![]() In Europe, carrots, spinach, and tomato products are the main contributors to β-carotene intake, while β-cryptoxanthin is most commonly taken from various citruses and citrus juices. ![]() It is mostly ingested through red and orange vegetables and partially through the same colored fruits and green vegetables ( Table 1). β-Carotene is the most abundant in the diet. These carotenoids have been identified in all groups of photosynthetic organisms, bacteria, fungi, and many animals. There are more than 50 provitamin A carotenoids, but only β-carotene, α-carotene, and β-cryptoxanthin are present in significant amounts in the human diet. The human body is not able to produce vitamin A, and therefore, it is necessary to obtain it from the diet either as preformed vitamin A or in the form of provitamin A carotenoids. Many of the compounds of all three classes are used clinically to some extent, with multiple indications, which are discussed in the corresponding sections of this review. Trifarotene is the only member of the fourth retinoid generation so far and has been approved only in the US. The third-generation of retinoids include adapalene, tazarotene and bexarotene. The second-generation retinoids were developed from the first generation, and the members of this group are etretinate and acitretin. First-generation retinoids are forms found in nature: retinol, retinal, ATRA (tretinoin), 9- cis-retinoic acid (alitretinoin) and 13- cis-retinoic acid (isotretinoin). Given the pleiotropic functions of retinoids, synthetic derivates have been developed, and therefore, retinoids can be classified into four generations. Only those molecules, which contain at least one unsubstituted β-ionone ring, have the nature of provitamin A. However, not all carotenoids can be converted into vitamin A in the human body. Carotenoids are tetraterpenoids in contrast to animal-origin diterpenoid retinoids but can be eventually metabolized to retinol. Some of the most well-known carotenoids are β-carotene, α-carotene, lutein, lycopene and cryptoxanthin. In addition to their relationship to vitamin A, they are known for their antioxidant activities. Ĭarotenoids are yellow- to orange-colored organic pigments found in several fruit and vegetables. ![]() Although the term vitamin A is mostly associated with retinol, and retinol is, in fact, the predominant form of retinoids in the human body, the main biologically active molecules are the oxidized derivates 11- cis-retinal and all- trans-retinoid acid (ATRA). The vitamin can be provided in the diet either via products of an animal origin in the form of vitamin A (retinol and its close derivatives) or as provitamin A (carotenoids) from vegetables. All retinoids possess a common structure and similar physicochemical properties, although their effects on the human body can vary greatly. Members of the 2nd, 3rd and 4th generations are synthetic derivates based on the original retinoic structure and are used in treating different diseases. First-generation compounds are found in the diet, except for some natural metabolites formed in the body. The retinoids represented belong to the four described generations. The structure of vitamin A and retinoids. All these compounds are liposoluble and, unlike water-soluble vitamins, are easily accumulated in the body, especially in the liver and adipose tissue.This represents, on one hand, an advantage since temporal deprivation of vitamin A intake is not associated with clinical symptoms, but on the other hand, accumulation with subsequent toxicity can appear. However, the basic vitamin A string is hidden in their structures, and they are similar to other retinoids in their interaction with retinoid receptors. Some synthetic derivatives do not resemble the natural isoprenoids from vitamin A class at first sight. These compounds can also be classified as retinoids, including compounds with a common structure of four isoprenoid units being of either a natural or synthetic origin ( Figure 1). All vitamin A forms share a similar structure and the same physiological functions in an organism. Vitamin A is a fat-soluble life-essential group of compounds both of animal and vegetal origin characterized by an unsaturated isoprenoid chain structure.
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