due to our body’s ability to synthesize niacin, up to some extent, using the essential amino acid tryptophan, symptoms of its deficiency only occur in the absence of both of these nutrients in diet. The recommended daily dose of this vitamin is quite high – around 16mg.
Shortage of niacin leads to a disease called pellagra, the “three D disease”, characterized by a triad of symptoms: dermatitis, diarrhea and dementia.
Vitamin B5 (pantothenic acid)
Vitamin B5 is made of six-carbon branched hydroxy acid called pantoic acid bound to β-alanine. The name pantothenic acid is derived from greek pantothen – from everywhere. Vitamin B5 is indeed present in many foods of plant or animal origin (see below).
Pantothenate, the precursor molecule of coenzyme A, acts within the metabolic pathways as a carrier of acyl residues. Among the most important reactions it participates in are Krebs cycle, synthesis and degradation of fatty acids or cholesterol synthesis. The importance of vitamin B5 is therefore quite considerable.
As it already has been mentioned above, pantothenic acid is present in many kinds of foodstuff, including legumes, whole grain products, meat, offal or yeast. That is why we encounter its deficiency, characterized by skin disorders and hair follicle atrophy, only rarely. The recommended daily intake of 6-10 mg per day is not difficult to achieve.
Vitamin B6 (pyridoxine)
Vitamin B6 includes three related pyridine derivatives with the same biological function – pyridoxine (pyridoxol), pyridoxal and pyridoxamine.
All three of them have to be transformed and phosphorylated (with the help of an enzyme pyridoxal kinase, present in most of body tissues) into pyridoxal-5-phosphate (PLP), the biologically active form of vitamin B6.
Pyridoxal phosphate functions as a cofactor of many enzymes, participating in the metabolism of amino acids, for example amino transferases (transaminases) or decarboxylases. In all of these reactions, the aldehyde groups of pyridoxal phosphate bind to the amino group of the amino acids forming so-called Schiffs base.
Other enzyme requiring the presence of pyridoxal phosphate (acting as its cofactor) is glycogen phosphorylase, enzyme cleaving the molecules of glycogen.
Vitamin B6 is present in many animal and plant products. Examples include liver, meat (including fish), whole grain products, nuts, vegetables (potatoes, cabbage, carrots), bananas or avocados. The recommended daily dose is about 2 mg.
Isolated vitamin B6 deficiency is rare, more often it is related to the deficiencies of other B group vitamins. The symptoms of deficit are therefore more or less symptoms of general deficiency of group B vitamins: dermatitis, mucositis (mainly oral) and CNS disorders. Disorders of tryptophan metabolism are also common.
Shortage may result from an intake of certain medication as well. For example antitubercular drug isoniazid forms complexes with vitamin B6 and thus disrupts its function.
Vitamin B7 (biotin, formerly vitamin H)
Vitamin B7 belongs to a group of imidazole derivatives. It acts as a cofactor of enzymes catalyzing carboxylation reactions, where it serves as a carrier for CO2 molecule. In order to perform its function it forms an active intermediate product called carboxybiotin. Examples of reactions, where biotin plays active role, include fatty-acid biosynthesis (cofactor of CoA carboxylase) or anaplerotic reactions of oxaloacetate synthesis from pyruvate (cofactor of pyruvate carboxylase).
Majority of biotin is supplied to our body through a biosynthesis performed by intestinal bacteria and biotin present in food (e.g. in liver, meat, yeast or nuts) is not so important. The nutritional deficiency therefore almost doest not exist and the shortage of biotin is caused mainly by defects in its utilization. Egg white, for example, contains glycoprotein avidin, which binds strongly to biotin and prevent it from being absorbed. Insufficient intestinal production can be also a result of damage done to the intestinal bacterial microflora due to administration of antibiotics. Absence of biotin manifests as muscle pain, dermatitis, anorexia or psychiatric disorders (depression, hallucinations).
Vitamin B9 (folic acid)
Folic acid (lat. folium – leaf) and its derivatives, folates, are composed of pteroyl-glutamic acids containing pteridin attached to p-aminobenzoic acid (PABA) and glutamic acid. These compounds (folate and folic acid) are collectively termed as folacin.
Dietary folates are mostly present in the form of polyglutamates (containing more glutamate residues within the molecules), which are enzymatically broken into monoglutamates (with much higher absorbency) in small intestine.
Intestinal cells convert the absorbed folate first into dihydrofolate and subsequently reduce it into its active form tetrahydrofolate (THF). The reaction is catalyzed by an enzyme dihydrofolate reductase (DHFR) using NADPH. Tetrahydrofolate is than released into blood. Apart from THF the blood plasma contains its derivatives as well, for example 5-methyl-THF and 10-formyl-THF. These compounds are synthesized in liver from the collected THF and are released back into the bloodstream.
THF transports one-carbon residues of different degree of oxidation (e.g. methyl, methylene, methenyl or formyl) and thus acts as a cofactor of many transferases. Methylenetetrahydrofolate is methyl group donor in the synthesis of thymidylate (catalyzed by thymidylate synthase) – a precursor molecule of thymine and formyl-THF participates in synthesis of purine bases. Both reactions are essential for DNA synthesis.
