## 3 Vitamin D's Structural Profile, Skincare Applications, and Functions
### Absorption of Vitamin D from Skincare Products  
Vitamin D is incorporated in skincare products offering a wide range of beneficial effects. However, it is important to note that topical vitamin D application does not replace the need for its systematic consumption, which is essential for overall health. Vitamin D\({}_{3}\) is more frequently incorporated in skincare formulations, improving the appearance of fine lines and wrinkles, decreasing inflammation, and enhancing skin moisture. During the winter, when skin is typically drier and more prone to irritation, skincare products with vitamin D\({}_{3}\) are quite helpful. Different skincare formulas enable personalized routines that target certain requirements. These products focus on key goals such as improving skin hydration, reducing inflammation, and enhancing overall skin health by promoting and providing better texture, tone, and resilience [53]. Calcipotriol, also known as calcipotriene, is commercially available in several products under the brand names Dovonex(r) and Diavonex(r), with comparable efficacy reported to that of calcitriol, due to its identical affinity for _VD_Rs. Calcipotriol has significant impact on the immunosuppression of inflammatory mediators and melanin formation enhancement [54]. Multi-hyaluronic antioxidant hydration serums [55], multivitamin supplies including vitamin D [56], vitamin D\({}_{3}\) orally administered coating tablets [57; 58], oral capsules [59], softgel formulations [60], topical calcineurin [61], cosmetic serums destined for facial skin improvement [62], and vitamin D\({}_{3}\)-phytochemical complex formulations [63] have been clinically approved and utilized in several acne, vitiligo, atopic dermatitis, pos  
Figure 4: The two common forms of vitamin D (D\({}_{2}\), D\({}_{3}\)), their origin, and their differences. (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)). riasis, rosacea, and anti-aging treatment strategies globally, and have offered multiple beneficial outcomes. Vitamin D as a natural photoproduct that is mainly administrated either as an oral supplement (e.g., tablet, syrup, soft gel capsules, and other supplements) or in an injectable dosage form is connected with many beneficial outcomes for the body [25; 64]. Nanotechnology has revolutionized medicine by encouraging targeted cellular drug delivery, enhancing the bioavailability of encapsulated vitamin D and its derivatives, and promoting several health-associated benefits. Organic, and especially lipid-based, nanocarriers for the delivery of vitamin D and its analogs have been extensively utilized over the past few years, stabilizing this hormone, improving its solubility, enhancing its absorption, and enabling and bettering targeted cellular release in specific tissues. Such innovative systems have overcome several adverse effects of oral administration including poor permeability in epithelial barriers and solubility [65]. Furthermore, photodynamic therapy supplemented with vitamin D\({}_{3}\) has increased 6-30% of the response of this treatment against squamous cell carcinomas (SCCs), actinic keratosis, and psoriasis, in a plethora of breast cancer cell lines and animal models [66]. For instance, topical vitamin D formulations enhanced with cholesterol, calcipotriol, or tacalcitol have been confirmed to stimulate melanin and melanogenesis during narrowband UV phototherapy (NB-UV\({}_{\text{B}}\)), in therapeutic treatments against vitiligo [54; 67]. However, in conjunction with psoralen long-wave UV radiation (PUVA) and monochromatic excimer light (MEL) vitiligo treatments, less beneficial effects have been recorded, in contrast to vitamin D combined with corticosteroid formulations, where vitamin D minimizes the adverse impact of corticosteroids and offers superior efficacy and vitamin D microneedle provision [54; 68; 69].