Image: AI Vitamin D is a fat-soluble micronutrient that also acts like a hormone precursor in the body. Part of it can be produced in the skin when UVB rays from sunlight are available. Only a few foods naturally contain meaningful amounts of vitamin D, including fatty fish, egg yolk, liver and fortified foods. It is best known for its role in calcium metabolism, bones, teeth and muscle function. It is also connected to immune function, inflammation regulation, mood and metab