## 3 Vitamin A, Its Vitaminoids and Carotenoids' General Profile and Health Benefits
### Chemical Structures, Origin, and Isolation of Vitamin A and Its Vitaminids  
The vitamin A group is comprised of compounds of several origins and a basic structure of an unsaturated isoprenoid chain that provides them with the same functional abilities. It is a 20-carbon (C) molecule comprised of a cyclohexenyl ring with methyl substitution (\(\beta\)-ionone ring) and a conjugated polyene (isoprenoid side chain) chain structure (-C=C-). Vitamin A compounds are, in fact, retinoids (vitaminoids) that own four isoprenoid units of natural or organic origin and share a mutual interaction with retinoid receptors [3; 20]. First-generation retinoids are dietary-derived metabolites, including retinol (vitamin A), retinal (all-_trans_-retinaldehyde), all-trans-retinoid acid (ATRA, tretinoin, or all-_trans_-RA), all-_trans_-retinyl ester, 9-_cis_-retinoic acid (alitretinoin), and 13-_cis_-retinoic acid (isotretinoin), while 2 nd (etretinate and acitretin), 3 rd (adapalene, tazarote, and bexarotene), and 4 th (trifarotene) generation ones are used in many diseases' treatments. Vitamin A is either obtained from animals (retinol and retinoids) or as provitamin A, otherwise carotenoids, from consumable plant sources [3; 20]. Moreover, it is mostly associated with retinol, and its main oxidized derivatives are 11-_cis_-retinal and ATRA. Other well-known retinoids with clinical significance in skin-protection cosmetics are 3,4-didehydroretinol, 11-_cis_-retinal, 5,6-epoxyretinoic acid, retro anhydrotineol, 4-oxoretinol, and retinosyl-\(\beta\)-glucuronide (retinylglucoronide) (Figure 2) [3; 20; 21]. A common vitamin A source derives from its preformed form and is obtained via animal-based food. Major retinol contributors include milk [22], dairy products [23],  
Figure 2: The structure of vitamin A and retinoids of the 1 st, 2 nd, 3 rd, and 4 th generations. meat [24], chicken [25], eggs (mainly egg yolks) [3; 26], and fish [27], while its general concentration ranges from 20 to 80 \(\upmu\)g/100 mL of whole milk (the higher the fat content in the dairy product, the higher the retinol content) [3; 20]. Retinol and ATRA are primarily obtained during breastfeeding, since vitamin A is responsible for mammary gland development and lactation, not only for the offspring but also for maternal health [22; 28]. Considering vitamin A content in meat and poultry, tens of mg of retinol and its esters have been reported (per 100 g), and the highest content was recorded in pigs' liver [3; 20; 29; 30]. It must be noted that the amount of vitamin A traced in such foods, is dependent on the amount of \(\upbeta\)-carotene included in the feed or food supplements the animal is provided with [3]. Vitamin A is also obtained by consuming fish, with predator species such as carnivorous fish (e.g., sharks) and liver oil from various marine species, owning the highest levels of retinol and retinoids [27; 31; 32]. Egg yolks, especially chicken egg yolks, are stated to contain almost up to 1 mg of vitamin A per 100 mg, in contrast to duck and quail eggs, which comprise it in lower and elevated amounts, respectively [25; 33]. Furthermore, retinoids also derive from several dietary groups like animal (e.g., chicken, egg yolks), marine (like salmon), and plant sources (e.g., broccoli, spinach, avocado, sweet potatoes), as well as whole grains, cereal, dry nuts, and other similar sources [34; 35]. In the early 20 th century (1932), after the discovery of its all-_trans_-retinol structure, scientists had focused on isolating vitamin A from several natural sources [36; 37]. However, specialized analytical methods had to overcome various obstacles, such as the multiple isomeric retinol forms. Traditional chromatographic and spectroscopic methods concerning vitamin A, vitaminoids, and carotenoids were improved over the years.