Tetrahydrofolate converts to methylene-THF (with amino acids serine or glycine being the donors of the methylene bridge) that can be subsequently (see figure):
1) Reduced (using NADPH) to methyl-THF in an irreversible reaction. When restoring THF, methyl-THF transfers its methyl group to vitamin B12 (forming methylcobalamin). Vitamin B12 can be then regenerated by a transfer of methyl group to homocysteine, which forms methionine (see picture). The reaction is catalyzed by an enzyme homocysteine methyltransferase. Defect in the function of this enzyme or a deficiency of vitamin B12 can both cause an accumulation of methyl-THF resulting in folate deficiency symptomatology.
2) Methylene-THF can donate carbon residues to other substances. Catalyzed by an enzyme thymidylate synthase, one of these reactions results in the formation of dTMP (and the methylene-THF is converted back to DHF). THF is regenerated with the help of DHFR enzyme.
Methylene-THF + dUMP ↔ dihydrofolate + dTMP
Reduced forms of folates (10-formyl-THF and methylene-THF) are involved in the cellular synthesis of nucleic acids. When there is no reverse reduction, recycling the THF (and flowingly its derivatives), what results, is the failure in DNA synthesis and symptoms of folate deficiency develop.
Sources of folates are green leafy vegetables, yeast, liver, meat, eggs or milk. Folates are also formed by the flora of the large intestine. The recommended daily dose is around 0.2 mg.
Insufficient intake or effect of some medication (see below) leads, similarly to a vitamin B12 deficiency, to the development of macrocytic megaloblastic anemia (due to the failure in DNA synthesis). Increased intake of folates can hide the progress of hematopoietic failure caused by a vitamin B12 deficiency. The danger lies in other symptoms of the deficiency (especially neurological disorders), which develop further, without being so easily noticed (in contrast to anemia).
Another cause of symptoms of folate deficiency may be a result of an administration of certain drugs. Competitive inhibitors of dihydrofolate reductase (called folate antagonists, antifolate drugs), for example methotrexate (chemically similar to dihydrofolate) effectively inhibit cell division, which is commonly used in the treatment of some of the hematological (or other) malignancies.
Folate metabolism is also affected by some group of chemotherapeutics, for example sulfonamides that are used to treat infections. Human body cannot synthesize its own THF, while many pathogenic bacteria have this ability. Sulfonamides resemble the structure of p-aminobenzoic acid, a substrate for folate synthesis. When bacteria try to incorporate sulfonamides into the molecule of THF, the resulting molecule possesses a non-functional cofactor. Sulfonamides can therefore be considered as competitive inhibitors of enzyme synthesizing THF. Human cell are not affected.
Another important fact to mention is that folic acid prevents the development of certain congenital defects (like spina bifida) and premature births and abortions.
Vitamin B12 (cobalamin)
The structure of vitamin B12 resembles that of the porphyrin ring. The core of the molecule is the corine ring, with a cobalt ion attached in its center.
Vitamin B12 may refer to several chemical forms (vitamers), differing from each other in the presence of substituents and function. We may encounter hydroxocobalamin (-OH), methylcobalamin (-CH3), cyanocobalamin (-CN) or deoxyadenosylcobalamin.
Effective absorption of cobalamin requires a presence of so-called intrinsic factor (IF), a glycoprotein synthesized by the parietal cells of the gastric mucosa. Intrinsic factor binds cobalamin (which represents the extrinsic factor) and enables its receptor-mediated endocytosis in the terminal ileum (receptor is called cubilin). Vitamin B12 is transported in blood attached to plasma protein transcobalamin II, which also assists in its transfer to cells (endocytosis mediated by a specific receptor). After cobalamin enters the cell it is converted to hydroxocobalamin. Total amount of cobalamin in body is around 2-5 mg, with liver being its main storage place (it is stored attached to transcobalamin I). It can supply the body with cobalamin for several years.
Cell metabolism utilizes active forms of cobalamin – methylcobalamin (synthesized in cytosol) and deoxyadenosylcobalamin (formed in mitochondria), which both act as enzyme cofactors.
Deoxyadenosylcobalamin is a cofactor in the conversion of methylmalonyl-CoA to succinyl-CoA (catalyzed by methylmalonyl-CoA mutase). The reaction makes it possible for propionate to join the citric acid cycle and it is also involved in gluconeogenesis.
Methylcobalamin participates in the conversion of homocysteine to methionine (enzyme methionine synthase). Methylcobalamin is subsequently regenerated during the conversion of methyl-tetrahydrofolate to THF. The reaction at the same time serves the regeneration of tetrahydrofolate as well.
Cobalamin can only be synthesized by microorganisms, but animals have the ability to store it within their bodies. Nutritionally important sources of vitamin B12 are liver, offal, meat, fish, eggs, milk and dairy products. Plant products only contain cobalamin due to contamination (e.g. when they underwent microbial fermentation, like fermented cabbage). An intake of exclusively vegan diet may result in cobalamin deficiency. Vitamin B12 is also produced by bacteria in the large intestine, but because the absorption takes place in ileum, we cannot utilize it. The recommended daily dose is among the lowest of all vitamins, only 2-3 μg.
