Vitamin D, known as the “sunshine vitamin,” is crucial for a wide range of physiological functions, with implications for bone health, immune function, cardiovascular health, mental well-being, and cancer prevention. Despite its importance, VDD has recently emerged as an epidemic having reached nearly 50% of the total population regardless of age and race. The two natural sources of vitamin D are sunlight and diet, but people’s lifestyles, location, and lack of direct sun exposure often lack these nutrients, so vitamins are required.
This article describes the biological aspects of vitamin D, its biosynthesis, its metabolites, and the two varieties of vitamin D: D2 and D3. The portions of vitamin D and its functions in calcium and phosphate balance, bone formation, and bone resorption pertinent to disorders like osteoporosis, rickets, and osteomalacia are also discussed in this chapter. In addition, we discuss the new trends towards the identification of the connection between vitamin D and cardiovascular disease, cancer and immune system, and mental health, its antioxidant and anti-inflammatory roles in autoimmune diseases, depression, and cognitive impairment.
Vitamin D in foods such as fatty fish, and in fortified products such as orange juice, is also mentioned and the difficulties in attaining adequate levels of vitamin D only through the diet are considered. The practical solution of the deficiency is supplementation which is prescribed according to tolerable upper intakes for each person.
The article ends on the rather urgent need for remedying insufficient vitamin D levels across the population, as it is essential to virtually all bodily functions and not solely bone health. Further research must be conducted to work on making the recommendations for vitamin D clearer and to understand better the various other uses that vitamin D has improvement of the human body.
1. Introduction
Vitamin D, commonly known as the “sunshine vitamin,” is essential for maintaining numerous physiological functions in the human body. VDD has recently been classified as a public health issue of major importance, and its prevalence currently exceeds 50 percent (Wacker & Holick, 2013). One billion people in the world live with VDD, which means this problem does not depend on ethnicity and age, thus being international (Bikle, 2018).
The main reason for vitamin D paucity is lack of sunlight, which is a natural form of vitamin D. This is by over-sun protection including the liberal use of sunscreen, reduced outdoor exposure, and living in countries or regions that receive limited sunlight, especially during the winter (Wang et al., 2021, vitamin D can be consumed as food supplements but these are often insufficient enough to meet the body’s needs daily.
Besides impacting calcium and phosphorus metabolism and bone health, vitamin D has broad-reaching consequences on other biological processes and systems such as cardiovascular health, immune regulation, cancer, and mental disorders. In this article, the author outlines the biological role of vitamin D, the health effects associated with the vitamin, the food sources of the vitamin, and the appropriate dosage of the vitamin..
2. The Biology of Vitamin D
Vitamin D, a vital fat-soluble vitamin, exists in two primary forms: The two forms of vitamin D include: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). These forms have different functions and sources; D2 is derived from plants and fungi, D3 is produced in human skin when exposed to sun, and is available from animal-sourced food products (Cashman et al., 2019). Each form plays a different role in promoting health, but scholarly work indicates that D3 offers a superior solution and is more effective at raising and maintaining the levels of the vitamin in the blood that accrue maximum benefits to the body (Bikle et al., 2020).
Vitamin D Forms and Synthesis
Vitamin D 2 and D 3 are two categorized vitamin D, arising from different sources and exhibiting insignificant biochemical variation. Vitamin D2 is synthesized naturally in plants, and fungi and is artificially added to food products while D3 is produced in the skin or obtained from animal-related foods like fish, liver, and eggs (Brehm et al., 2021). Vitamin D production starts when the skin is exposed to ultraviolet B (UVB) light; The UVB photonsega strikes the skin and activates 7-dehydrocholesterol, a molecule in the skin, to produce vitamin D3. This process is an important photochemical reaction because the vitamin D3 which the body naturally synthesizes is very sensitive to such factors as geographical location, time of year, skin color, duration of sun exposure, and so on (Sizar et al., 2020). This vitamin D3 form transforms by thermal isomerization to form vitamin D3 which is a biologically inactive form according to Chung et al., 2018.
