## 3 Vitamin A, Its Vitaminoids and Carotenoids' General Profile and Health Benefits
### Anti-Aging and Photo-Protective Properties of Vitamin A, Retinoids, and Carotenoids  
#### 5.1.1 Anti-Aging Effect--Skin Regeneration  
Vitamin A and carotenoids have been pointed out to present remarkable anti-aging properties. Skin aging is a result of a plethora of intrinsic and extrinsic factors. Intrinsic reasons are unavoidable (i.e., genetics, the passage of time), but extrinsic factors like UV\({}_{\text{R}}\) exposure, bad diet, stress, or toxins can be avoided. Such factors lead to the breakage of collagen fibers, resulting in early aging [128]. Vitamin A, as an immunomodulatory compound, though, is capable of regulating and proliferating several cells, like skin cells, including keratinocytes and fibroblasts, leading to collagen and elastin synthesis, which is in turn responsible for minimized wrinkles and reversed photo-aging [129]. Retinoids are capable of inhibiting MMPs and modulating gene expression while also regulating the activity of growth factors and cytokines present in complex extracellular matrix protein (ECM) exchange and inflammation [111]. MMPs present in continuous exposure to UV\({}_{\text{R}}\) partake in breaking down the ECM in the dermis, along with collagen and elastin. This situation results in premature photo-aging, with unlikable visual skin spots such as wrinkles and fine lines, dryness, and irregular pigmentation [128]. Retinol and the other retinoids, namely retinal and ATRA, that occur via a sequential conversion (Figure 5), are able to eliminate both intrinsic and extrinsic factors by simultaneously promoting collagen synthesis and preventing collagen breakdown through RARs and RXRs (Figure 8) [111]. Regarding retinoic acid, a study conducted on ATRA confirmed that it can increase types I, II, and III collagen while reorganizing elastic fibers and modulating GAGs of the ECM [117]. Also, ATRA has the capability to enhance epidermis thickness by promoting the growth of epidermal keratinocytes and increasing the formation of endothelial cells and blood vessels in the papillary dermis, all contributing as anti-aging factors for skin protection by visibly decreasing wrinkle formation, skin roughness, and relaxation. As a result, an improvement of the skin's elasticity, hydration, and resilience refinement, as well as a barrier function enhancement, has been observed with long-term use of vitamin A-based cosmetics [111]. Adding to retinoids beneficial impact, the application of antioxidant and anti-aging substances, like carotenoids, may also aid in the protection of the human skin from environmental factors [130]. The carotenoids present in the human skin are mostly \(\alpha/\beta\)-carotene, zeaxanthin, lycopene, and lutein. In some cases, the synergistic effect of such compounds, along with some vitamins, is highly encouraged as the best way of protecting the skin from aging [123]. For example, in a study where the combination of vitamins E and C with \(\beta\)-carotene was examined, this conjunction of active vitamin forms concluded in an advanced scavenging ability against reactive nitrogen species (RNS), rather than this of single antioxidants [131]. Moreover, a study about the xanthophyll fucoxanthin demonstrated that it can inhibit tyrosinase activity, an enzyme whose hyperactivity leads to hyperpigmentation and skin spots, melanogenesis in melanoma, and UV\({}_{\text{B}}\)-induced skin pigmentation, factors notably contributing to aging [132]. Astaxanthin, additionally, is believed to increase the elasticity factor and improve the skin's texture while reducing the size of age spots [117]. Furthermore, non-colored carotenoids, namely phytoene and phytofluene, are capable of absorbing UV\({}_{\text{A}}\) and UV\({}_{\text{B}}\) radiation, acting as a photo-protector for the skin, while at the same time lutein and zeaxanthin, also present in the human epidermis, can protect the skin from the blue light and subsequently from early aging [117].