Vitamin D may be one of the few vitamins we really need to take as a supplement. In July 2016, Public Health England issued new advice that everyone aged four and over needs an average daily intake of 10 micrograms of vitamin D, however, Professor Hilary Powers has said that "we can store enough vitamin D produced by sunlight during the summer months to last through-out the year."

Vitamin D is a fat-soluble vitamin whose primary function is to regulate the absorption of calcium and phosphorus in the body, which is essential for the formation and maintenance of healthy bones and teeth. Additionally, vitamin D plays a significant role in modulating the immune system, reducing inflammation, and may have protective effects against diseases such as type 1 diabetes, multiple sclerosis, and certain forms of cancer.

The body can produce it when the skin is exposed to sunlight, specifically ultraviolet B (UVB) rays. Vitamin D is also obtained through dietary sources, including fatty fish, fish liver oils, egg yolks, and fortified foods, as well as through supplements.

Vitamin D production depends on geographic location, skin pigmentation, time of day, season, and the use of sunscreen, which can all affect UVB exposure. In regions with less sunlight, especially during winter months, or for individuals who spend limited time outdoors or wear sunblock, achieving adequate vitamin D levels can be challenging.