## 3 Vitamin A, Its Vitaminoids and Carotenoids' General Profile and Health Benefits
### Vitamin A-Related Bioactive Compounds: Provitamins (Carotenoids)
#### 4.1.3 Other Absorption Pathways of Vitamin A, Its Vitaminoids, and Carotenoids
The cycle involves the conversion of 11-_cis_-retinal to all-_trans_-retinal upon light activation and recycling via the retinoid cycle in the cytoplasm of retinal cells, by the aid of several enzymatic reactions, such as retinol dehydrogenase and isomerase enzymes [92]. All-_trans_-retinal is then reduced by nicotinamide adenine dinucleotide (NADH) to NAD\({}^{+}\) and, via ADHs in the liver, is converted to all-_trans_-retinol. All-_trans_-retinol is subsequently transformed through isomerases (liver) to 11-_cis_-retinol, which is then again converted by NAD\({}^{+}\) to NADH transformation and ADHs to 11-_cis_-retinal during the Wald's visual cycle [112]. As for the antioxidant activity, retinoids and carotenoids like \(\beta\)-carotene scavenge ROS, protecting cells from oxidative stress and damage, supporting cardiovascular (CV) health, skin protection, and immune defense, while also being able to combat oxidative modification of LDLs (oxidized LDLs), thereby mitigating thrombo-inflammatory stimuli and atherogenesis, as well as supporting CV health, through inhibiting the inhibitor of \(\kappa\)B\(\alpha\) (lxB\(\alpha\)) pathway [19; 113]. Moreover, platelet activating factor (PAF) inhibition is induced by both retinoids and carotenoids, able to counteract the pro-inflammatory effects of PAF, which is implicated in skin inflammation, allergic responses, and vascular changes. PAF via binding to the G-protein-coupled membrane receptors (GPCMRs) and PAF-receptor (PAF-R/PAR signaling), via pro-inflammatory genes and proteins like phospholipase C\(\beta\) (PLC\(\beta\)), phosphatidylinositol 4,5-biphosphate (PIP2), and Gq protein i, \(\gamma\) and \(\beta\) subunits(Gq,i/\(\gamma\)/\(\beta\)) and via partaking in mitogen-activated protein kinase (MAPK), inhibits PAF-cholinetransferase (PAF-CPT) and lysophosphatidylcholine (LPC) acyltransferase (LPCAT) thrombo-inflammation [114; 115; 116]. Thrombin, eicosanoids (cyclooxygenenses (COX)), collagen (type I and II), and adenosine diphosphate (ADP), implicated in several inflammatory cascades, are restrained by carotenoids action against the nuclear factor \(\kappa\) light-chain enhancer of activated B cells (NF-\(\kappa\)B) and gene expression [105; 111; 117; 118]. Another biochemical pathway retinoids influence is the phosphatidylinositol 3-kinase (PI3K)/protein kinase (Akt) pathway, implicated in cell survival, metabolism, and proliferation. By regulating this cascade, vitamin A, retinoids, and carotenoids contribute to tissue repair (wound healing) and protection against ROS formation and oxidative stress. More specifically, via the kelch-like ECH-associated protein 1 (keap1), an adaptor subunit of cullin 3-based E3 ubiquitin ligase, the activity of the nuclear factor erythroid 2-related factor 2 (Nrf2) that acts as a sensor for oxidative/electrophilic stress via PI3K/Akt is regulated [110; 119; 120]. All the above biochemical mechanisms of the beneficial action of vitamin A, retinoids, and carotenoids are thoroughly depicted in Figure 9.  
The metabolism of carotenoids, retinoids, and vitamin A prepares and regulates the availability of vitamin A by converting it into active or stored forms, while biochemical  
Figure 9: Biochemical mechanisms of action of vitamin A, its vitaminoids, and carotenoids. cascades describe how these active forms exert their physiological effects. Both processes are interdependent but have distinct purposes in preserving health and cellular function. Vitamin A, Vitaminoids, and Carotenoids' General Role in Nutricosmetic, Cosmeceutical, and Cosmetic Applications  
As aforementioned, both retinoids and carotenoids have a lot of beneficial impact on human health. Regarding carotenoids, more specifically, both their bioactive groups, carotenes and xanthophylls, depending on the presence of oxygen in their structure, own an antioxidant and anti-aging effect, sometimes working synergistically with other antioxidants too.