Vitamin D is a fat-soluble nutrient belonging to the group of secosteroids, known to have a “broken” ring in their chemical chain, as shown in Figure 1 (Bickle, 2011). The two types of vitamin D present in humans are vitamin D2, known as ergocalciferol, and vitamin D3, also named cholecalciferol. Vitamin D2 is present in food (mainly in fish, egg yolk and fish oils) and used as a dietary supplement as well, while vitamin D3 is synthesized by the skin after exposure to UVB radiations (Calvo, Whiting & Barton, 2005).
As shown in Figure 2, vitamin D3 synthesis starts by converting cholesterol to 7-dehydrocholesterol by DHCR7 enzyme. 7-dehydrocholesterol is then affected by UVB radiations which convert it into pre-vitamin D3. Then, pre-vitamin D3 is automatically converted into vitamin D3, at room temperature in the body. Subsequently it is bound to vitamin D binding protein (DBP) in the bloodstream to be transported to the liver. Here, it is hydrolyzed by liver 25 hydroxylases and transformed in 25-hydroxycholecalciferol (25(OH)D3) which requires another hydroxylation step, which occurs in the kidneys by 25 hydroxyvitamin D3-1-α hydroxylase (1α OHase), yielding the final active form of vitamin D (1α,25(OH)2D3) named calcitriol. Vitamin D production in the kidney is also regulated by parathyroid hormone and calcium/phosphate blood concentration. Calcitriol is present in the bloodstream and functions as a hormone, it is subsequentially bound to its receptor (VDR) in the intestine and in bones to regulate calcium homeostasis (Holick, 2003).
The main functions of vitamin D are to control the reabsorption of calcium to give structure and to maintain the skeletal structure in good health. Therefore, vitamin D deficiency may lead to several conditions such as osteoporosis, osteopenia, osteomalacia, rickets and muscle weakness. Usually, vitamin D deficiency is common in both adults and children and in many cases linked to limited sun exposure. According to the national institute of heath (NIH) the recommended daily intake of vitamin D for an adult is 600 international units (IU), which decreases to 400 IU for children until 12 and increases to 800 IU for people older than 75. In case of deficiency, patients should immediately be treated, especially those suffering from kidney or liver pathologies which have reduced absorption capability and therefore need higher dosages (Holick, 2003).
Another problem might arise in case of excessive vitamin D production and accumulation. Indeed, unlike vitamin C excess which can be excreted easily via urine, vitamin D is not excreted and can lead to hypervitaminosis. This is usually caused by a misuse of food supplements: vitamin D excess itself does not cause any harm when it is absorbed or metabolised by the skin as the body is able to detect any excess in advance and therefore it is only transformed the needed dose of inactive form into active. The main issue could arise when an elevated consumption is present. In this case the over calcification of soft tissue could lead to skeletal malformation if it is not promptly cured (Cline, 2012).
To conclude, vitamin D is very important for human life and sometimes underestimated, it can boost the immune system, prevent osteoporosis and improve the general wellness and mood of human beings, although it needs to be monitored due to the potential link between sun exposure and dermatological and neurological disorders (Kechichian & Ezzedine, 2018).
Figure 1: Vitamin D structures (Bikle, 2011).
Figure 2: Vitamin D metabolism
References
Cline, J. (2012). Calcium and Vitamin D Metabolism, Deficiency, and Excess. Topics in Companion Animal Medicine, 27(4), 159-164. doi:https://doi.org/10.1053/j.tcam.2012.09.004
Holick, M. F. (2003). Evolution and Function of Vitamin D, Berlin, Heidelberg.
Bikle, D. D. (2011). Vitamin D metabolism and function in the skin. Molecular and Cellular Endocrinology, 347(1), 80-89. doi:https://doi.org/10.1016/j.mce.2011.05.017
Office of Dietary Supplements - Vitamin D. (2021). Retrieved 28 March 2021, from https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/#en1
Calvo, M. S., Whiting, S. J., & Barton, C. N. (2005). Vitamin D Intake: A Global Perspective of Current Status. The Journal of Nutrition, 135(2), 310-316. doi:10.1093/jn/135.2.310
Kechichian, E., & Ezzedine, K. (2018). Vitamin D and the skin: an update for dermatologists. American journal of clinical dermatology, 19(2), 223-235.