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Vitamins are valuable micronutrients that are key players in different physiological processes, such as metabolism, immunity, neurological functions, and bone health. The study is an exploration of classification, biological importance, and novel ways to use vitamins. Vision, immunity, bone mineralization, antioxidant defense, and the blood clotting process are some of the functions of fat-soluble vitamins such as A, D, E, and K in the body. On the other hand, water-soluble vitamins (B complex and C) are of utmost importance for energy metabolism, red blood cell formation, and immune modulation. Vitamin deficiencies are the cause of serious health conditions such as night blindness, osteoporosis, anemia, scurvy, and neurological impairments. Recent studies have established that the use of vitamins in the treatment of chronic diseases like cardiovascular conditions, neurodegenerative disorders, and cancer is a possibility. Besides, vitamin D and B-complex vitamins are being checked for their role in fighting autoimmune diseases and COVID-19, as well as the neuroprotective effects of the latter. This research points out the necessity of consuming adequate amounts of vitamins and supplementation to enhance overall health and prevent diseases.
Vitamins are essential micronutrients that play critical roles in various physiological processes, including metabolism, immunity, and cellular function [1]. Despite being required in small amounts, their deficiency or excess can lead to significant health issues [2]. This chapter explores the importance of vitamins in maintaining health, their biological roles, dietary sources, deficiency disorders, and emerging research on their therapeutic applications [3].
1.1 Classification of vitamins
Vitamins are split into two broad groups, namely, according to their solubility. Some are soluble in water while others in fat (Figure 1) [4].
Figure 1.
Classification of vitamins.
1.2 Fat-soluble vitamins
Vitamin A is a fat-soluble vitamin that plays an important part in keeping sight, immune response, and skin integrity. It can be found in two forms: preformed vitamin A and provitamin A. Preformed vitamin A is made of some materials like retinol, retinal, and retinoic acid, which are obtained from liver, fish, dairy, and eggs. These types of vitamin A are quickly used for different processes in the body. In contrast, provitamin A has beta-carotene and other carotenoids, which are economic resources, most of which are from plants like carrots, spinach, sweet potatoes, and mangoes [5]. The human body uses beta-carotene from the provitamin and converts it into vitamin A whenever it is necessary. Thereby, it is a crucial element of a diet, especially for those who choose to be vegetarian. Both are major contributors to the good sight, the immune system, healthy skin, and cell growth [6].
1.2.1 Role of vitamin A in immune function
Vitamin A is essential for modulating innate and adaptive immunity by fortifying the integrity of mucosal barriers and controlling immune cell function [7].
Vitamin A is among the important players in defending the immune system, primarily doing so by maintaining the mucosal barrier integrity, assisting white blood cells in their activity, and controlling cytokines. Including skin, respiratory, gastrointestinal, and urinary tracts, the new epithelial tissues are preserved due to the vitamin A-induced regeneration, and their relationship with the strong mucosal barrier is thus the first line of defense against infections. By helping strengthen the mucous membrane, vitamin A converts the body’s primary defense mechanism from passive to active [8]. Apart from this, retinoic acid, a metabolite of vitamin A, is indispensable in the regulation of white blood cells (WBCs) through T-cell differentiation so that a balanced immune response is manifested between pro-inflammatory and anti-inflammatory mechanisms. It also increases B-cell activation, which results in efficient antibody production. Moreover, vitamin A is responsible for cytokine regulation, thereby controlling the production of key immune signaling molecules such as IL-6, TNF-α, and IFN-gamma. These cytokines are essential in the inhibition of the overexpression of such common inflammatory diseases while promoting innate immunity against such diseases, thus modulating a resistant immune system through vitamin A [9].
Vitamin A deficiency destroys the immune system, increasing the vulnerability to respiratory infections, measles, and diarrhea. This, in turn, results in delayed wound healing and reduces the effectiveness of inflammatory responses, thus making the body less capable of fighting infections and recovering from injuries [10].
1.2.2 Role of vitamin D
Vitamin D (Calciferol) is another necessary component. Vitamin D regulates the level of calcium in the body. It enhances the absorption of calcium from the intestines into the blood and helps to mineralize the bones. It also maintains the proper level of calcium in the blood [11].
Vitamin D is a significant factor in the balance of calcium and the skeleton through some complex biological processes. The intestinal tract or dietary consumption is responsible for the distribution of vitamin D3 (cholecalciferol) to the body, which is first produced in the skin by taking in 7-dehydrocholesterol in the presence of sunlight. However, these two options are not the only ones: both vitamins D2 and D3 can be found in the food you eat, such as fish, dairy, and eggs. In the body, vitamin D is first processed by the liver into a substance called 25-hydroxyvitamin D (25(OH)D), which further contributes to the formation of 1,25-dihydroxyvitamin D, a steroid hormone with the influence of the parathyroid hormone (PTH), which is the chief regulator of calcium metabolism [12].