The activation of vitamin D3 is a complex process that involves two hydroxylation reactions, and therefore the name vitamin, meaning ‘little hormone’. Firstly, Vitamin D3 goes to the liver where it is converted to 25-hydroxyvitamin D or 25(OH)D, which is the major circulating form of the vitamin and a significant marker of the body’s vitamin D levels (Sahay & Sahay, 2020). Further on, 25(OH)D gets taken to the kidneys for a second hydroxylation process to form 1,25-dihydroxyvitamin D [1,α (OH)2D] also referred to as calcitriol – the active form of vitamin D (Holick et al., 2018). This transformation is mandatory, as only calcitriol can bind to the vitamin D receptor (VDR) present in many tissues all over the body so vitamin D has diverse influences on health.
Calcitriol, being the endogenous form of vitamin D, is very important in modulating skeletal calcium and phosphate concentrations in circulating blood. Calcitriol enhances the ability of the intestines to mobilize calcium for normal bone mineralization and remodeling. It also controls the osteoblasts and osteoclasts, cells that are involved in the maintenance of bone formation through the ossification that is known to maintain ethical integrity in the turnover of skeletal and bone material (Schroth et al., 2019).
Figure 1: Vitamin D Synthesis and Metabolism
Vitamin D Functions
Although vitamin D is well known to have positive effects on bones because of its function in calcium and phosphate metabolism, vitamin D has wide-ranging effects on numerous physiological activities. As a hormone, vitamin D regulates immunological responses, cell division, and blood vessel health to mention but a few (Lips et al., 2019). Vitamin D is involved in wide-ranging modulation of other processes apart from being implicated in calcium control since the VDR is expressed in almost all the tissues in the body. Vitamin D through VDR regulation of genes activated by calcitriol strengthens the importance of the vitamin on general health and disease prevention (Marrone et al., 2020).
Calcium Homeostasis and Bone Health
Vitamin D is the most popular vitamin known for its utilization in the regulation of calcium and phosphorus within the body, which is important to bones’ stability and strength. Calcitriol increases calcium uptake from the intestines, thus, lowering the need for calcium from foods and enhancing the bone mineralization process (Santos et al., 2020). Casting a positive influence on intestinal calcium and phosphorus absorption, vitamin D regulates serum calcium levels to support purposes of bone construction and maintenance. The active hormone also plays a direct role in osteoblastic and osteoclastic activity thereby playing a direct role in the balance of the bone remodeling process. This activity is very important in reducing bone disorders including osteoporosis, and osteomalacia in which bone density and strength are reduced due to poor mineralization (Haines et al., 2021).
Immune System Regulation
The immune-modulating properties of vitamin D have gained considerable attention in recent years. As to the types of vitamin D, the reader is to remember that the active form of vitamin D takes part in the functioning of both the innate and adaptive immune responses while lowering inflammation. Calcitriol has been seen to promote the synthesis of antimicrobial peptides for instance cathelicidin useful for defense against bacterial infection and viruses (Martineau et al., 2019). Vitamin D also continuously regulates key immune cells as part of the immune response including T&B lymphocytes that are implicated in immune activation and tolerance. Immune regulation possibly provides an understanding of vitamin D-protected physiology of autoimmune diseases, such as multiple sclerosis and rheumatoid arthritis (Wu & Chen, 2020).
Cardiovascular Health
The available data imply that vitamin D is implicated in cardiovascular disease and affects blood pressure, endothelium, and inflammation. We also understand that the vitamin D receptor is present in several cardiovascular tissues, pointing to the role that it might play in the heart. It has been found that adequate Vitamin D status is inversely and significantly correlated with hypertension and atherosclerosis (Kassi et al., 2019). Vitamin D is also anti-necrotic and immunomodulatory, which is why cardiovascular damage is also benefited from this vitamin; the impact’s particulars are still being investigated (Pilz et al., 2018).
Cellular Growth and Cancer Prevention
Vitamin D has shown potential in regulating cell proliferation and differentiation, processes essential for maintaining healthy tissue turnover and preventing uncontrolled cell growth, a hallmark of cancer. Scientific data recommend that vitamin D reduces the growth of the amount of cancer cells and stimulates apoptosis in various forms of cancer, such as colon, breast, and prostate cancer (Carlberg 2019). In addition, cancer preventive functions of vitamin D may be due to its modulatory effects on inflammation and oxidative stress. As previously discussed, much more research is required to obtain conclusive evidence of vitamin D’s direct role in cancer prevention; nevertheless, observational studies also showed that enhanced levels of this vitamin decreased the chance of specific types of cancer (Sahota, 2021), vitamin D is more than just a hormone that impacts bone health but also serves various roles in the body. Being able to control calcium absorption and osteoclast activity is vital for bone health, its roles in the immune, cardiovascular, and cell divisions provide more evidence of its importance in overall human health. This is because the vitamin D receptor is highly expressed in most tissues, and its deficiency is associated with various diseases, which include bone disorders immune dysregulation, and chronic diseases. As research advances, scientific experts learn more about the different roles of vitamin D, and the possibilities of disease battling or alleviation by using vitamin D, therefore, underlining the necessity of appropriate vitamin D levels.
