## 3 Vitamin A, Its Vitaminoids and Carotenoids' General Profile and Health Benefits
### Vitamin A, Vitaminoids, and Carotenoids from Microorganism Sources
Nanoemulsions improve the solubility of lipophilic compounds like retinol, enhance their stability, amplify their bioavailability, and promote rapid penetration, while being created via ultrasonication, high-pressure homogenization, and/or micro-fluidization (Figure 13) [15; 197; 252; 259; 260]. Limitations and Future Perspectives of Vitamin A, Vitaminoids, and Carotenoids in Nutticosmetics, Cosmeceuticals, and Cosmetics Applications  
Despite the recognized potential of vitamin A, its vitaminoids, and carotenoids in several scientific fields, certain limitations hinder their widespread application in nutticosmetic, cosmeceutical, and cosmetic formulations. Vitamin A, vitaminoids, and carotenoids are highly sensitive to light, heat, and oxygen; thus, they are prone to oxidative degradation and loss of potency during formulation, storage, and use (photodegradation) [3; 136; 261]. Due to their chemical instability [262], vitamin A and its derivatives also demonstrate chemical instability that requires the use of advanced encapsulation and stabilizers, which may subsequently increase production costs and manufacturing complexity. Moreover, such compounds are lipophilic, resulting in poor solubility in aqueous systems and limited absorption in the GI, while their low skin penetration is also an important disadvantage [105; 127]. Additionally, several adverse effects follow systemic retinoids and carotenoids excessive usage in some cases, counting skin irritation, oxidative stress, elevated cancer risk, hypervitaminosis, carotenemia [123], build-up of tolerance, dermatological issues (e.g., erythema) [100], and concurrently, inconsistent clinical efficacy, questionable bioavailability, safety aspects, and environmental sustainability play a significant role in boundless utilization [53; 125; 127]. Concerns regarding the lack of robust and recent clinical evidence to support the anti-aging, photo-protective, and antioxidant claims of many retinoids and carotenoids pose another significant limitation. Many strategies have been put out to slow down the aging process and rejuvenate older skin, but as of right now, there are no 100% safe and effective treatment solutions to prevent skin atrophy in older people [242; 263]. More specifically, the cellular and molecular processes ATRA uses to prevent aging are currently unknown exactly, while there is a significant lack of studies examining the molecular basis of topical retinol anti-aging benefits in aged human skin, in contrast to ATRA. As a consequence, it has been proven challenging to comprehend the molecular pathways by which retinoids exacerbate aging in human skin, due to an absence of suitable in vivo and in vitro models [111]. Numerous studies evaluating retinoids and carotenoids suffer from methodological limitations like small sample sizes, inappropriate control groups, little to no rigorous scientific backing, and a lack of standardization, which create uncertainty regarding the true efficacy of retinoids currently used in cosmetics [242; 263]. The future of vitamin A and its derivatives in cosmetics lies in addressing these limitations and exploring new frontiers. With a view to overcoming all limitations and bettering the clinical experience for patients, many advances, primarily in the delivery systems of vitamin A and vitaminoids, are necessary. More specifically, nanotechnology improvements, including nanoemulsions, liposomes, solid lipid nanoparticles, and micelles, offer promising solutions for developing more stable, bioavailable systems capable of controlled release of such compounds [15; 125; 127; 197; 242; 253; 254]. For instance, retinyl retinoate, retinyl N-formyl aspartate, and other chemically stable ester derivatives are being thoroughly investigated to enhance resistance to degradation [242].