Calcitriol mission in the intestine is to induct the synthesis of calcium-binding proteins (Calbindin-D9k and TRPV6) by intestinal cells, and it can reabsorb the calcium as well as the phosphorus. Additionally, mineralization of bones is promoted when vitamin D stimulates osteoblast activity, thus regulating the even deposition of calcium necessary for the strength and density of the bones. For calcium balance, vitamin D drives calcium resorption in the kidney thus, it reduces calcium excretion in the urine. These coherent strategies prove that vitamin D is a tool in the field of bone development, maintenance, and overall skeletal health [13].
A state of hypocalcemia resulting from vitamin D deficiency is diagnosed with weak bones, muscle cramps, and feeling of numbness. The depletion of vitamin D in the body is found to cause the release of parathyroid hormone (PTH) in increased quantities, which in turn induces secondary hyperparathyroidism leading to the bio resorption of the bones and increasing the risk of osteoporosis, respectively [14].
1.2.2.1 Role of vitamin D in bone health
Vitamin D is needed for the proper growth and development of the bones and for the prevention of bone diseases [15].
Vitamin D is a very important player in the metabolism of the bones by overseeing the action of osteoblasts and osteoclasts, thus, proper bone formation and remodeling happens. Osteoblasts, these bone-making cells, use vitamin D to enable the deposition of calcium and phosphate into the bone matrix, therefore, the skeletal structure becomes stronger. Moreover, osteoclasts, which are cells that degrade bone, are indirectly controlled by vitamin D through parathyroid hormone (PTH), so that a balanced bone remodeling process is maintained to keep skeletal integrity [16].
Vitamin D shortness can cause very severe bone disorders. In infants, a vitamin D shortage leads to rickets, a phenomenon that is the result of the softening of bone, deformities, and slow growth owing to the lack of calcium and phosphate in the body. Apart from that, the lack of it leads to osteomalacia in adults, presenting with bone weakness, chronic painful sensations, and the tendency for easily getting fractures as the mineralization of bones is not properly carried out. On the other hand, the role of vitamin D in preventing osteoporosis, particularly in old people, is crucial. Vitamin D at good levels increases bone mineral density; that way, the possibility of fractures is rare, and the bones become stronger and healthier. Calcitriol, which is the active form of vitamin D and calcium-binding protein, is needed to balance the calcium concentration and the activity of bone cells. So, the results are obvious – strong and resilient bones throughout our existence [17].
Vitamin D deficiency is responsible for rickets in children, which is the process of softening, is of the bony tissues, the bone gets curved and looks like a bow-legged person. For example, in adults, the consequence is osteomalacia, a disease of the bone which is characterized by bone pain, fractures, and muscle weaknesses. In the long term, a lack of vitamin D also contributes to osteoporosis by reducing bone density and increasing the probability of fractures in old individuals [18].
1.2.2.2 Other health benefits of vitamin D
Vitamin D is not only important for bone health, but it also has other significant roles in immune function, muscle strength, and chronic disease prevention.
Vitamin D owns a significant position in the immune system by stimulating the production of antimicrobial peptides such as cathelicidins and defensins that help the system fight bacterial, viral, and fungal infections [19]. Furthermore, it influences the T-cell, affecting the immune system so that the risk of autoimmunity is decreased. Apart from immunity, vitamin D has a direct impact on muscle strength and cardiovascular health too. A deficiency of vitamin D has been found in the past to be related to the weakening of muscles, thus increasing the susceptibility of old people to falls and fractures. Additionally, vitamin D is involved in the regulation of blood pressure and cardiac function through the improvement of vascular health and inflammation reduction; vitamin E functions like another vitamin. Vitamin D along with tocopherol is a potent antioxidant that helps maintain the integrity of cells by preventing oxidative damage to cell membranes. This antioxidant activity is crucial in the prevention of chronic diseases, including cardiovascular disorders, neurodegenerative conditions, and some cancers [20].
Vitamin E is a fat-soluble antioxidant that is critical to the protection of cells from oxidative stress, maintenance of immunity, and skin and cardiovascular health. There are various forms of vitamin E found on the market, but α-tocopherol is the most biologically active form in humans.
1.2.3 Role of vitamin E as an antioxidant
Vitamin E is a lipid-soluble antioxidant that protects cell membranes from damage caused by reactive oxygen species (ROS) and free radicals [21].
Vitamin E indispensably contributes to the integrity of the cell by inhibiting by-products produced during the oxidative decomposition of cell membranes. Consequently, polyunsaturated fatty acids (PUFAs) that are van overcrowded are the ones that are the most easily oxidized lipids. This is due to the fact that free radicals attack them. Thus, a process known as lipid peroxidation begins which might result in membrane damage, increased permeability, and cell dysfunction. Vitamin E, on the other hand, in the form of a lipid-soluble antioxidant, tries to lessen lipid peroxyl radicals produced in the membrane, and, as a result, it stops the Series of Oxidations-corresponding chain of reactions and maintains membrane integrity [22].