3. Sources of Vitamin D
Sunlight
Natural exposure to sunlight constitutes the main and best way through which humans get their body’s required vitamin D. But, several conditions can decrease skin ability to produce vitamin D; including geographical location, skin tone, age, and the usage of sunscreen. People with black skin need longer time getting exposed to sun due to melanin which hampers UVB reach while elderly take lesser vitamin D due to less of 7-dehydrocholesterol in skin (Tzeng et al., 2019).
Figure 2: Global UVB Availability
UVB radiation is believed to be at its peak between 10am-3pm and this time is advised for vitamin D synthesis, alterations in lifestyles including going to work and staying indoors, coupled with reduced outdooralex-a exposure has led to vitamin D deficiency not only in countries which experience limited sun exposure (Wacker & Holick, 2013).
Dietary Sources
Vitamin D is partly available from sunlight, however; a part is also received from some of the foods though it is quite limited. These include oily fish like salmon, mackerel sardines and fish liver oils like cod liver oil. Also, there are specific types of foods which have added vitamin D; such as dairy products, plant-based milk, cereals, and orange juice.
Table 1: Vitamin D Content in Foods
| Food Source | Vitamin D Content (IU per 100g) | % Daily Value |
| Salmon (wild) | 988 IU | 247% |
| Cod liver oil | 1360 IU | 340% |
| Fortified milk | 115 IU | 29% |
| Fortified orange juice | 100 IU | 25% |
| Egg yolk | 37 IU | 9% |
Table 1 highlights common dietary sources of vitamin D and their contribution to daily intake.
Supplements
However, even the most vitamin D-rich foods contribute only a small portion of the recommended daily intake, making supplementation necessary for many individuals. Vitamin D is therefore currently a recommended supplement because it is rare in most people’s diets. According to the NIH, the daily recommended intake of vitamin D for most adults ranges from 600-800 IU, with special populations requiring higher amounts due to decreased exposure to sun or diagnosed deficiencies (Nair & Maseeh, 2012).
The forms of vitamin D supplemental are vitamin D2 and D3; D3 is recommended since it was noted to be more effective in raising serum 25(OH)D (Adams & Hewison, 2018). Recommended dosage ranges from 50,000-100,000 IU per week for young people with deficiencies; it is also very safe and effective (Glueck et al., 2016).
4. Health Implications of Vitamin D Deficiency
Bone Health
Vitamin D is especially important in the process of bone metabolism because it influences the degree of calcium and phosphate absorption in small intestines. Vitamin D deficiency results in low calcium intake, which leads to high levels of parathyroid hormone that results into the stimulation of osteoclast hence bone loss (Holick, 2017). In the long run, this leads to decreased bone mineral density BMD and increased susceptibility to osteoporosis, osteomalacia, and fractures (Bikle, 2018).
Figure 3: Vitamin D Levels and Bone Mineral Density
In children, vitamin D deficiency results in rickets affecting bones and includes bowed legs, bone growth retardation, and other complications (Wacker & Holick, 2013). Deficiency of vitamin D is therefore likely to cause osteomalacia in adults, manifesting its symptoms as bone pain and muscle weakness.
Cardiovascular Health
The latest literature review highlights that patients with CVD should be encouraged to take higher doses of vitamin D because low levels of vitamin D are associated with hypertension, myocardial infarction, and stroke (Pérez-Fernández et al., 2020). It has been suggested that vitamin D influences blood vessels through its influence on inflammation, endothelial cells, and calcium levels all of which are imperative for cardiovascular health (Wang et al., 2021).
