## 4 Natural Antioxidants and Their Role in Cosmetics
### Vitamins
#### 4.1.3 Vitamin A (Retinol)  
Retinol (Figure 2) is a vital member of the vitamin A family, which includes several forms with distinct biological activities. These forms are categorized into three generations: Generation I: alitretinoin (9-cis-retinoic acid), isotretinoin, retinaldehyde (13-cis-retinoicacid), retinol, and tretinoin (retinoic acid); Generation II: acitretin; and Generation III: adapalene, bexarotene, and tazarotene [36]. Vitamin A occurs naturally in its active form, retinol, as well as in the form of its precursor, \(\beta\)-carotene. Retinol is primarily obtained from animal-based foods, whereas \(\beta\)-carotene, a provitamin A, is converted into its active retinoid form in the liver [37]. Both retinol and \(\beta\)-carotene play crucial roles in mitigating oxidative damage due to their potent antioxidant properties. These compounds contribute to cellular protection, particularly in combating oxidative stress and maintaining skin health. Vitamin A functions as an indirect antioxidant by influencing the transcription of genes involved in the body's standard antioxidant defense mechanisms [38]. Vitamin A was the first vitamin formally approved by the Food and Drug Administration (FDA) as an anti-wrinkle agent, recognized for its ability to improve the surface appearance of the skin and exhibit significant anti-aging properties. Over the years, vitamin A, its derivatives, and \(\beta\)-carotene have been extensively used in cosmetic formulations due to their proven efficacy. \(\beta\)-carotene is a potent lipid-soluble antioxidant capable of neutralizing singlet oxygen. However, its instability in cosmetic formulations has led to the more frequent use of retinol, which is considered one of the most effective agents for reducing signs of aging [39; 40]. Retinol-based products are commonly recommended for preventing skin aging, particularly after the age of 35. Studies have demonstrated that retinol enhances gene expression associated with collagen synthesis, while also inhibiting the activity of enzymes that degrade collagen, a process exacerbated by ultraviolet (UV) radiation. Topical application of retinol has been shown to stimulate the production of glycosaminoglycans (GAGs) in the skin, thereby improving hydration, strengthening the epidermal barrier, and enhancing the skin's resilience against environmental stressors, including oxidative damage induced by free radicals. Furthermore, retinol promotes angiogenesis, contributing to improved skin tone through new blood vessel formation [41]. For visible results, consistent use of retinol-containing formulations is necessary over a period of three to six months. Clinical studies have confirmed that a 0.1% retinol concentration significantly ameliorates signs of photoaging, including increased levels of hyaluron and procollagen type I, which are markers of improved skin hydration and collagen synthesis [42; 43]. Vitamin A, particularly in the form of retinoids, is widely incorporated into topical formulations due to its significant impact on skin physiology. Topical retinoids modulate key processes in keratinocyte differentiation and keratinization, reducing cellular adhesion and promoting corneocyte exfoliation. These actions contribute to the activation of collagen biosynthesis and angiogenesis, which are essential for maintaining skin structure and function. Vitamin A enhances skin smoothness, improves elasticity, and diminishes fine wrinkles, while also reducing hyperpigmentation. It stimulates fibroblast activity, thereby increasing collagen fiber production [41; 44]. By influencing the stratum corneum, vitamin A strengthens the epidermal barrier, reducing transepidermal water loss (TEWL). These properties make retinoids beneficial in treating various dermatological conditions, including atopic dermatitis, psoriasis, acne, and photoaging. In cosmetic formulations, retinoids accelerate collagen and elastin synthesis, activate the production of natural moisturizing factors, and elevate glycosaminoglycan levels.