When vitamin E carries out the process of neutralizing free radicals, it undergoes oxidation and thus is absolutely ineffective in its antioxidant role. However, if it connects with vitamin C and glutathione and they donate electrons to return it to its active, reduced state, it can be transformed into its original form. The recycling mechanism involved means that there exists a building of a protective screen to shield cell membranes from oxidative stress while making use of vitamin E to support cell functioning and restoring cell health. This is vital for a long life.
Vitamin E, if appropriately protected, also contributes to cellular health; thus, the destruction of the cell membrane, an effect if it happens, is the reason for various neurological disorders (headaches, fatigue, mood changes, memory difficulties) and muscle weakness. Also, apart from that, elevated oxidative stress from deficiency of vitamin E has been associated with age-related and chronic diseases including cancers and heart problems to mention just a few. The role of cell membrane stabilization and the protection of red blood cells are the most recognized features of vitamin E in the human body [23]. This is observable in oxidative areas like the brain, skin, and red blood cells. It has been documented that it modifies red blood cells to experience hemolysis induced by oxidants and that it also shields nerve fibers to be harmed and flexes DNA changes, so cancer risk can be decreased in that way. However, the lack of vitamin E can lead to increased free radical formation (oxygen toxicity), and this can lead to the neurons being damaged and causing neuropathy. What is more, the muscles are also weakened, the red blood cells may undergo hemolytic anemia and oxidative stress can also be attributed to triggering various chronic diseases [24].
Apart from the function of vitamin E as a cellular protector, there exist other benefits of this essential nutrient. The immune function of the organism is brought to a high level provided by the arrival of T-cells. Those who fight off bacteria and viruses act as a security to the human body. It also has influences on the CV system. Vitamin E is highly involved in the process of the oxidation of LDL cholesterol reduction in the heart. Different studies have shown that in addition to the alleviation of ultraviolet rays, the presence of vitamin E in the skin cells can delay the aging process and decrease the incidence of AMD (age-related macular degeneration), which is among the frequent degenerative disorders of the visual perception.
1.2.4 Role of vitamin K
Vitamin K is a necessary fat-soluble vitamin that is involved in blood clotting, bone health, and cardiovascular function. While the body is made up of two substances called Phylloquinone (vitamin K1), found mainly in green leafy vegetables, on the other hand, there is another element, Menaquinone (vitamin K2), which is present in fermented foods and also synthesized by the gut bacteria. Vitamin K’s chief responsibility lies in blood clotting, where it forms clotting factors, the net result of which is the prevention of uncontrolled bleeding. When the body gets insufficient vitamin K, the effective clot formation in it will be affected leading to hemorrhages. Furthermore, vitamin K takes part in the proper action of bones because it helps to utilize calcium and thus build new bones, which are necessary for prevention of osteoporosis, stressful conditions, and fractures [25].
Vitamin K is one of the crucial elements that are the leaders in the blood clotting process because the entire process can only start if the vitamin K goes through the chain of the catalysis. The liver is actively involved in the production of clotting factors II (prothrombin), VII, IX, and X, which all require a process called γ-carboxylation for them to be fully effective. γ-carboxylation is a vitamin K-mediated process. Without γ-carboxylation, clotting factors do not bind to calcium ions at their surface. For γ-carboxylation to occur, the final event is taking place in the liver, and this process needs to be completed if there is to be no excessive bleeding and the wound is going to heal properly.
After the γ-carboxylation process, the inactive epoxide form is obtained by vitamin K, so it becomes metabolized. The body works in a way that a preliminary substance, a vitamin K Cycle, is converted to the full active vitamin K state, which is done with the help of the enzyme vitamin K epoxide reductase (VKORC1) that regenerates vitamin K, and then the necessary step is carried out. This recycling method of clotting contributes to the prevention of deficiencies, and as a result of this process, problems with increased bleeding tendencies or clotting disorders such as vitamin K deficiency bleeding (VKDB) in newborns are eradicated [26].
Vitamin K deficiency may cause bleeding (hemorrhage) among other things, because of the clot reaction, after which the clumps become torn off. This situation is significantly prevalent in newborns since they may have very low stores of vitamin K, which can be a cause of the hemorrhagic disease, and thus, they may need some extra supplementation. Apart from its primary function in blood clotting, vitamin K holds a special place in the process of bone metabolism. It works on the regulation of calcium-binding proteins. Those proteins are very important for bone structure and mineralization since they build new tissue and strengthen it [27].
1.3 Water-soluble vitamins
These are vitamins that are particularly enriched in water, and which must be replenished frequently:
Vitamin C (ascorbic acid): Enhances the immune system defense mechanism and is a natural antioxidant that is a key element of the synthesis of the intercellular substance.
A water-soluble vitamin like vitamin C has been known for a long time to have several functions in the body among which are the immune system support, antioxidant activity, and the synthesis of collagen. It is vital for human beings to consume a healthy diet that contains fruits like citrus, berries, peppers, and airborne vegetables to be able to get vitamin C since it cannot be produced by the body [28].
1.3.1 Role of vitamin C in immune system support
Vitamin C strengthens the protective function of the body through its effects on both the innate and the adaptive immune systems.
