## 4 Natural Antioxidants and Their Role in Cosmetics
### Carotenoids
#### 4.2.4 Astaxanthin  
Astaxanthin (ASX, 3,3\({}^{\prime}\)-dihydroxy-\(\beta,\beta\)-carotene-4,4\({}^{\prime}\)-dione) (Figure 3) is a xanthophyll, a class of oxygen-containing carotenoids. This reddish-orange pigment naturally occurs in microalgae, yeast, and bacteria. The most important commercial source is the cryophilic algae _Haematococcus pluvialis_. The structure of ASX contains ketone and hydroxyl groups, which contribute to its greater stability and polarity compared to other carotenoids. Studies have shown that carotenoids with a higher number of oxygen molecules and the same number of double bonds exhibit greater photostability and enhanced antioxidant potential.  
ASX functions as a potent antioxidant without exhibiting pro-oxidative properties. It effectively neutralizes reactive oxygen species (ROS) while remaining gentle on the body's cells. Its unique structure, which includes 13 conjugated double bonds and hydroxyl groups at positions 3 and 3\({}^{\prime}\), allows it to neutralize free radicals from both hydrophilic and hydrophobic regions of the cell membrane, making it a highly effective antioxidant. ASX combats free radicals/ROS through energy quenching, electron transfer, or hydrogen abstraction (scavenging) [67]. Compared to other carotenoids, it is also more resistant to high temperatures and light [68,69].  
ASX is believed to be 14 times stronger than vitamin E, 54 times stronger than beta-carotene, and 65 times stronger than vitamin C as an antioxidant. It is fat-soluble, allowing it to penetrate the stratum corneum efficiently. It normalizes keratinocyte differentiation, influences protein synthesis, cell division, metabolism, and the secretion of transcription and growth factors. ASX accelerates cell renewal, strengthens the epidermal defense function, and reduces transepidermal water loss (TEWL). In the dermis, it increases the production of collagen and elastin. Studies have shown that ASX supplementation helps prevent skin damage and supports healthy skin maintenance [70].  
ASX supplementation has been shown to rejuvenate the skin by reducing lipid oxidation and corneocyte desquamation in individuals over 40. Human studies indicate that ASX reduces wrinkles and age spots, improves skin elasticity, moisture content, and texture and exhibits enhanced effects when combined with topical ASX application [71]. Additionally, ASX protects the skin from solar radiation-induced damage by neutralizing singlet oxygen, increasing optical density, and facilitating the formation of all-trans retinoic acid, which is used to treat UV-induced dermatoses.  
Astaxanthin is widely used in sunscreens due to its high antioxidant potential and its anti-aging, anti-inflammatory, and anti-cancer properties. It provides comprehensive protection against UV radiation-related skin problems, ranging from acute sunburn, discoloration, and redness to chronic inflammation, photoaging, and skin cancer [69]. ASX quenches peroxyl radicals (ROO\(\bullet\)), hydroxyl radicals (OH\(\bullet\)), and singlet oxygen (\({}^{1}\)O\({}_{2}\)). Additionally, it reduces redness and lipid oxidation. ASX is commonly used in the treatment and care of oily and combination skin and is included in mattifying preparations. It is frequently incorporated into moisturizing, nourishing, toning, and firming creams [72].  
Currently, astaxanthin used in cosmetics is primarily derived from microalgae (_Haematococcus pluvialis_) or microbial fermentation (_Phaffia rhodozyma_, _Paracoccus carotifiaciens_). Synthetic astaxanthin is less popular due to its lower bioavailability and weaker biological activity. Biotechnological production methods are gaining increasing popularity as they ensure purity, stability, and sustainability, aligning with trends in anti-aging and protective cosmetics [73].