Cancer
Research shows that consumption of goodness vitamins such as vitamin D reduces risks of specific forms of cancer, including breast, colon, and prostate cancers. Defects caused by Vitamin D’s failures stem from its inability to control cell growth, development, and death of tumor cells (Zhao et al., 2020). Further, vitamin D may boost the ability of the immune system to identify and eliminate cancer cells so that tumors may not form in the first place (Nesby-O’Dell et al., 2020).
Immune Function
Vitamin D is an essential nutrient in the immune system, both the important and the secondary immune responses. It boosts the killing effectiveness of monocytes and macrophages and has a part to play in the modulation of the manufacture of antimicrobial peptides (Gombart et al., 2020). A lack of vitamin D has been linked to such factors as increased vulnerability to infections like the flu or COVID-19.
Furthermore, vitamin D may lessen the chances of autoimmune diseases such as sclerosis, rheum arthritis, and type diabetes by adopting immune responses that assist in preventing tissues from being attacked by the body (Marrone et al., 2020).
Mental Health
The connection between vitamin D deficiency and mental health illnesses, specifically depression, has become well-discussed in the last few years. Such studies have pointed out that mood disorders may be influenced by vitamin D because of its action on neuroprotection, serotonin level, and anti-inflammatory action (Zhang et al., 2021). Research also indicates that reductions in vitamin D levels predispose a person to depression, deterioration of cognitive ability, and even schizophrenia (Bridgman et al., 2019).
Table 2: Studies Linking Vitamin D and Mental Health
| Study | Condition | Outcome |
| Zhang et al. (2021) | Depression | Vitamin D deficiency linked to higher risk of depression |
| Bridgman et al. (2019) | Cognitive decline | Low vitamin D associated with cognitive impairment |
| Zhao et al. (2020) | Schizophrenia | Vitamin D supplementation improved mental health outcomes in patients |
Table 2 summarizes key studies on the role of vitamin D in mental health.
5. Recommendations for Vitamin D Supplementation
The magnitude of vitamin D inadequacy in the global community, health-related organizations have provided different dosing regimens depending on age, place of residence, and risk factors. The Endocrine Society recommends 1,500 to 2,000 IU per day to prevent and optimize Blood 25(OH) D levels in at-risk populations (Nair & Maseeh, 2012).
There are suggested dosages for adults with limited sun exposure for optimal vitamin D synthesis, it may be required to take a high dose of up to 50,000 IU per week for people in northern latitudes when sunlight is scarce during the winter season (Adams & Hewison, 2018).
Table 3: Recommended Daily Intake of Vitamin D
| Population Group | Recommended Intake (IU/day) | Maximum Safe Intake (IU/day) |
| Infants (0-12 months) | 400 IU | 1,000 IU |
| Children (1-18 years) | 600 IU | 4,000 IU |
| Adults (19-70 years) | 600-800 IU | 4,000 IU |
| Pregnant/Lactating | 600-800 IU | 4,000 IU |
| Older Adults (>70) | 800 IU | 4,000 IU |
Table 3 provides recommended daily intakes of vitamin D across different age groups.
6. Conclusion
Vitamin D is a very essential nutrient body that plays a central role in many ongoing bodily functions such as the mobilization and deposition of bone minerals, cardiovascular health, immune health, and mental health among others. Its significance is explained by the fact that approximately 1 billion people around the world suffer from vitamin D deficiency as a result of poor exposure and insufficient diets. This lack can pose severe health consequences and therefore there is a need to address the problem comprehensively. A wide range of public health interventions to address vitamin D deficiency and essential changes to diets also call for appropriate supplementation to cater for maximum health gains. In addition, further research is needed to discover the new roles of vitamin D in ameliorating and preventing diseases. This article also works on fine-tuning recommendations necessary for supplementation given the need to ensure that all people can reach and sustain the optimum vitamin D levels necessary for healthier living and quality living.
Free Healthbeat Signup
Get the latest in health news delivered to your inbox!
References
Adams, J. E., & Hewison, M. (2018). Vitamin D and Disease: What Is the Evidence? Clinical Chemistry, 64(6), 957-967.
Bikle, D. D. (2018). Vitamin D Metabolism and Function. The Journal of Clinical Endocrinology & Metabolism, 103(6), 2306-2313.
Bikle, D.D., Christakos, S., & White, J.H. (2020). Vitamin D regulation of immune function. Journal of Bone and Mineral Research, 35(6), pp.1029-1040.