Vitamin C (ascorbic acid) works as a significant immune enhancer by augmenting white blood cell activity, stimulating cellular antiviral immunity, and curtailing inflammation.
Vitamin C boosts not only the number and efficiency of the destroying cells (for example, neutrophils and macrophages) but also the other immune cells, such as T-cells and B-cells, to a great extent, which are partially responsible for the body’s immune reaction to infections.
Vitamin C is partly responsible for the production of interferons, which are in reality proteins signaling the nearby cells to become more resistant to viruses and thus helping in antiviral defense. While this mechanism facilitates the detection and removal of viruses, which replicate on the body, the effective development of antivirus mechanisms and top-notch immune conditions appear [29, 30].
Vitamin C functions as an opposing modulator of the immune response and, thus, it is the potent anti-inflammatory presence that regulates immune responsiveness and reduces tissue damage caused by hyperactivation. It helps to curb the production of pro-inflammatory cytokines such as IL-6 and TNF-α, which otherwise would cause sterile inflammation. This feature is particularly useful in the management of chronic inflammatory diseases, the treatment of infections, and the reduction of infection severity.
Therefore, vitamin C is a significant factor in immunity to infections and recovery of speed. As a result, the risk for immune-related diseases is lowered.
The lack of vitamin C is known to cause the deficient functioning of the immune system which may result in the body being prone to infections and also may result in delayed wound healing due to immunity cell dysfunction. Being a strong antioxidant, vitamin C resists free radicals and contributes to the protection of cells from oxidative damage, thus reducing the incidence of chronic diseases [31].
Vitamin C plays an essential part in the development of collagen, which is a necessary wound-healing component, and it also maintains skin elasticity. While working alongside prolyl hydroxylase and lysyl hydroxylase enzymes, it plays as a cofactor for these enzymes in hydroxylating proline and lysine residues in collagen. Collagen is vital as it gives stability to the tissue, and if there is a lack of vitamin C, then the collagen will not be formed properly which may result in the development of a disease such as scurvy, which is typically characterized by bleeding gums, easy bruising, and wound slow healing. Moreover, vitamin C activates the fibroblast cells, which are the cells that the body sends to the wound to repair it. Treated wounds will get new collagen by reducing one of the factors that cause cell injury: oxidative stress [32]. The post-surgical period, as well as burns and ulcers, all benefit from the treatment due to the capacity of vitamin C to disperse free radicals. Last but not least is the support of dermal extracellular matrix stability, which provides skin with a taut appearance and prevents the formation of fine lines and wrinkles. Also, its antioxidative features are useful as an ultraviolet light photoprotector which in turn results in the reduction of hyperpigmentation and aged skin condition, thereby protecting against UV-induced photodamage. Hence, vitamin C is very important for good skin structure and overall body repair [33].
Deficiency Consequences (Scurvy): It deteriorates blood vessels and causes frequent bruising, bleeding gums, and wounds not healing due to the inefficiency of collagen synthesis because of its low level. Moreover, one may also experience loose teeth and joint pain because the connective tissue would be breaking down. Vitamin C, besides, has a critical role in cardiovascular health since it lowers the risk of atherosclerosis by reducing blood pressure and inhibiting LDL cholesterol oxidation. It also supports iron absorption; ascorbic acid intervenes by changing iron levels from a non-absorbable to an absorbable form; by doing so, the development of anemia is inhibited. Moreover, vitamin C has a neuroprotective effect, thus limiting cognitive impairment and neurodegenerative diseases such as Alzheimer’s [34].
1.3.1.1 B-complex vitamins: Involved in energy metabolism and neurological functions
B-complex vitamins are micronutrients that are soluble in water and perform crucial activities in the mobilization of energy, the formation of red blood cells, the neural system, and DNA synthesis. They need to be reinstated regularly via diet since they are not mostly stored in the human body.
1.3.2 B1 (thiamine): Essential for carbohydrate metabolism and nerve function
Thiamine functions as a coenzyme (thiamine pyrophosphate, TPP) in the metabolic process of carbohydrates. It is required for energy production, as it is a mediator through the Krebs cycle and the pyruvate dehydrogenase complex, which are crucial in converting glucose to energy. Furthermore, it is essential for the regulation of choline acetyltransferase, which modulates cholinergic nervous conduction. Thiamine insufficiency can be followed by other diseases such as beriberi which is presented by muscle weakness, neuropathy, and heart failure and Wernicke-Korsakoff syndrome, a neurological disorder usually among alcoholics. The major dietary sources of thiamine are whole grains, legumes, nuts, and pork [35].
1.3.3 B2 (riboflavin): Helps in energy production and red blood cell formation
Riboflavin, a very important element of the coenzymes FMN (flavin mononucleotide) and FAD (flavin adenine dinucleotide), which are necessary to produce ATP, serves a key role in the electron transport chain (ETC). Furthermore, riboflavin is required for efficient iron absorption and the formation of red blood cells, without which oxygen transport will be compromised. With this, riboflavin, which is an antioxidant, serves the function of a producer of glutathione, the element that is responsible for safeguarding the cells from oxidative damage from the body.
