## 3 Vitamin D's Structural Profile, Skincare Applications, and Functions  
### Chemical Structure and Types of Vitamin D  
Vitamin D refers to a group of fat-soluble secosteroids essential for various bodily functions. Particularly, vitamin D plays an important role in calcium and phosphorus metabolism, which is crucial for maintaining bone health. The two primary forms of vitamin D are vitamin D\({}_{2}\) (C\({}_{28}\)H\({}_{44}\)O) and vitamin D\({}_{3}\) (C\({}_{27}\)H\({}_{44}\)O) (Figure 3). Both forms derive from different sources, and their own individual chemical structures, and exhibit distinct biological activities, as vitamin D\({}_{2}\) is present in plants and yeast while vitamin D\({}_{3}\) derives from animal sources. Vitamin D's chemical structure is mainly depicted as a steroid molecule. Sex hormones, adrenal hormones, and cholesterol are placed amongst the many chemical molecules that make up the huge family known as steroids. Interestingly, a unique feature of vitamin D's structure is its secosteroid nucleus, which resembles a steroid nucleus but lacks one of its rings. Four rings designated as A, B, C, and D make up the secosteroid nucleus as important components of the cyclopentanoperhydrophenanthrene skeleton. The B ring especially is broken at the 9, 10 carbon-carbon bond with a view to form pre-vitamin D. Small differences comprise the molecular structures of vitamin D\({}_{2}\) and D\({}_{3}\). Specifically, the methyl group at carbon 24 and a double bond located between carbons 22 and 23 are present in vitamin D\({}_{2}\). In comparison to vitamin D\({}_{3}\), the methyl group at  
Figure 2: Article selection, inclusion, and exclusion methodology.  
carbon 24 is absent and only one bond is found between carbon 22 and 23. These structural variations are primarily responsible for their stability and metabolism within the human body and impact the sequence of many biochemical processes [48,49]. The two primary forms of vitamin D are depicted in Figures 3 and 4.  
Sunlight is the most significant and natural source of vitamin D. Sunlight's UV\({}_{\text{B}}\) rays cause the skin's 7-dehydrocholesterol to automatically transform into provitamin D\({}_{3}\), which subsequently changes into vitamin D\({}_{3}\). Latitude, season, time of day, skin pigmentation, age, and usage of sunscreen are only a few of the variables that might affect the skin's ability to produce vitamin D\({}_{3}\)[50]. In the case of vitamin D\({}_{2}\), the main sources of vitamin D\({}_{2}\) are fungus and plants formed by subjecting ergosterol--a sterol present in plant and fungal cell membranes--to ultraviolet radiation [51]. As vitamin D\({}_{3}\) reaches the liver, it is converted by the aid of enzymes to 25(OH)D\({}_{3}\). 25(OH)D\({}_{3}\) is then transformed to 1\(\alpha\),25(OH)\({}_{2}\)D\({}_{3}\) in the kidneys, and this process will be explained in more detail in the following section (Figure 2) [30]. Vitamin D\({}_{3}\) is also present in animal-based foods like fatty fish (salmon, mackerel, and sardines), cod liver oil, egg yolks, and fortified dairy products (20%). For those at risk of deficiency, and especially people dealing with limited sun exposure, older adults, and those with specific medical problems that influence their metabolism of vitamin D, vitamin D supplements are vital (Figure 3) [52].  
Figure 3: Metabolism pathway of vitamin D\({}_{3}\). (Parts of this figure were obtained by [[https://smart.servier.com/](https://smart.servier.com/)]([https://smart.servier.com/](https://smart.servier.com/)) (accessed on 29 December 2024) and structures from PubChem: [[https://pubchem.ncbi.nlm.nih.gov/](https://pubchem.ncbi.nlm.nih.gov/)]([https://pubchem.ncbi.nlm.nih.gov/](https://pubchem.ncbi.nlm.nih.gov/)) (accessed on 29 December 2024)).