## 8 Discussion--Challenges and Controversies  
The use of vitamin D in cosmetic products has garnered noteworthy interest owing to its possible advantages for the skin's wellbeing, such as better wound healing, improved barrier function, and anti-inflammatory properties [242; 243; 244]. However, issues and disagreements have emerged, regarding the inclusion of vitamin D in cosmetic products, that must be resolved. Stability, bioavailability, efficacy, regulatory difficulties, and possible interactions with other components are only a few of the challenges, where especially vitamin D's low stability poses a major obstacle for its use in cosmetic products. When exposed to light, heat, and air, vitamin D is prone to deterioration, chiefly in its active forms (D\({}_{2}\) and D\({}_{3}\)), while over time this instability may cause the product's efficacy to decline. Cosmeticians and cosmetic industries, as a result, need to take great caution when creating such formulations, to shield the vitamin D content from any external elements that may degrade it. Stabilizing chemicals and encapsulation techniques are common approaches engaged to improve vitamin D's stability in cosmetic products. It is difficult to guarantee whether vitamin D cosmetic formulations will benefit our skin, while also owning longer shelf life, and this is the main reason why further investigation upon improving both efficacy and longevity issues is highly necessitated [245]. Another point of contention is the bioavailability of vitamin D when applied topically [54; 246; 247]. Although the systemic benefits of vitamin D are well established, its efficacy in topical preparations remains uncertain. The composition of the substance and the skin state might affect how well it absorbs and implements vitamin D. Studies have claimed that topical vitamin D may not have a substantial impact on systemic vitamin D levels but may enhance localized effects like lowering inflammation and enhancing skin hydration [216; 248]. This limitation raises questions about the overall effectiveness of vitamin D in cosmetics and whether it can provide safe, substantial benefits. Regulatory issues also complicate its cosmeceutical use. Different countries have established varying regulations regarding the use of vitamin D in skincare products, as, for example, while some countries have allowed the inclusion of vitamin D at certain concentrations, others have imposed stricter guidelines [249]. Additionally, concerns about its potential for adverse reactions or interactions with other cosmetic ingredients have recently been raised [250]. The safety profile of vitamin D at high concentrations or when used in combination with other active ingredients, needs to be conclusively and thoroughly assessed [249]. Vitamin D incorporation into cosmetic formulas can be rather expensive, as maintaining its stability, efficacy, and proper function is vital. This may result in higher retail prices, potentially limiting the availability of certain products. Furthermore, market acceptance is also crucial, due to the fact that while many customers recognize vitamin D's benefits, others may still be skeptical of its effectiveness when applied topically, given its well-established advantages when ingested or produced directly after sun exposure. An additional point of contention is how vitamin D interacts with other active chemicals utilized in cosmetic formulations, such as retinoids, peptides, or antioxidants. While some combinations might increase the product's overall efficacy, others might cause negative side effects or lessen the final product's benefits. In order to guarantee that vitamin D stays effective, and that the formulation does not cause unforeseen side effects, understanding these interactions takes substantial research and testing. Consumer education is essential in addressing misconceptions about the role of vitamin D in skincare. There is often confusion regarding the differences between vitamin D's systemic effects and its topical benefits.