The skin plays a crucial role in the synthesis of vitamin D, a vital nutrient for various physiological processes in the body. Vitamin D is unique because it can be synthesized in the skin through a series of chemical reactions triggered by exposure to ultraviolet B (UVB) radiation from sunlight.

The solar UV B rays (290-315 nm) are absorbed by the skin, and 7-dehydrocholesterol (provitamin D₃) photolyzes it to previtamin D₃, which undergoes a rapid transformation to vitamin D₃(cholecalciferol). Once vitamin D3 is formed in the skin, it diffuses into the blood vessels of the dermis, the layer of skin beneath the epidermis. From there, it enters the bloodstream and is transported to various tissues and organs in the body.

Vitamin D₃ is lipid soluble, so adipocytes can store it during excessive production. The vitamin has both inhibitory and stimulatory effects on the differentiation of adipocytes. The result depends on the differentiation stage of the cell and cell type.

It is important to note that while sunlight is a significant source of vitamin D, other factors can influence the skin's ability to produce it. These factors include the latitude, the season, the time of day, skin pigmentation, clothing, sunscreen use, and age. For example, people with darker skin have higher levels of melanin, which can reduce the skin's ability to synthesize vitamin D compared to individuals with lighter skin.

Vitamin D is vital in energy metabolism modulating fatty acid oxidation, insulin resistance, and even adipokine production. It aids absorption of calcium and phosphates in the intestine to improve bone health.

This text is adapted from https://openstax.org/books/anatomy-and-physiology-2e/pages/17-10-organs-with-secondary-endocrine-functions