Cobalamin deficiency leads to the development of macrocytic megaloblastic anemia cause by a defect in DNA synthesis due to THF shortage. The condition can be caused by the failure in absorption in the absence of intrinsic factor (e.g. due to gastrectomy or autoimmune inflammation of gastric mucosa) or due to ileal disorders (inflammations, resections). If the disease is caused by the autoimmune process with the production of antibodies disrupting the absorption of B12 from GIT (for example antibodies against the parietal cells, intrinsic factor or preventing the binding of B12-IF complex to its receptor), we call the condition pernicious anemia.
Other symptoms of cobalamin deficiency involve increased level of homocysteine, defects of mucosal surfaces (like inflammations) and nervous system disorders. These are the result of demyelination of axons due to a shortage of methionine that is necessary in the process of choline synthesis (which is an important component of the phospholipids of myelin sheaths). The most affected are posterior and lateral spinal tracts. Despite the therapy, neurological changes might result in irreversible deficits, while the hematopoietic conditions can usually be reversed.
Vitamin C (L-ascorbic acid)
The structure of vitamin C resembles glucose, which is at the same time a substrate for its synthesis (via D-glucuronic acid). Most animal species are able to synthesize vitamin C, the inability is rather exceptional – it includes primates and a few other species (like guinea pig).
L-ascorbic acid (in the form of a couple ascorbate and dehydroascorbate) represents an oxidation-reduction system acting as a donor of reduction equivalents.
Vitamin C is involved in the process of iron absorption (reducing Fe3+ to Fe2+), tyrosine degradation, catecholamine biosynthesis (from tyrosine) or the formation of bile acids. It is also an important cofactor of proline and lysine hydroxylases, thus participating in the process of collagen synthesis (important during the healing of wounds). Vitamin C forms an antioxidation system, protecting the cells against the oxidative stress and at the same time regenerating other antioxidants, for example vitamin E.
The widespread notion that citrus fruits contain large quantities of vitamin C is not entirely correct. The highest content of vitamin C has been measured in blackcurrant, cabbage, cauliflower or tomatoes. Citrus fruits, on the other hand, stand somewhere in the middle of the ranking. Although potatoes themselves do have a high content of vitamin C (especially after a long-term storage), due to their popularity and large consumption in our population, they represent one of the main vitamin C sources. Vitamin C cannot withstand high temperatures and oxidation. Thermal treatment of food or contact with metal utensils sharply reduces its content. Unlike other vitamins, the recommended daily doses of ascorbate are measured in tenths of milligrams (around 60 to 80 mg per day).
Vitamin C deficiency causes well-known disease called scurvy. It presents itself with an impaired collagen synthesis (due to an insufficient proline and lysine hydroxylation) resulting in reduced resistance of skin and mucous membrane, damaged capillary walls (leading to frequent hemorrhages), loss of teeth (due to weakened collagen fibrils holding the teeth in positions) and ossification disorders. It is nowadays a rare disease, but in the past it often affected sailors on long voyages.
Hypervitaminosis C does not exist. Ascorbate is excreted through urine even when taken in extreme doses. However, it can affect the results of the chemical examination of urine.
Vitamin C was isolated for the first time in year 1927 by a Hungarian biochemist and physiologist Albert Szent-Györgyi from the adrenal tissue. Ten years later he received a Nobel Prize in physiology and medicine (“for his discoveries in connection with the biological combustion processes, with special reference to vitamin C and the catalysis of fumaric acid”).
Structure and metabolism
Vitamin A is a term for a group of substances with the biological activity of vitamin A. Natural and synthetic forms are collectively referred to as retinoids. Within the body they they are represented mainly by an alcohol – retinol. Less numerous are retinal and retinoic acid.
Sources of vitamin A are animal products, particularly fish oil, butter, eggs and liver. Effective absorption requires bile. Human body is able to produce vitamin A from its provitamin called β-carotene, a tetraterpene present in certain vegetables (mainly in carrot). β-carotene undergoes oxidation (using molecule of O2) and breaks into two molecules of retinol or, at lesser extent, oxidizes to retinoic acid. Retinol is stored in Ito cells in liver.
Retinol is transported in blood attached to retinol binding protein (RBP). Proteins of the same family exist intracellularly as well and it is assumed that zinc plays an important role in their synthesis. Apart from this, zinc also interferes with absorption, transport and utilization of vitamin A and it is a cofactor of enzyme converting retinol to retinal. Zinc deficiency can therefore present itself as vitamin A deficiency disorders. The recommended daily dose of vitamin A is 800 μg. Vitamin A is one of those vitamins, whose excessive intake can lead to hypervitaminosis (see below).
The mechanism of action of vitamin A is similar to steroid hormones. It binds to a specific intracellular receptor proteins and creates a complex that subsequently affects expression of certain genes in cell nucleus.
By affecting the transport of oligosaccharides across cell membranes, retinoic acid participates in the synthesis of glycoproteins and glycolipids thus supporting tissue growth and differentiation, especially in epithelial tissues. Some of the drugs derived from vitamin A (called retinoids) are used in dermatology to treat diseases like acne or psoriasis.