The primary symptoms of ariboflaninosis, which is a shortage of riboflavin, are dry and cracked lips, sore throat, and anemia. In addition to these, it is likely to bring about different skin diseases and the inflammation of mucous membranes, thus decreasing the overall health status [36, 37].
1.3.4 B3 (niacin): Supports DNA repair and cardiovascular health
Vitamin B3 is niacin. As part of NAD+ and NADP+, niacin not only plays the role of ATP synthesis, but it is also the essential coenzymes for this. It is also an ingredient of the DNA repairing process, which, in turn, is responsible for mutations that could lead to chromosomes demolition being avoided. As an additional benefit, along with the fact it permits the decrease of LDL (also known as “bad” cholesterol), the effects of which are offset by encouraging HDL (or “good” cholesterol) are obtained – this is done through a reduction of the chances of heart disease.
Pellagra that is characterized by the “four Ds”: dermatitis, diarrhea, dementia, and in severe cases that lead to death is the result of a niacin deficiency. Lack of niacin can also promote a weak immune system, tiredness, and anemia. Foods that are rich in niacin such as meat, fish, whole grains, and peanuts are good sources to ensure proper niacin intake [38, 39].
1.3.5 B5 (pantothenic acid): Crucial element in the formation of coenzyme A
Pantothenic acid is a very important part of metabolism by being the coenzyme A (CoA) producer that is necessary in creating and destroying fatty acids. It is an oxidative process of the citric acid cycle, which provides ATP for cellular fuel. Also, it is the connection point between the production of neurotransmitters and steroidal hormones and the maintenance of the correct function of the nervous system and the hormonal balance in the body.
While pantothenic acid deficiency is extremely rare, fatigue, numbness, and metabolic disorders can still be observed, seeing that energy production is diminished. Foods such as whole grains, eggs, avocados, and meats are rich in pantothenic acid, thus promoting the adequate intake of the same for metabolic and neurological health [40].
1.3.6 B6 (pyridoxine): Contributes to neurotransmitter formation and hemoglobin production
Vitamin B6 is a major player in the production of neurotransmitters, such as serotonin, dopamine, and GABA, which are absolutely necessary for the functioning of the brain, by changing the amino acids into them. Besides, it is essential in hemoglobin production, thereby aiding in the transport of oxygen and in the regulation of homocysteine metabolism which in return reduces the risk of cardiac diseases.
Deficiency of vitamin B6 may lead to anemia, irritability, seizures, and depression, as it is a necessary component of neurotransmitter balance and the development of red blood cells. Poultry, bananas, potatoes, and nuts are examples of excellent dietary sources that help improve the levels of B6 in the body for the needed metabolic and neurological activities [41].
1.3.7 B7 (biotin): Assists metabolic activities and skin health
Biotin (vitamin B7) acts as a coenzyme for carboxylase enzymes, thus, having a great influence on the metabolism of glucose, fatty acids, and amino acids. Biotin is also involved in preserving healthy skin and nails.
Insufficient biotin can be the cause of hair loss, dermatitis, and neurological disorders, for example, fatigue, depression, and numbness. The preventive measures from the side of the nutrient are the due-biotin amino acids-rich foods, eggs, nuts, dairy, and liver, which provide the most support for metabolic and skin health [42].
1.3.8 B9 (folate): Fundamental in DNA synthesis and fetal growth
Folate is an important nutrient in the synthesis of DNA and RNA and thus, it is indispensable in the process of transcription leading to genetic stability and mutation prevention. It is also involved in the cell division phase which is vital in pregnancy for the baby’s development and the safe delivery of the baby. A mother’s good intake of folic acid is a great helper in the preventing of neural tube defects in newborns and therefore it is a crucial element in maternal and infant health.
Folate is the indispensable component of DNA and RNA synthesis thus being an only good cell division and genetic stability guarantor. When you hear about the changes in DNA during it, you need to know the crucial role folate plays in the whole process, namely the production of the processes that guide cell and genetic formation accurately. Moreover, the main reason for deficiency on this ground is megaloblastic anemia, where the red blood cells are large and immature, therefore, the oxygen transport is hampered. And the key food sources include leafy greens, beans, citrus fruits, and grains that are fortified with vitamins [43, 44].
1.3.9 B12 (cobalamin): Needed for neurological processes and red blood cell synthesis
Vitamin B12 is crucial in the formation of the protective myelin sheath to ensure the enveloping and thereby working of the neurons. It also plays a prime role in the generation of new red blood cells in the bone marrow by preventing the onset of cellular anemia. Additionally, B12 plays a vital part in the conversion of homocysteine to methionine, which winds down the occurrence of heart and other artery-related diseases.
A lack of vitamin B12 may lead to the development of pernicious anemia, characterized by megaloblastic anemia, exhaustion, and weakness. Also, be prepared for the numbness of your limbs, loss of memory, and cognitive decline, since its dreamy nerve function and brain health will provide such services. Mostly, vitamin B12 can be obtained from all kinds of animal products like meat, fish, dairy, and eggs [45].
