## 3 Vitamin A, Its Vitaminoids and Carotenoids' General Profile and Health Benefits
### Vitamin A-Related Bioactive Compounds: Provitamins (Carotenoids)  
Carotenoids are vibrant yellowish-orange-red organic pigments mainly isolated from plants, with remarkable antioxidant activity (only 40 are traced in the diet). These metabolites are tetraterreponoids but may also be metabolized to retinol. In carotenoids, two 20-carbon structures with \(\beta\)-ionone rings and a poly-isoprenoid side chain are linked together, tail-to-tail [16]. Carotenoid molecules can be converted into retinol (provitamin A derivatives) due to having at least one unsubstituted \(\beta\)-ionone ring and are \(\beta\)-carotene, \(\alpha\)-carotene, and \(\beta\)-cryptoxanthin, while those unable to be transformed by any metabolic pathway to vitamin A but also owning vital activity are xanthophyls like lutein, lycopene, astaxanthin, zeaxanthin, canhaxanthin, fucoxanthin, phytoene, phytofluene, and capsanthin (Figure 3), with oxygen atoms in their containing hydroxyl groups [3; 20]. Carotenoid content in vegetables, fruits, and plant species is strongly influenced by flesh color, degree of ripeness (fully ripe fruits display higher carotenoid content), method and location of cultivation, post-cultivation modification, storage techniques (sun favors carotenoid loss, while storage in dry places at high temperatures favors their perseverance), intracellular carotenoids' position, purification method, cooking way, and heat treatment [3; 29; 30]. Photosynthetic organisms, including plants, herbs, animals, marine organisms, and microorganisms (e.g., bacteria, fungi), may produce carotenoids in different amounts. Red and orange vegetables on a great scale (tomato, red chili pepper, red pepper, carrots, pumpkins, sweet potatoes, etc. ), green vegetables (like kale, spinach, broccoli, parsley, lettuce, etc. ), mushrooms, fruits such as apricots and mango, herbs like basil, cereals, grains, and spirulina entail high carotenoid levels, with \(\beta\)-carotene being mostly present. \(\alpha\)-carotene is similarly to \(\beta\)-carotene obtained via carrots, sweet potatoes, pumpkin, mangoes, broccoli, oranges, and spinach [69], while \(\beta\)-cryptoxanthin, either as a free compound or esterified with fatty acids (lauric, myristic, palmitic), is administered via the same sources as \(\beta\)-carotene and \(\alpha\)-carotene (Figure 3) [3; 70]. Figure 3: The structure of carotenoids able and unable to be converted into vitamin A.  
Indicative Role and Functions of Vitamin A, Its Vitaminoids, and Carotenoids--Vitamin A Deficiency, Hypervitaminnosis, and Toxicity  
Vitamin A is an essential bioactive metabolite with multiple biological functions in the human body. Initially, it plays a vital role in the cycle of vision primarily in the form of retinal, which partakes in several enzymatic and isomerization reactions. In fact, loss of night vision (night blindness) is associated with vitamin A deficiency [71]. Retinol and ATRA act as signaling molecules for gene expression in vertebrates (i.e., of growth hormones) and are involved in cell growth and differentiation [72]. Retinol is crucial for normal immune system function of both innate and adaptive immunity, while it is an immune booster resulting in antibodies vast response, preserving and reestablishing integration of all mucosal surfaces [73; 74]. Moreover, minimal amounts of vitamin A and \(\beta\)-carotene synergistic effects may prevent cancer and scavenge ROS. \(\beta\)-carotene's anti-cancer and anti-proliferation properties especially delve from its antioxidant profile and conversion into vitamin A [3; 75]. Plus, red blood cells require retinoids and vitamin A for proper differentiation, while retinol contributes to spermatogenesis, glycoprotein synthesis, taste and hearing function, and retinoids partake in maintaining normal epithelial homeostasis by promoting the differentiation of keratinocytes into mature epidermal cells [3; 10]. Carotenoids, vitamin A, and vitaminoids have emerged as pivotal components in synthesizing nutraceuticals, nuticosmetics, cosmeceuticals, and cosmetics.