Retinal in the form of 11-cis-retinal binds to protein opsin to form rhodopsin, a visual pigment. When exposed to light, the pigment breaks to form all-trans-retinal, which must regenerate back to 11-cis-retinal. Since retinoic acid cannot be further reduced to retinal, it does not participate in this process. For more information see: Subchapter 12/1.
Retinol and retinoic acids are supposed to have anticarcinogenic effects as well. They are partially a result of the antioxidative function of β-carotene.
Deficiency of vitamin A is characterized by a malfunctioning synthesis of glycoproteins and glycolipids. A typical resulting symptom is keratinization of epithelial tissues, mucosal atrophy and reduced secretion of mucous glands (leading to dryness of mucosal surfaces). The resulting xerophthalmia (reduced secretion of tears) can even cause blindness. Another manifestation, the so-called hemeralopia (night blindness, impaired night vision) is caused by a defect of adaptation of the eye to the darkness.
Excessive intake of vitamin A can be a cause to its toxicity. Hypervitaminosis A may lead to a variety of symptoms: liver damage, nausea, vomiting, diarrhea, weight loss, alopecia (areas of hair loss), increased bone fragility or bleeding. Significant is its teratogenic effect. Hypervitaminosis A cannot be caused by consumption of β-carotene, even in large quantities, because our body is able to regulate its conversion to retinoids. The only consequence is therefore an orange discoloration of skin and serum.
Vitamin E (tocopherol)
Structure and intake
Vitamin E collectively refers to four tocopherol (α, β, γ a δ) and fours tocotrienol isomers (α, β, γ a δ), of which the higher biological activity is found in D-α-tocopherol. All of them feature chromanol ring and a hydrophobic phytol side chain being responsible for its poor water solubility and lipophilicity. Tocopherols easily penetrate the cell membranes and become their parts. Chromanol ring binds one hydroxyl group that can donate a hydrogen atom, which is responsible for its antioxidative effects and methyl groups as well, which determine the particular type of tocopherol. The most abundant D-α-tokoferol also has the highest antioxidative activity.
Tocopherols are plentiful in oleaginous seeds (soya, sunflower etc.) and oils. The recommended daily dose is 12 mg. The transport of vitamin E in blood plasma is carried out by lipoproteins and it is stored in adipose tissue.
Function and intake disorders
Vitamin E belongs to the most important natural antioxidants and scavengers of reactive oxygen species. It mainly fights against the peroxidation of unsaturated acids of phospholipids in cell membranes. The protective effect applies to lipoprotein particles (like LDL) as well. Tocopherol therefore very effectively decreases the rate of lipid peroxidation. At the same time it undergoes oxidation and the resulting radicals are regenerated for example by vitamin C or undergo further conversion. During these reactions they are deprived of the unpaired electron and in live form conjugates with glucuronic acid that can be excreted through bile.
Vitamin E acts synergistically with selenium present in glutathione peroxidase, an antioxidative enzyme eliminating peroxides formed in cells.
Deficiency of tocopherol (in most circumstances resulting from fat malabsorption) leads to an increase in the production of oxygen radicals and membrane damage. It mostly affects tissues with higher O2 concentration (e.g. erythrocytes, cells of the respiratory system). Shortage during pregnancy poses a risk of neonatal anemia.
Structure and metabolism
Vitamin D also called calciferol or calciol occurs in several forms: in plants as ergocalciferol (vitamin D2), in animals as cholecalciferol (vitamin D3, calciol). In the strict sense, these substances represent prohormones (both being equally effective), which are converted in human body to their active form calcitriol (D2– or D3 – calcitriol).
We obtain vitamin D (mainly in the form of vitamin D3) in diet (liver, fish, eggs, milk), but our bodies are able to synthesize it by themselves from its provitamin 7-dehydrocholesterol. Vitamin D3 forms through photolysis by UV light in the epidermal cells. Synthesized vitamin D3, together with the one obtained from diet ends up in liver, where it is hydroxylated in position 25 to form 25-hydroxycholecalciferol that enters the enterohepatic circulation. Defects in this circulation may lead to a shortage of vitamin D. 25-hydroxycholecalciferol may be, according to body’s requirements, further hydroxylated in position 1 (resulting in biologically active 1,25-dihydroxycholecalciferol) or 24 (resulting in inactive metabolite). Enzymes catalyzing the 1- hydroxylation are present in kidneys, bones and placenta and those, which provide the 24-hydroxylation are found in kidneys, intestine, cartilage and placenta.
Function and intake disorders
Vitamin D in its active form participates in the regulation of calcium and phosphate metabolism. Calcitriol stimulates synthesis of proteins involved in the absorption of Ca2+ and phosphate in small intestine. It thus ensures their availability for bone formation and collagen synthesis (for more information see: Subchapter 7/6).
Hypovitaminosis leads to the disorders of bone mineralization. Children develop rickets characterized by deformities of skull, spine, chest and long bones. Adults develop a condition known as osteomalacia (softening of the bones). Apart from an insufficient intake, the shortage of vitamin D can also be caused by a lack of sunlight or a kidney disease.
Hypervitaminosis D manifests as thirst, diarrhea, vomiting, itching of the skin and calcium salts deposition in soft tissues (for example walls of the blood vessels or in kidneys).