1.3.10 Role of vitamins in health
1.3.10.1 Immunity boosting
Vitamins, such as C, D, and E, promote immune responses by the modulation of white blood cells and the reduction of inflammation.
Vitamin C: The presence and functional abilities of phagocytes and lymphocytes, which are involved in the destruction of dangerous agents, are increased by the presence of vitamin C. It also stimulates the production of interferons which are proteins that are involved in antiviral defense.
Vitamin D: It is involved in both innate and adaptive immunities through the induction of antimicrobial peptides like cathelicidins and defensins. Besides, it controls the differentiation and function of T-cells and macrophages, implying that the hypersensitivity to inflammatory agents are minimized.
Vitamin E: This is a strong antioxidant that defends immune cells from oxidative stress. The compound is an inhibitor of T-cell proliferation and an enhancer of natural killer (NK) cell activity which is crucial in the body’s resistance to fight off infections [46].
1.3.11 Metabolism and energy production
B-complex vitamins take part in the process of the metabolism of carbohydrates, proteins, and fats which are indispensable for cellular functions by providing energy.
It is an essential factor for amino acid metabolism and the production of neurotransmitters.
It has a role in glucose synthesis.
Vitamin B7 (biotin) → carboxylase coenzyme
It is one of the vitamins that is needed for the synthesis of fatty acids and the metabolism of amino acids.
It also helps with gluconeogenesis.
Vitamin B9 (folate) and vitamin B12 (cobalamin)
Code for DNA and RBC construction in order to avoid anemia.
It plays the role of methionine & homocysteine metabolism that affects the energy balance [47].
1.3.12 Cardiovascular health
1.3.12.1 Niacin (vitamin B3) and cholesterol regulation
Niacin takes a particular place in the improvement of lipid profiles and the reduction of cardiovascular risks through the following ways:
Inhibition of triglyceride synthesis: The enzyme diacylglycerol acyltransferase-2 (DGAT-2), which is compulsory for the synthesis of triglycerides in the liver, is inhibited by niacin. This process lowers the production of VLDL, which is the precursor of LDL or “bad” cholesterol.
Reduction of LDL cholesterol: Thanks to the reduction of hepatic VLDL production, niacin indirectly lowers circulating LDL cholesterol and vocals the shortening of the peripheral artery, which makes it possible to avoid serious losses of blood functioning.
Increase in HDL cholesterol: Niacin strengthens the good cholesterol (HDL), a high-density lipoprotein, by breaking down its catabolism, ensuring the formation of reverse cholesterol transport and reducing atherosclerosis.
Lowering of plasma free fatty acids: Niacin stops the function of lipolysis in adipose tissue, which leads to the release of free fatty acids into the bloodstream, leading to the decrease of hepatic triglyceride synthesis and lowering the levels of LDL at the same time [48].
1.3.12.2 Vitamin K and blood clotting/arterial health
Vitamin K is important for default blood coagulation and arterial health through the following networks:
Activation of clotting factors: Vitamin K serves as a co-complete for the enzyme gamma-glutamyl carboxylase, which converts inactive clotting factors (II, VII, IX, and X) into their active forms, ensuring the right coagulation and reducing excessive bleeding.
Regulation of anticoagulant proteins: On the other hand, it also takes part in the synthesis of anticoagulant proteins C and S, which are responsible for the regulation of clot formation and thrombosis prevention.
Prevention of arterial calcification: Vitamin K stimulates Matrix Gla Protein (MGP), which is an inhibitor of arterial calcification. A timely administration of vitamin K helps in the inhibition of the side effects of MGP and hence vascular calcification that leads to cardiovascular risks is greatly minimized. Diabetes, kidney diseases, Congenital heart defects, and genetic disorders are some of the cardiovascular diseases that vitamin K can help to prevent [49].
1.3.13 Bone and joint health
1.3.13.1 Vitamin D and calcium absorption
Vitamin D has a significant influence on calcium homeostasis and the process of bone mineralization by several mechanisms. It will catalyze the absorption of intestinal calcium through its transformation of cholecalsiferol to be calcitriol, the bioactive form of vitamin D. The said conversion happens in the liver and kidneys. Then calcitriol binds to vitamin D receptors (VDR) in intestinal epithelial cells, hence increasing the expression of calcium-binding proteins (called calbindin-D) and calcium channels (the so-called TRPV6), which make calcium absorption from the gut mucosa better [46]. The level of vitamin D that the body gets will be determined by the extent to which calcium absorption and bone remodeling are affected. Vitamin D for instance deals with the maintenance of bone function by inducing osteoclast maturation with the blood calcium levels inconsiderable, and its manifestation is the release of such bone material that is then used to release calcium outward. Conversely, in the presence of enough calcium, vitamin D helps osteoblast to produce new bone material, thus enhancing its mineralization and formation. Vitamin D does play a very important role in the prevention of rickets in children, and also in the development of osteomalacia in adults. Vitamin D also helps to keep the amount of calcium in the bones at a proper level; therefore, it minimizes the risk of osteoporosis and increases the likelihood of fractures, especially in old people [45].
