## 5 Vitamin D Health-Promoting Activities
### Anti-Aging Activity  
An intricate process of both internal and external variables contributes effortlessly to skin aging. Like all other organs, the physiological, morphological, and functional characteristics of the skin gradually deteriorate with age. In general, aging is a multifactorial process, as a natural and genetically predisposed phenomenon [27]. More specifically, genetically predetermined intrinsic aging, commonly called chronological aging, is driven by genetic factors and the passage of time and starts around the mid-20s [28]. It entails a slower synthesis of collagen and elastin, two proteins necessary to preserve skin firmness and suppress, as well as a progressive reduction in the skin's capacity to regenerate. As a result, the skin thins, loses volume and elasticity, and develops fine lines and wrinkles, whereas dryness and a lifeless complexion result from cellular turnover deceleration, where the skin becomes more brittle and less able to preserve moisture [27]. Conversely, extrinsic aging results from environmental factors such as pollution, UV radiation, and also lifestyle choices like smoking and bad dietary habits [175]. The main cause of photoaging is UV radiation, and particularly UV\({}_{\text{A}}\) rays that induce collagen fibers to break down and aberrant elastin to be produced, which results in deeper wrinkle formation, a leathery texture, and pigmentation changes like age spots or hyperpigmentation [27]. Additionally, UV radiation produces ROS that expedite further extracellular matrix (ECM) damage, which accelerates the aging process of the skin by degrading collagen and elevating the expression of enzymes like matrix metalloproteinases (MMPs). Additionally, air pollution, smoking, and oxidative stress are in accordance with the breakdown of collagen and elastin, worsening skin texture and elasticity. Smoking reportedly reduces blood flow of the skin, depriving it of oxygen and nutrients, while pollution introduces free radicals that damage skin cells [28; 175]. Chronic stress, poor diet, and sleep deprivation can all hasten these consequences by interfering with skin regeneration processes [27]. As a sequel, the skin becomes drier and thinner due to collagen loss [28; 175; 176]. The impairment of the skin's collagen partakes in a condition called dermatoprosis [177]. Normally, young skin contains more than 75% of collagen [178; 179]. As collagen is the protein of the skin responsible for strength and elasticity, aging leads to thinning and fragility. Studies have shown that vitamin D can act as a stimulating agent for the synthesis of collagen that promotes skin's thickening and resilience [177]. Also, vitamin D influences the collagen turnover, by repairing and replacing collagen, reducing collagenase, and stimulating collagen production. Collagen is particularly an enzyme that breaks down collagen [27]. Thus, vitamin D acts equally both as an antioxidant and an anti-aging agent, with great efficacy and promising outcomes [25]. Vitamin D participates dexterously in slowing the aging process, acting as a powerful \"anti-aging shield\" at both a cellular and a molecular level. Its anti-aging effects are attributed to its regulatory functions in immune modulation, inflammation control, and the maintenance of skin homeostasis. 1\(\alpha\),25(OH)\({}_{2}\)D\({}_{3}\) impacts a wide range of biological processes, indisputably crucial for healthy aging [27; 34]. Cellular senescence is another hallmark of aging, characterized by the cessation of cell division and the secretion of inflammatory mediators, known as the senescence-associated secretory phenotype (SASP). As people age, senescent cells proliferate and contribute to tissue malfunction and persistent inflammation [34]. It has been demonstrated that vitamin D prevents the accumulation of senescent cells and modulates pathways involved in cell cycle regulation, in order to counteract cellular senescence [34; 180].