## 3 Vitamin A, Its Vitaminoids and Carotenoids' General Profile and Health Benefits
### Vitamin A-Related Bioactive Compounds: Provitamins (Carotenoids)
\(\beta\)-carotene, lutein, and astaxanthin act as potent antioxidants by combating oxidative stress, reducing photoaging, and promoting a healthy complexion. On the other hand, retinoids such as retinol and retinoic acid accelerate skin renewal, reduce hyperpigmentation, and mitigate aging impact. Such vitaminoids extend the scope of cosmetic applications by supporting hydration, skin elasticity, and barrier repair [76]. Their role in the cosmetic field and importance in cosmetic formulations will be further discussed in Section 4. Intake recommendations on vitamin A differ by age and are typically measured in retinol activity equivalents (RAE). Reportedly, one RAE corresponds to 1 \(\mu\)g of retinol, 12 \(\mu\)g of \(\beta\)-carotene, or 24 \(\mu\)g of \(\alpha\)-carotene/\(\beta\)-cryptoxanthin [3; 77]. For infants and children, the suggested intake ranges from 400 to 500 RAE; adult males are advised to consume 900 RAE daily, and females, pregnant, and lactating women should consume 700 to 1300 RAE, with lactating women requiring the highest doses. Sufficient vitamin A levels are crucial for proper fetal development and ensuring adequate nutrition for the newborn [3; 77]. Vitamin A deficiency impairs vision, and in severe cases, prolonged deficiency may lead to xerophthalmia and even irreversible blindness [71; 78]. Such deficiency affects various body systems (inadequate plasma retinol (<0.52 \(\mu\)m) or liver levels (<5-20 \(\mu\)g/g)). The respiratory system displays increased susceptibility to infections due to epithelial changes, skin keratinization, and dryness [3; 77], while impaired sperm production and testicular degeneration are observed in the reproductive system [79]. Reduced intestinal goblet cells and alterations in the epithelium lead to poor digestion (gastrointestinal system) [80], while nerve lesions and diminished taste/smell (nervous system) [81], weakened immunity and risk of infection due to malnutrition (immune system) [73], iron metabolism disruption [82], and anemia, particularly in children and pregnant women [83], are only a few indications implying vitamin A deficiency. Either because of a poor diet (primary), malabsorption, or chronic diseases (secondary), hypovitaminosis affects around one-third of children worldwide [3; 77]. Vitamin A supplementation [84], biofortification (i.e., transgenic golden rice and fortified samples (e.g., sweet potatoes)) [85], and a balanced diet are effective solutions against hypovitaminosis [3; 77]. Hypervitaminnosis, otherwise known as vitamin A toxicity, on the contrary, is rare but can occur due to excessive vitamin A or retinoid intake (plasma retinol levels > 2.09 \(\mu\)m). Toxicity plus arises from supplement overuse or vitamin A-rich food overconsumption and may be either acute or chronic. Retinoid acid syndrome, hypertriglyceridemia, teratogenicity, impaired bone health, skin redness, peeling, irritation, and allergic reactions [3; 77; 86] may follow vitamin A's toxicity. Plus, excess carotenoid intake can subsequently cause a range of comorbidities, from harmless, easily dissolved carotenoderma (yellow/orange skin) to complications like nephrotic syndrome, cancer, or liver issues [3; 25; 71; 77]. Vitamin A, Vitaminoids, and Carotenoids as Nuticosmetic and/or Cosmeceutical and Cosmetic Factors: Absorption, Conversion, Storage, Distribution, Metabolism, and Biochemical Mechanisms of Action  
As the human body is unable to produce vitamin A, it is of great importance to obtain it from the diet either in its preformed version (70-90% absorption) or as provitamin A in the form of carotenoids (\(\leq\)3% absorption), and mainly \(\beta\)-carotene, \(\alpha\)-carotene, and \(\beta\)-cryptoxanthin [3; 16]. Vitamin A is implicated in several mechanisms of absorption and mainly partakes in oral, skin, parenteral, sublingual, and rectal absorption, which means that additional administration routes, like intramuscular and topical ways besides one's diet, are also possible.