## 4 Skin Biology, Vitamin D Synthesis, and Its Impacting Factors
### Factors That Affect the Synthesis of Vitamin D  
Many factors affect the process by which the skin makes vitamin D when exposed to UV light, known as cutaneous vitamin D synthesis. Maintaining appropriate levels of vitamin D is critical for immune system performance, bone health, and general wellbeing. This mechanism makes this possible. The amount of UV radiation, skin type, location, season, time of day, and use of sun protection are some of the major variables that affect how well vitamin D is produced in the skin. The main factor influencing the synthesis of vitamin D in the skin is the UV light intensity. As mentioned previously, 7-DHC in the skin is changed by UV\({}_{\text{B}}\) radiation to pre-vitamin D\({}_{3}\), which is then transformed into vitamin D\({}_{3}\). UV\({}_{\text{B}}\)'s radiation intensity, in general, varies with height, latitude, and cloud cover, among other factors. For instance, UV\({}_{\text{B}}\) radiation is stronger near the equator and at high altitudes. On the other hand, UV\({}_{\text{B}}\) intensity drops in the winter and in areas distant from the equator, where vitamin D synthesis is reduced [140].  
Figure 6: Vitamin D\({}_{3}\)’s 25-hydroxylation metabolism and the anti-inflammatory and pro-inflammatory roles of vitamin d\({}_{3}\) and its active form calcitriol in immune response. (Reproduced from [30], while all the structures utilized were obtained from the free databases PubChem ([[https://pubchem.ncbi.nlm.nih.gov/](https://pubchem.ncbi.nlm.nih.gov/)]([https://pubchem.ncbi.nlm.nih.gov/](https://pubchem.ncbi.nlm.nih.gov/)) (accessed on 29 December 2024)), and Molview ([[https://molview.org/](https://molview.org/)]([https://molview.org/](https://molview.org/)) (accessed on 29 December 2024)).  
The effectiveness of vitamin D production is significantly influenced by skin type as well. The pigment melanin, which gives skin its color, absorbs UV rays and lessens their ability to penetrate the skin's deeper layers. As a result, those with darker skin display higher melanin levels, and are less able to synthesize vitamin D than people with lighter skin. Skin types are categorized on the Fitzpatrick scale from I (very light) to VI (extremely dark), with darker skin types needing more sun exposure to generate the same amount of vitamin D in contrast to lighter skin types [141]. In addition, geographical differences in solar UV\({}_{\text{B}}\) exposure have a significant impact on vitamin D production. More direct sunshine is received year-round in areas near the equator, which promotes steady vitamin D synthesis. Higher-latitude regions, on the other hand, undergo notable seasonal fluctuations. For instance, there may be insufficient UV\({}_{\text{B}}\) sunlight during the winter in northern latitudes like Scandinavia or Canada, which increases the risk of vitamin D deficiency. UV\({}_{\text{B}}\) exposure is also notably influenced by day and season. During the summer, when the sun is at its maximum point, UV\({}_{\text{B}}\) radiation is more intense. The angle of sunlight decreases the amount of UV\({}_{\text{B}}\) rays that reach the skin in the winter or in the early morning and late afternoon. For efficient vitamin D synthesis, one must understand the idea of the \"solar noon\", or the time when UV\({}_{\text{B}}\) rays are at their strongest. Hence, exposure to the sun at midday, when UV\({}_{\text{B}}\) radiation is at its highest, is ultimately more effective in producing vitamin D in individuals [142].