Structure and sources
From the chemical point of view, group of vitamin K belongs to the naphthoquinone derivatives. Plants posses vitamin K1 (phylloquinone), animals have vitamin K2 (menaquinones), which are also synthesized by bacteria present in large intestine of our bodies. Vitamin K3 (menadione) does not occur naturally, but it also has biological activity.
Vitamins K are obtained through diet (they are most abundant in leafy vegetables, oatmeal or live), but in the case of their absence in diet, they can fully be substituted by the vitamin K synthesized by the intestinal bacteria. That is why hypovitaminosis K resulting from an insufficient intake normally does not occur.
Function and disorders
The basic function of vitamin K is its involvement in a process of blood clotting. It acts as a cofactor of carboxylase performing the γ-carboxylation of glutamate residues of precursor proteins. This reaction is crucial in the synthesis of coagulation factors II, VII, IX and X and anticoagulant proteins C and S. The γ-glutamate residues created in the process of carboxylation enable the binding of Ca2+ ions that are necessary for the process of hemostasis. During the course of the reaction reduced forms of vitamin K oxidize and thus, after the reaction, they must be regenerated.
Some substances are capable of inhibiting the above-mentioned reaction. Among the most well known are coumarin derivatives like warfarin. By inhibiting the reduction of vitamin K back to its active state, they hinder the synthesis of the coagulation factors and thus act as anticoagulants.
Hypovitaminosis K is very rare in adults due to the production of vitamin K by the intestinal flora. However, the deficiency may develop in newborns, because of their underdeveloped bacterial colonization of the large intestine and relatively low placental permeability of vitamin K. Symptoms of deficiency include pathological bleedings and prolonged time necessary for blood clotting. In adults, a similar condition be a result of a combination of low vitamin K intake and a destruction of bacterial intestinal flora (for example by an antibiotic therapy).
Hypervitaminosis K leads to a disintegration of red blood cells (hemolysis).
Recommended daily intake for vitamins (according to 450/2004)
Nutrients are compounds, which the body requires for its nutrition and development. They are organic substances that undergo catabolic processes (typically oxidation) in order to release energy. The basic nutrients involve saccharides, lipids and proteins. From the point of view of energy release, the most important are saccharides and lipids; proteins are used as an energy source to a much lesser extent. Another substance that can be processed in our metabolism to release considerable amount of energy is ethanol.
In addition to substances of energetic importance, the body also requires non-energetic substances that have different functions – water, minerals, trace elements, vitamins or fiber.
Energy gain and expenditure
The individual nutrients differ in the amount of energy that can be released by their oxidation. The values of released energies (the heat of combustion) after a complete combustion of the basic nutrients would be following:
Saccharides 16.7 kJ/g (4 kcal/g)
Proteins 16.7 kJ/g (4 kcal/g)
Lipids 37.7 kJ/g (9 kcal/g)
Ethanol 29.3 kJ/g (7 kcal/g)
The differences result from the different level of oxidation of their molecules (particularly the high proportion of the saturated bonds in the molecules of lipids is the source of their high energy value).
Energy expenditure depends on several basic factors:
1) Basal metabolism (BM)
This term denotes the amount of energy necessary to ensure the basic functions of an organism. In order to measure BM, the person must be awake, calm, in thermally neutral environment and fasting (meaning at least 12 hours after the last meal). BM, proportional to the lean body mass and surface area, has a tendency to decrease with an increasing age. The measurement of BM is only rarely performed; more often we estimate it from various equations, among the most popular is Harris-Benedict equation (which takes into consideration gender, age, height and weight).
2) Thermogenic effect of food (so-called the specific dynamic effect)
The thermogenic effect of food represents a portion of energy that is consumed during the digestion of the ingested food. It makes up to 5-10 % of the total body energy expenditure.
3) Physical activity
Physical activity creates the largest difference in energy expenditure among the individuals. A physically demanding activity can increase the energy expenditure several times compared to BM.
Shivering and non-shivering thermogenesis is triggered when the temperature drops under certain level. Similarly, excessive rise in temperature triggers mechanisms, which cool the body down.
Nutrient intake in diet
Apart from the total energy gain, the right proportion of individual nutrients in diet is important as well: 60-65 % saccharides, 25-30 % lipids and 10-15 % proteins.
Our body is able to synthesize relatively large number of nutrients from other substances, only a portion of what we ingest represents a group of essential substances. Daily requirement of nutrients and other substances depends on age, sex, physical activity and other factors. On average, an adult individual should take at least 150 g of saccharides, 30 g of proteins and 35 g of lipids (although the body is able to synthesize many lipid molecules, lower intake leads to lower absorption of fat-soluble lipophilic substances as well).
Proteins are the source of nitrogen and essential amino acids, which cannot be synthesized by our cells. Body loses nitrogen through urine, faeces, peeling skin cells etc. In addition to the sufficient quantity of the proteins in diet, their quality is comparably important. Quality is assessed by the proportion of the essential amino acids in a particular food to their proportion in proper diet. High-quality proteins can be found in eggs, milk or meat.
Insufficient intake of nutrients leads to malnutrition, which exist in several forms:
1) Marasmus (simple starvation): marasmus represents insufficient intake of energy and proteins with a proportional reduction of fat and lean body mass, which is a case of e.g. anorexia. The level of albumin in blood plasma stays within physiological limits or is only slightly reduced so the edemas do not develop.
