Vitamin D is a hormone that can be found in the foods we eat as well as produced by our own bodies. It is a long-known, fat-soluble vitamin that assists the body in absorbing and retaining calcium and phosphorus, both of which are important for bone formation. Additionally, laboratory studies show that D vitamin supplementation may reduce the growth of cancer cells, help control infections, and reduce inflammation. Many organs and tissues in the body have D vitamin receptors, suggesting that it may have functions beyond bone health, and scientists are actively researching other potential functions.

There are only a few foods that naturally contain vitamin D, but some foods have been fortified with this vitamin. For most people, the best way to get enough vitamin D is to take a supplement, as it can be difficult to get enough through diet alone. Vitamin D supplements come in two forms: D2 vitamin ("ergocalciferol" or vitamin D precursor) and D3 vitamin ("cholecalciferol"). Both are naturally occurring forms and are produced in the presence of ultraviolet-B (UVB) rays from the sun, hence the nickname "sunshine vitamin", but D2 is produced in plants and mushrooms, while D3 is produced in animals, including humans.

The production of vitamin D in the skin is the main natural source of vitamin D, but many people have insufficient levels because they live in places where sunlight is limited in winter or they spend most of their time indoors. Additionally, darker-skinned individuals tend to have lower blood levels of vitamin D, as their skin pigment (melanin) acts as a shade and reduces the production of vitamin D (as well as reducing the harmful effects of sunlight on the skin, such as skin cancer).

To help determine the level of vitamin D: https://www.kocyasa.com/en/corensis-diagnosis-kits/d-vitamin-hizli-test-en