## 7 Vitamin D in Cosmetics and Cosmetics
By slowing down the skin cells' fast proliferation, calcipotriol, namely a synthetic derivative of vitamin D\({}_{3}\), is topically applied to treat the symptoms of psoriasis. Plus, the active 1\(\alpha\),25(OH)\({}_{2}\)D\({}_{3}\) is used as well in a plethora of medical applications, due to its strong potency against skin diseases [121]. Vitamin D is categorized as an active ingredient in medicine. The European Union prohibits the use of vitamin D\({}_{3}\) and D\({}_{2}\) in cosmetic formulations (No. 335 in Annex II of Regulation (EC) No. 1223/2009) [213]. Nevertheless, in the CosIng database, a vitamin D precursor, 7-dehydrocholesterol, is characterized as an emulsion-stabilizing, skin-conditioning, and viscosity-controlling ingredient [214]. Vitamin D provitamins are allowed to be used in cosmetic and cosmeceutical formulations. Notably, the European Cosmetics Directive explicitly bans the use of both ergocalciferol (vitamin D\({}_{2}\)) and cholecalciferol (vitamin D\({}_{3}\)). However, this does not apply to provitamins that are licensed in the US, for their utilization in skincare products [25; 215]. Several examples of products utilizing vitamin D in the form of provitamins exist, including anti-aging day and night creams, serums, and many other formulations deriving from numerous global companies. Moisturizers enriched with vitamin D improve hydration and reduce transepidermal water loss, benefiting dry and sensitive skin [216]; vitamin D-based anti-aging serums regulate collagen synthesis and reduce oxidative stress [34]; and topical vitamin D formulations may regulate melanogenesis, addressing melanoma and uneven skin tone [217; 218]. As mentioned before, a whole series of interesting effects in the skin are described in the context of vitamin D. To begin with, calcitriol has an impact on keratinocyte development and differentiation, playing a crucial function in the therapy of psoriasis [219]. In parallel, the skin's calcium gradients are properly maintained by calcitriol [25]. Stimulation for the formation of anti-microbially effective peptides, like defensins and cathelicidins, is another function activated by 1\(\alpha\),25(OH)\({}_{2}\)D\({}_{3}\). The impact of such peptides is interesting in the context of inflammatory processes in neurodermatitis cases [220]. Furthermore, calcitriol prolongs the self-protection of the skin during the UV\({}_{\text{B}}\) radiation exposure, by stimulating heat shock proteins. Lastly, several studies [221; 222; 223] describe a repignmentation in vitiligo cases, after the application of calcitriol or vitamin D analogous products. In Table 4, various cosmetic and cosmeceutical formulations are described. \begin{table}
\begin{tabular}{p{85.4 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Cosmetic Type** & **Purpose of Vitamin D** & **Effects** & **Examples** & **References** \\ \hline Moisturizers & Enhances hydration and skin barrier function & Improve moisture retention and south dry skin & Vitamin D\({}_{3}\) creams, hydrating lotions & [216] \\ \hline \multirow{3}{*}{Anti-aging products} & Supports skin homeostasis, redox oxidative stress, improves elasticity & Stimulate DNA repair, enhance collagen production, antioxidant and anti-broading property, reduce wrinkles & \multirow{3}{*}{D\({}_{3}\) anti-aging serums} & \multirow{3}{*}{D\({}_{3}\) anti-aging serums} & \\  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Possible cosmetic and cosmeceutical formulations containing vitamin D.