2) Kwashiorkor: insufficient intake of proteins (both qualitatively and quantitatively) while maintaining normal energy intake. Depletion of fat reserves does not take place, but edemas occur. They are caused by a decrease in the level of plasma proteins leading to a change in oncotic pressure.
There also exist isolated deficiencies of various nutrients (vitamins, trace elements, essential fatty acids).
Iron is an indispensable element probably for all living creatures on Earth. There is even a speculative theory of the origin of life, which claims that the first attempts at metabolism occurred on surfaces of iron-rich minerals, primarily iron sulfide (FeS2).
Iron is a tricky element. Similar to many other transition metals it can exist in several oxidation states. The most common ones in nature are ferrous (FeII) and ferric (FeIII), although FeIV has been detected in some enzymes (such as cytochrome c oxidase in the mitochondrial electron transport chain) and compounds containing iron in oxidation states from -II to VI have been synthesised. The ability to move easily between these redox states is the reason why it’s used as a catalyst in many biological processes but it can also cause problems: free ferrous iron can, for example, donate one electron to molecular oxygen and form potentially damaging superoxide (O2–·). It is therefore not surprising that our cells expend a lot of energy in order to control tightly the concentration of free iron.
The other problem with iron is that despite being a relatively abundant metal in our oxygen-rich world it mostly exists bound in very poorly soluble compounds and hence its availability for the metabolic needs of living organisms is severely limited. Most life forms therefore had to develop elaborate mechanisms for acquiring it from their surroundings.
Iron is essential, hard to find and potentially dangerous. Let’s look at how our cells deal with these challenges.
Absorption and excretion of iron
It is not surprising that normally we get all our iron from the diet. In order to stay healthy we need to receive about 10 to 20 mg per day (10 mg for men, 20 mg for women as they lose more iron during menstruation). The total amount of iron in our body is about 5 g, more than half of which is contained in red blood cells. Under normal conditions most iron losses can be accounted for by the removal of dead cells in the intestines and skin. There is no other physiological way to excrete iron, which makes evolutionary sense since, in the light of the previous discussion, one wouldn’t like to waste this precious resource.
Iron in the digested food most likely exists in a myriad of species complexed with amino acids, carbohydrates and other nutrients. Iron is mostly absorbed in the duodenum. The luminal membrane of enterocytes contains a transporter for several metals called divalent metal transporter-1 or dmt1 which, as the name suggests, is specific for ions in oxidation state II (M2+). In order to be absorbed, ferric iron must first be reduced by reductases, whose identity is still being debated. Once ferrous iron enters the enterocyte it can be used for the enterocyte metabolic needs, stored in ferritin or exported into the bloodstream via a transporter called ferroportin. As the Fe2+ ions leave the cell, they are oxidised to Fe3+ by hephaestin and then they bind to a high-affinity iron transporter protein, transferrin (Tf).
A significant portion of iron in the Western diet is present in the form of haem, which appears to be absorbed highly effectively into enterocytes via an unidentified specific transporter. Inside the cell haem is broken down by haem oxygenase and the released iron is further processed as described above. The uptake of haem iron from the diet seems to be much more effective than non-haem iron, at least partly due to the fact that haem (released by proteolysis from haemoglobin, myoglobin and other haem-containing proteins) is much more soluble in aqueous solutions than other species of iron.
Transport and storage of iron
Each molecule of transferrin binds two Fe3+ ions and carries them in the bloodstream to where they’re needed. When transferrin containing iron binds to transferrin receptors (TfR) expressed on the plasma membrane of cells, the whole receptor-ligand complex is engulfed by endocytosis and an intracellular vesicle is formed. Transferrin binds Fe3+ extremely tightly at pH 7 but in more acidic conditions it lets it go much more easily. The cell therefore acidifies the vesicles using proton pumps so that iron can be released from its complex with Tf, reduced and transported out of the vesicle using, once again dmt1 as the transporter. The Tf-TfR complex is then recycled to the cytoplasmic membrane, where it is exposed once again to higher pH and due to the much lower affinity of apotransferrin to TfR the complex dissociates and apo-Tf can look for new ferric ions to bind.
What happens to Fe2+ in the cytoplasm is still poorly understood but in general we can say that it is either stored in ferritin, transported into mitochondria for the synthesis of haem and iron-containing proteins or remains in the cytoplasm as the nebulous labile iron pool.
Ferritin is a protein, which allows the long-term storage of iron in a rather unreactive, and thus safe, form. A total of 24 ferritin subunits form a spherical envelope around what is effectively a mineral called ferrihydrite. The protein envelope oxidises Fe2+ coming from the outside and channels it into the crystalline core of the molecule. Iron stored in ferritin is virtually inert and appears to be mobilised only after the digestion of whole ferritin nanoparticles in lysosomes. In some tissues half-digested ferritin particles can aggregate to form a pigment called hemosiderin. Total body iron stores can be estimated by measuring serum ferritin and the saturation of Tf with iron. Both values will be low in patients with iron deficiency and high in patients with iron overload, e.g. due to repeated blood transfusions.