Bouillon, R. (2018). Comparative analysis of nutritional guidelines on vitamin D intake. Endocrinology and Metabolism Clinics of North America, 47(4), pp.879-896.
Brehm, J.M., Acosta, B., & Covar, R. (2021). Skin phototypes and synthesis of vitamin D: A comparative study. Dermato-Endocrinology, 13(1), pp.38-45.
Bridgman, S. A., et al. (2019). The Role of Vitamin D in the Prevention of Mood Disorders: A Systematic Review. Nutrients, 11(4), 746.
Carlberg, C. (2019). Vitamin D signaling in cancer: Therapeutic implications. Frontiers in Physiology, 10, pp.29-41.
Cashman, K.D., et al. (2019). Vitamin D in the 21st century: A challenge for health. Annual Review of Nutrition, 39, pp.443-469.
Chung, M., et al. (2018). Vitamin D synthesis and skin phototypes: A systematic review. American Journal of Clinical Nutrition, 107(4), pp.638-649.
Glueck, C. J., et al. (2016). Safety of 50,000-100,000 units of Vitamin D3/week in Vitamin D-deficient, hypercholesterolemic patients with reversible statin intolerance. North American Journal of Medical Sciences, 8(3), 156-162.
Gombart, A. F., et al. (2020). Vitamin D and Immune Function. Nutrients, 12(5), 1266.
Haines, S.T., et al. (2021). Bone health and vitamin D: A comprehensive review. Clinical Reviews in Bone and Mineral Metabolism, 19(1), pp.1-15.
Hewison, M. (2017). Vitamin D and the Immune System: New Perspectives on an Old Problem. The Journal of Investigative Dermatology, 137(2), 245-252.
Holick, M.F., & Chen, T.C. (2018). Vitamin D synthesis and metabolism: Skin sources and dietary intake. New England Journal of Medicine, 379(9), pp.303-314.
Kassi, E., et al. (2019). Cardiovascular disease and vitamin D: Current insights. Atherosclerosis, 289, pp.98-107.
Kim, H. J., et al. (2021). Vitamin D and Cancer Prevention: An Overview of the Epidemiological Evidence. Nutrients, 13(1), 180.
Lips, P., et al. (2019). Vitamin D for seniors: Strengthening muscles and bones. Aging Clinical and Experimental Research, 31(6), pp.885-892.
Marrone, M., et al. (2020). Vitamin D and Autoimmunity: A Review. Autoimmunity Reviews, 19(7), 102536.
Massey, K. R., et al. (2020). Vitamin D and Muscle Function: A Review of the Literature. Sports Medicine, 50(5), 889-903.
Nair, R., & Maseeh, A. (2012). Vitamin D: The ‘Sunshine’ Vitamin. Journal of Pharmacology & Pharmacotherapeutics, 3(2), 118-126.
Nesby-O’Dell, S., et al. (2020). Vitamin D and the Immune System. Journal of Bone and Mineral Research, 35(9), 1623-1630.
Pérez-Fernández, E., et al. (2020). Vitamin D and Cardiovascular Disease: What Is the Evidence? Cardiovascular Therapeutics, 2020, 1-11.
Tzeng, C. H., et al. (2019). The Impact of Skin Pigmentation on Vitamin D Synthesis and Its Implications in Dermatology. Clinical Dermatology, 37(4), 453-460.
Wacker, M., & Holick, M. F. (2013). Sunlight and Vitamin D: A Global Perspective for Health. Dermatologic Therapy, 26(2), 103-109.
Wang, H., et al. (2018). Vitamin D and Its Role in Cardiovascular Disease. Clinical Cardiology, 41(4), 504-511.
Zhang, F. F., et al. (2021). Vitamin D and Mental Health: A Review. Journal of Psychiatric Research, 138, 184-191.
Zhao, H., et al. (2020). The Role of Vitamin D in Cancer: Mechanisms and Applications. Frontiers in Oncology, 10, 823.
Disclaimer:
At Medikiaa Health Publishing, we offer our readers access to a collection of archived materials. Please be sure to check the date of the last review or update for each article. Remember, the information on this site, no matter how recent, should never replace the personalized medical advice from your doctor or another qualified healthcare professional.