Melanin acts as a natural barrier to the UV rays needed for Vitamin D production.

Melanin is a pigment in our skin cells. Its main job is to absorb ultraviolet B (UVB) light from the sun. These same UVB rays are the exact trigger our skin needs to start manufacturing Vitamin D. Because dark skin has much more melanin, it absorbs these rays before they can kickstart the Vitamin D process. It is a highly effective shield, but it means the chemical reaction in the skin happens at a much slower rate.