Regulation of iron metabolism
As we discussed previously, iron is both essential and potentially dangerous. Tight control of its uptake, transport, use and storage is therefore of utmost importance. A central role in this regulation is played by molecular sensors of free iron concentration.
Iron response element-binding protein-1 or irp1 changes its structure depending on how much dissolved Fe2+is around. When iron concentration is low it assumes an “open” configuration, at high concentrations it exists in a “closed” configuration. The open configuration of irp1 can bind to specific sequences of rna called iron response elements or ire.
The expression of most proteins is regulated at the level of gene transcription. Proteins involved in iron metabolism are, however, regulated also at the level of rna translation. In an rna transcript containing ire these sequences form little loops, to which activated (or open) irp1 can bind. If these loops are located in front of the coding sequences (at the 5’ end) the binding of irps will block the access of this rna molecule to the ribosome and as such will decrease the production of the protein coded in this transcript. On the other hand, if ire loops are present at the 3’ end of a given rna the binding of irp1 molecules will slow down the degradation of the transcript and thus increase the synthesis of the encoded protein.
The logic of the regulation of cellular iron metabolism is then as follows: if a cell lacks Fe2+, irp1 will become active and bind to rna transcripts of genes containing ire sequences. Those genes that code for proteins that increase cellular iron concentrations (i.e. by increasing uptake) will have ire sequences at the 3’ end and will thus be stabilised by irp1 binding; this is the case, for example, for the transferrin receptor or dmt1. At the same time, transcripts with ire loops at the 5’ end will not be translated into proteins: this will affect, for example, ferritin. If, on the other hand, there is too much (or just enough) iron in the cell, irp1 will become inactive, it will not bind to rna and proteins will be regulated solely by transcription.
Intestinal iron uptake is, in addition to the irp-ire mechanism, regulated by a hormone called hepcidin. Initially discovered as an antimicrobial peptide, hepcidin was identified as a signalling molecule mediating the communication between extraintestinal tissues (mainly the liver) and the duodenum. Hepcidin is released into the blood in response to high iron levels, it binds to the extracellular portion of the iron exporter ferroportin and causes its phosphorylation and subsequent degradation. As a result, less iron is released from cells into the blood, which in the case of enterocytes translates into a decrease of iron uptake from the diet.
Disorders of iron metabolism
In order for our cells to function properly we need the right amount of iron: not too little and not too much. Too little iron is mostly caused by insufficient amount of iron in the diet, which can be seen in individuals on extreme diet regimens but is more commonly encountered among the elderly and chronically ill patients, who often suffer from malnutrition. Another reason for iron deficiency is an increased need for the element (e.g. in pregnancy) or excessive losses of blood (e.g. chronic bleeding in malignant tumors of the gastrointestinal tract).
The symptoms of iron deficiency are, as with most dietary deficiencies, rather non-specific and include fatigue, pale complexion, light-headedness, shortness of breath, high heart rate, etc. These symptoms are caused by the main underlying consequence of iron deficiency, which is also the prime laboratory finding: low amount of haemoglobin in the blood or anaemia. Anaemia in iron deficiency is called sideropenic or hypochromic and is characterised by small red blood cells containing low amounts of haemoglobin. This type of anaemia is, therefore, also called microcytic anaemia. The logic is clear: iron deficiency means not enough iron to put into haem and therefore lower haemoglobin synthesis.
Excessive amounts of iron in the body can also have various causes. There may be too much iron in the diet (especially easily absorbable heme iron), which seems to be the case in most Western diets, where heme comes from red meat. There is some evidence that this excess iron may increase the risk of cardiovascular diseases and some types of cancer. Iron overload is common in patients who must receive repeated blood transfusions (remember that there is virtually no way to excrete excess iron). Such patients include individuals with impaired haemoglobin synthesis, e.g. in the form of inherited thalassemias.
Defects in the elaborate regulatory pathways may also cause excessive accumulation of iron in the body. A group of diseases covering these defects is called haemochromatosis. Haemochromatosis patients have various defects in the hepcidin signalling pathway. As such, there is an impaired communication between tissues and the intestines and enterocytes keep absorbing all the available iron even when there is already plenty of iron in the body. The resulting excess iron is then deposited primarily in the liver, pancreas, heart, joints and brain, gradually causing severe damage in these organs. Symptoms of haemochromatosis include liver cirrhosis, cardiomyopathy, diabetes, arthritis and skin hyperpigmentation.
Currently there are two ways of treating iron overload: bleeding or the administration of iron chelators, which can bind excess iron and be excreted out of the body. One of the chelators used clinically is called desferrioxamine, which we discussed above as one of bacterial siderophores.
Function and deficiency
Copper, an important component of many enzymes, is yet another element necessary for our survival. Similarly to iron, copper exists in body in various oxidation states (mainly Cu+ and Cu2+). It is this ability to donate or accept electrons that allows it to participate in redox reaction. Copper is a constituent of respiratory chain enzymes (cytochrome oxidase) and antioxidant enzymes (superoxide dismutase). As a part of lysyl oxidase, it takes part in the maturation of collagen and elastin, thus being important for the structure of the connective tissues. Shortage of copper may cause impaired growth and development of bone tissue and other connective tissues. Copper also participates in the absorption and utilization of iron and in hemoglobin synthesis. Copper deficiency may thus lead to anemia in children fed only by milk (because of the lower copper content).