1.3.13.2 Vitamin K and bone mineralization
Vitamin K is necessary for bone strength, which is, at the very least, a prerequisite to all the mechanisms connected with its maintenance and the lack of fracture. The activation of osteocalcin, a vitamin K-dependent osteoblast-derived protein that binds calcium to the bone matrix, is by that process. Vitamin K will be the cause for matrix Gla protein to transform into bone material (which is necessary for bone remodeling functions) and thus to make up mineralized bone material stronger. It even reduces bone loss by inhibiting osteoclast activity. If you want to know what benefits vitamin K has, it is to be mentioned that it directly relates to the improvement of bone strength of the participant through the activation of osteocalcin and the influence of the free protein MGP on the bone formation process. Additionally, MGP processing gives stability to the vasculature. Vitamin K for its part regulates MGP, a bone-specific matrix protein that is a central inhibitor of vascular calcification. Therefore, bone calcium is incorporated and not the arterial tissue. The health of the skeletal system is not only benefited, but the flexibility of the arteries is also improved, by decreasing atherosclerosis, which is important for the health of the heart and the body [49].
1.3.14 Neurological function
B12, B6, and folate, whose involvement in the synthesis of neurotransmitters, the formation of neuronal tissues, and protection from neurodegenerative diseases is of tremendous value, play key roles in the above-described situations. These B vitamins and mental health are connected. They regulate how the neurotransmitters are classified, especially dopamine and serotonin [50].
1.3.15 Vitamin deficiencies and associated disorders
A balanced diet is required to reach the desired vitamin intake. Carrots, sweet potatoes, liver, etc., are some of the rich sources [51].
Vitamin A: Carrots, sweet potatoes, liver
Vitamin D: Fish, fortified dairy products, sun exposure
Vitamin E: Nuts, seeds, spinach
Vitamin K: Leafy greens, fermented foods
Vitamin C: Citrus fruits, bell peppers, strawberries
B vitamins: Whole grains, dairy, meats, and legumes (Figure 2) [52].
Figure 2.
(Sources of Vitamins Chart – Google Search, n.d.) [53].
1.4 Emerging research and therapeutic applications
The understanding of vitamins has gone beyond their conventional roles, as per recent research. Besides their traditional roles, it has been found that they can be used in managing chronic diseases, providing immune system support, and thus, in general, improving health. Below are some key areas where vitamins are being explored for therapeutic applications [54].
1.4.1 Vitamin D in autoimmune diseases and COVID-19
1.4.1.1 Role in autoimmune diseases
The main interest has been in the existing link between vitamin D and autoimmune disorders such as:
Multiple Sclerosis (MS): Factor X as well as vitamin D are the two main elements in protecting the patient’s brain, so vitamin D acts largely due to X. Mostly, vitamin D decreases cytokine production and acts as an apoptosis inhibitor, thus preventing the differentiation of immune cells to memory of the autoantigen through decreasing the generation of cellular memory. In the meantime, vitamin D also promotes differentiation and transfer of Tregs as well as Bregs.
Rheumatoid Arthritis (RA): Lower vitamin D levels in the blood may lead to the occurrence of a specific type of white blood cell (macrophage) in the body. The vitamin D-deficient macrophages release a protein molecule known as IL-23, which in turn, makes them hyperactive and thus, their function will be over-activated, thereby they will migrate and finally infiltrate the joints. This autoimmune inflammation will eventually lead to the development of rheumatoid arthritis (RA) [55].
Type 1 diabetes (T1D): Lower vitamin D levels are related to the autoimmunity attack and development of type 1 diabetes. Pancreas islet cells are damaged by the fall in the efficacy of the regulatory T cells, which are a potent force shielding insulin from immune system attack. It has been shown that owing to their immunoregulatory role, Tregs are involved in the homeostasis of autoimmune diseases in addition to the anti-inflammatory cytokines and regulatory T cells. Tregs are most prevalent in infectious diseases, allergies, asthma, autoimmune diseases, and cancer [56].
1.4.2 Role in COVID-19
The emergence of studies reporting that vitamin D provides protection against various respiratory infections, including COVID-19, through the following means:
Vitamin D supports the immune system by enhancing the natural defense mechanism through the production of antimicrobial peptides like cathelicidins and defensins, which kill the virus. It also decreases the cytokine storm, a condition that happens when there are too many pro-inflammatory cytokines, such as IL-6 and TNF-α, while it also allows the production of anti-inflammatory cytokines. Besides, it assists the lung in working correctly by preventing acute lung injury (ALI) which is linked to the onset of ARDS. This, in turn, is one of the most prevalent causes of hospitalization and mortality. A survey of people has indicated that the levels of vitamin D were inversely linked to hospitalization and death from COVID-19. However, research is ongoing to ascertain the preventive and adjunctive treatment roles of the supplementation of vitamins [57].
