## 9 Future Directions and Research  
The application of vitamin D in cosmetics is an area of growing interest due to its potential benefits for skin health. However, to fully realize and harness these benefits, future research and development must focus on several key areas, including optimizing formulation techniques, enhancing bioavailability, comprehending the mechanisms of action, evaluating long-term safety, and exploring innovative delivery systems. Future research should focus on advancing formulation techniques to improve the stability and efficacy of vitamin D in cosmetics. Vitamin D, particularly in its active forms, is vulnerable to degradation from UV, heat, and air exposure. Innovative encapsulation methods, such as nanoencapsulation or liposomal delivery systems, could provide enhanced protection against environmental factors, thereby extending the shelf life and stability of vitamin D in cosmeceuticals [251]. Moreover, developing new stabilizing agents and excipients that preserve vitamin D's potency, without also compromising the product's texture and feel, is crucial, as this will not only improve the quality of vitamin D-based cosmetics, but also ensure their maximum benefits [251]. Additionally, vitamin D could be a key ingredient in orally administered nutricosmetics, designed to improve skin health and appearance. Future research may focus and explore optimal formulations, combining vitamin D with other skin-beneficial nutrients, including collagen, hyaluronic acid, omega-3 fatty acids, polyphenols, biotin, and antioxidants, able to enhance skin hydration, elasticity, and overall vitality [252; 253; 254; 255]. Moreover, the synergistic activity of antioxidants like vitamin C and E with vitamin D [254; 255; 259], or niacinamide-vitamin D [260] combinations, could offer several cosmeceutical and nutricosmetic benefits against dermatological and skin health conditions. Sunscreens, a type of cosmetic product, can provide protection to the skin against UV and dermatologists recommend their use [261]. It is unfortune to mention that some of these cosmetic preparations contain chemicals, something that is not safe and can even lead to systemic absorption [262]. To add, not all sunscreens offer protection against UVA, pollution, infrared light (IR), or blue light and so as to produce vitamin D, UVB is necessary. More specifically, the role of vitamin D in mitigating oxidative stress could be harnessed in anti-pollution skincare formulations. Plus, as blue light exposure from screens becomes a growing concern, combining vitamin D with other protective agents may aid in counteracting its effects, promoting healthier skin [263]. There are currently no recommended sunscreen usage guidelines or ideal sun protection agents [264]. New active compounds, stable and safe to add to cosmetic compositions [265] that may also function as antioxidant agents preventing photoaging like vitamin D, are therefore needed [266]. One major challenge in the topical application of vitamin D remains as its bioavailability [267]. Enhancing the skin's capacity to absorb and utilize topical vitamin D should be a primary research goal. By comparing the effectiveness of various vitamin D forms, such as vitamin D\({}_{3}\) and its metabolites, in topical applications, researchers may be able to identify the most suitable variants for topical use. Moreover, its delivery and absorption could also be improved by researching synergistic effects with other compounds, such as penetration enhancers or advanced vehicle systems that enhance skin permeability. Future research on the optimal vitamin D content for various skin types and conditions will help create products tailored to specific dermatological needs [267].