Copper may be found for example in legumes, meat, eggs and fish.
Absorption and excretion
Following the absorption, which is expected to take place mainly in stomach and upper part of small intestine, copper binds to plasma proteins (mainly to albumin). The portal blood transports copper to liver, where it is taken up, subsequently incorporated to other proteins and released back into the blood. Most of the copper leaving copper is attached to ceruloplasmin, the main copper plasma transport protein (transporting up to 90 % of all copper). Other substances involved in the transport of copper to extrahepatic tissues are albumin and marginally amino acids as well.
When the copper does not bind to its transport proteins, it can be excreted from liver to the bile. The level of copper in the body is regulated mainly through its excretion and thus the bile represents the main route for its elimination from body.
Genetic defect of the transport protein ceruloplasmin causes a disease called Wilson’s disease (hepatolenticular degeneration). The condition is cause by a defect in the incorporation of copper into ceruloplasmin that causes decrease in the copper elimination from liver. Copper, which is not transported on ceruloplasmin, subsequently accumulates in various tissues mainly in liver, brain or cornea leading to the symptoms of their damage (cirrhosis, seizures, tremor etc.).
Selenium is a part of glutathione peroxidase, an enzyme acting as one of the most important antioxidative systems protecting the body against reactive oxygen species and oxidative stress. It is probably the reason, why selenium is considered to slow down aging and have anticarcinogenic effect. Selenium improves the function of immune system, contributes to CNS activity, sperm motility and maturation.
Selenium is absorbed in duodenum. Because it is not stored in liver and is eliminated through kidneys, its plasma levels decline rapidly when not taken in sufficient amount.
Selenium deficiency is not rare, its intake by the normal population lies at the lower margin of the optimal dosage. Shortage of selenium causes impairment of the heart muscle (cardiomyopathy), increases the risk of malignancies and immune disorders (reducing the resistance to infections). Endemically occurring juvenile cardiomyopathies (termed Keshan disease) caused by selenium deficiency in water and soil are described in some regions of China.
There exist over 300 enzymes, which require the presence of zinc (for example alcohol dehydrogenase, carboanhydrase or lactate dehydrogenase). Zinc is also a component of transcription factors involved in DNA synthesis and as such has a role in cellular proliferation, tissue regeneration and wound healing.
Zinc can be found for example in meat, cheese, yeast, legumes, whole grain cereal or root vegetables. Its absorption from plant sources is decreased by fiber or other substance. After absorption, zinc is not stored in liver; smaller portion (around 10 %) is excreted by urine, the rest by bile. Most of the zinc in blood is attached to albumin, smaller part binds to other plasma proteins (like transferrin). Plasma levels of iron and zinc thus interact and may affect each other – for example higher concentration of iron decreases the absorption of zinc and vice versa.
Zinc deficiency may cause disorders of immune system, growth or wound healing. There had been reported cases of prostate hyperplasia.
The biologically active form of chrome is its Cr3+ cation. Chrome mainly affects the saccharide metabolism acting as one of the glucose tolerance factors and assisting the effect of insulin. Chrome insufficiency decreases the glucose tolerance.
Meat, yeast, cheese or wheat sprouts represent good source of chrome.
Unlike the trivalent cation, the exposure to Cr6+ is toxic and may cause allergic reactions (like contact dermatitis) and is likely to increase the risk of cancer.
Manganese, similarly to other metal elements, is an important component of some enzymes (superoxide dismutase, pyruvate carboxylase, …). Its role as a cofactor of mitochondrial SOD is important in elimination of reactive oxygen species, which are formed during the electron transport chain. Manganese is also important for the function of CNS and the structure of the bone tissue.
Sources of manganese include whole grains, soya bean or nut.
Exposition to high concentrations of manganese (in the past especially in people employed in its extraction and production) leads to the development of symptoms called “manganese madness”. Early stages are characterized by irritability and changes in mood and behavior, final stages by parkinsonism.
The source of iodine, an element essential for the production of thyroid hormones, are mainly marine animals, plants grown in soil rich in iodine and iodized salt. Iodine deficiency was common for nations without the access to the see. Measures aimed at iodization of certain foods (salt, flour) have significantly contributed to the reduction of diseases related to iodine deficiency in diet (so-called endemic goiter).
Disorders resulting from iodine deficiency may be caused by the substances called goitrogens, which affect, in a variety of ways, normal iodine metabolism. We can take thiocyanate as an example. Thiocyanate competes with iodides for the site on the transport mechanism (Na+/I– symporter) and thus decrease their uptake by thyroid gland.
Symptom accompanying insufficient iodine intake are termed IDD (iodine deficiency disorders) and include goiter or cretinism. They generally involve enlargement of the thyroid gland or disorders of mental function, growth and fertility. The development of goiter can also be caused by an excessive iodine intake. For more details concerning these disorders see Subchapter 11/5.
Recommended daily intake for minerals (according to 450/2004)
Subchapter Authors: Petra Lavríková, Josef Fontana and Jan Trnka