1.4.3 Vitamin C in cancer therapy
Vitamin C is an antioxidant that has been known for a long time, but more recent studies suggest it as an adjunct therapy in treating cancer.
1.4.3.1 Mechanism of action in cancer therapy
Vitamin C can be both an antioxidant and a pro-oxidant; therefore, it occupies a dual role in cancer therapy. Under normal circumstances, it serves as an antioxidant, thus protecting the normal cells from oxidative stress. Nevertheless, it can also behave as a pro-oxidant by increasing the level of hydrogen peroxide (H2O2) which is the main component of oxidative stress and apoptosis in cancer cells. Vitamin C contributes to the effective operation of chemotherapy and radiotherapy by lowering the sensitivity of normal cells to radiation damage while increasing the sensitivity of cancer cells to drugs. Its activity is also determined to some extent by cancer cell metabolism, which gets disordered because glycogenolysis is disturbed so that normal metabolism cannot be maintained, and the cells will rely on glycolysis in the Warburg effect. Consequently, cells will lose energy and die [58].
1.4.3.2 Clinical evidence and trials
Studies performed on animals have proved that by using a high dose of intravenously injected vitamin C, cancers such as lung, pancreatic, and colorectal cancers can grow at a slower pace. Ongoing clinical trials are looking into whether or not this use of high-dose vitamin C as an auxiliary therapy may be effective in cancerous growths that are resistant to the action of chemotherapy. Besides, it also indicates that this supplement may boost the body’s defensive mechanism and make it able to attack the tumor cells more efficiently. Furthermore, in integrative oncology, patients are being administered with high-dosage intravenous vitamin C, which is believed to be a breakthrough in the treatment of chemotherapy-induced side effects such as fatigue, nausea, and oxidative stress, which in turn improves overall treatment tolerance and patient well-being [59].
1.4.4 Vitamins in mental health
B-complex vitamins are responsible for brain health, neurotransmitter synthesis, and the regulation of mood. On the other hand, research has shown that vitamin B deficiencies are connected with cases of depression, cognitive dysfunction, and neurodegenerative diseases in older adults.
1.4.4.1 Role of key B vitamins in mental health
Vitamin B12 (cobalamin) is a worrisome problem as it is necessary in the production of myelin sheaths, which are protective to the nerve cells and aid not only in the effective transmission of the neurotransmitter. However, one of the initial signs that a deficit in vitamin content is visible is that people may suffer from neurological symptoms such as memory loss and depression as well as they may be diagnosed with Alzheimer’s disease as well as other neurological disorders. Vitamin B6 (Pyridoxine) is vital in neurotransmitter synthesis, which in turn helps to produce serotonin, dopamine, and GABA [60]. These compounds that are present in the brain are responsible for mood and anxiety control. A lack of B6 deficiency may result in irritability, moodiness, and depression. Vitamin B9 (folate) is involved in the synthesis and repair of dopamine in brain cells, and low levels of the compound are the cause of depression and cognitive decline. This is compounded by the fact that it is also involved in the metabolism of homocysteine, and along with B12, it can regulate the synthesis of the latter therefore reducing the risk of neurodegeneration [61].
1.4.4.2 Clinical evidence
Research has proven that a deficient intake of B vitamins is responsible for the occurrence of depression, anxiety, and dementia. It is a well-known fact that the intake of supplementation with B12 leaf and folic acid has been found to enhance the thinking process and even relieve depression. Further to this, long-term observational studies have established that B-complex vitamins are efficacious in combating the progression of neurodegenerative diseases like Alzheimer’s and Parkinson’s [62].
Vitamins are essential micronutrients, which are required for the proper functioning of the body, metabolism, immune response, neurological health, and cell growth. The classification of these essential nutrients into fat-soluble (A, D, E, and K) and water-soluble (B-complex and C) vitamins demonstrates their critical roles in the maintenance of homeostasis and the prevention of various diseases. The lack of these essential vitamins can be associated with the development of serious health problems such as phobias, infections, anemia, osteoporosis, and neurodegenerative disorders. One of the emerging areas in science is the use of vitamins as the predominant therapy that can cure diseases like cardiovascular conditions, cancer, and autoimmune disorders. The rising inclination toward vitamin supplementation underlines the preventive healthcare relevance of this nutritional area. Getting an optimum level of vitamins, whether from regular food intake or supplementation, is essential for the normal functioning of the body and the individual’s overall well-being. Research in the future must focus on the personal need for vitamins and their role in disease modulation, so that this knowledge can be utilized for the betterment of public health.
I must express my great thanks to my teacher, Dr. R. R. Melinkeri, a retired professor at BVDUMC, Pune, for the support and valuable guidance required for this work conceptually. I am also very grateful to Dr. Meghana K. Padwal, who is a Professor and Department Head and other senior mentors in the Biochemistry Department at the same institution, for her valuable contributions and steadfast support.
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Written By
Pradnya Padalkar and Prakash Zende
Submitted: 27 January 2025Reviewed: 06 March 2025Published: 18 August 2025
Open access peer-reviewed chapter - ONLINE FIRST
