4. Causes of vitamin D deficiency

Vitamin D is also known as the sunshine vitamin. The importance of sunlight for human health came into light with the industrial revolution in Northern Europe [7]. Sniadecki first published an article in 1822 about high prevalence of rickets in children who lived in the inner city in comparison to those who lived in the rural areas [8]. Many observations regarding the sun exposure and rickets have been published in the course of time. Studies have also revealed the high prevalence of vitamin D deficiency in general population, mostly owing to lack of sun exposure.

Vitamin D is a fat-soluble seco-steroid made from four cholesterol rings IOM (Institute of Medicine) [9]. It has two bioequivalent forms, D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D2 is derived from the plant sterol ergosterol [1, 9]. Vitamin D from the diet or dermal synthesis is biologically inactive [10]. Vitamin D3 is synthesized mainly from 7-dehydrocholesterol in keratinocytes of the skin stimulated by UVB of sunlight [3]. Under the influence of sunlight (ultraviolet radiation, action spectrum 280–320 nM, or UVB), 7-dehydrocholesterol in the epidermis is converted to vitamin D (Figure 1). Keratinocytes express the vitamin D receptor (VDR) due to which they are capable of responding to the 1,25(OH)2D produced [11]. Both UVB intensity and skin pigmentation level contribute to the rate of D3 formation [12]. D3 is converted to 25(OH)D (calcidiol) in the liver by a number of enzymes. 25-Hydroxyvitamins D2 and D3 produced by the liver enter the circulation and the kidney bound to vitamin D-binding protein. The kidney

More than 90% of vitamin D requirement comes from sunlight. According to Holick, exposure of ≥20% of the body's surface to either direct sunlight or tanning

2. Vitamin D structure, synthesis, and metabolism

Fads and Facts about Vitamin D

metabolizes 25(OH)D to the active metabolite 1,25(OH)2D3.

3. Sources of vitamin D

Overview of vitamin D, UpToDate 2019.

Figure 1.

84

There are numerous causes of vitamin D deficiency. Decreased synthesis from the skin is one of the main causes. Less exposure to sunlight, lifestyle, skin pigmentation, abundant use of sunscreen lotion, and geographical variation owe to less dermal synthesis. Decreased bioavailability due to malabsorption, decreased synthesis of active form of vitamin D due to liver or renal failure, or increased catabolism with the use of various medications like glucocorticoids and anticonvulsants also cause deficiency. VDD is also seen in diseases like rickets, osteomalacia, hyperparathyroidism, and granulomatous disorders [14].
