## 5 Vitamin D Health-Promoting Activities
### Antioxidant Activity  
Vitamin D was first recognized as an antioxidant in 1993, and since then, research has clarified its antioxidant function mechanisms [35]. Interestingly, vitamin D enhances the expression of antioxidant genes and inhibits oxidative stress pathways, while reducing the accumulation of harmful molecules like advanced glycation end-products. Vitamin D, like other antioxidants, plays a crucial role in neutralizing ROS and RNS, which are major mediators of oxidative stress [168]. This oxidative damage contributes to skin aging, inflammation, and various skin diseases' onset. Antioxidants aid in maintaining skin integrity by reducing ROS and RNS levels, thus protecting cellular structures like lipids, proteins, and DNA from oxidative damage [35].  
The vitamin D hormone contributes to neutralizing oxidative stress by activating the Nrf pathway, a critical/master regulator of the antioxidant response [168]. This activation subsequently enhances the expression of antioxidant enzymes, which protect against oxidative damage in skin cells [34]. Nrf also induces the action of cytoprotective genes essential for redox balance and detoxification, which partake in protecting against various diseases and aging. Moreover, vitamin D increases total antioxidant capacity by raising superoxide levels [169]. Additionally, vitamin D plays a protective role in mitochondria by regulating ROS production and preventing lipid peroxidation, a crucial activity for the insurance of cellular integrity. A deficiency in vitamin D can lead to increased ROS and oxidative damage, especially in mitochondria. By activating the Nrf pathway, vitamin D helps mitigate mitochondrial damage, reduces oxidative stress, and favorably supports the regulation of several inflammatory responses [169].  
Human skin naturally produces vitamin D upon exposure to UV rays. This hormone serves as a regulator in maintaining skin homeostasis by influencing various cellular processes, like proliferation, differentiation, and apoptosis processes of skin cells. As an antioxidant, vitamin D can mitigate the damage caused by excessive UV exposure, which leads to ROS formation. By reducing this UV-induced oxidative stress, vitamin D aids in preventing skin damage, inflammation, and even skin cancer development [170].  
Furthermore, vitamin D enhances the expression of antioxidant enzymes, such as GSH, which generally neutralizes ROS. Furthermore, it boosts the skin's immune responses and enhances the activity of enzymes such as superoxide dismutase (SOD) that disarms superoxide radicals, with great efficacy. Such a protective intervention is crucial in maintaining the skin's barrier function and preventing conditions like eczema, psoriasis, and acne [170]. Vitamin D's antioxidant properties extend to repairing oxidative damage by modulating DNA repair mechanisms [171]. It also ensures the removal or repair of damaged DNA segments caused by ROS, further preventing mutations and carcinogenesis' onset in skin cells [170; 171]. Studies have highlighted the broader antioxidant framework that includes other vitamins like E and C, flavonoids, and polyphenols, which work synergistically and collaboratively to protect ones' skin from oxidative stress, and thus the overall skin health [170].