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Vitamin D is one of the four fat-soluble vitamins (vitamin A, D, E, and K). Actually, it is not a true vitamin because it acts as a hormone and individuals with adequate sunlight exposure do not need vitamin D from diet. Vitamin D is essential for healthy bones and teeth. The main source of natural vitamin D is produced in the inner layer (dermis) of skin after sunlight exposure. Globally, there is an estimation that 1 billion people across all age groups and ethnicities have vitamin D deficiency. This is mostly due to people getting less sun exposure and/or using sunscreen because of climate, lifestyle, and concerns about skin cancer.
The most important health bene t of vitamin D is to maintain bone strength, one of its endocrine functions. Vitamin D promotes calcium and phosphorus absorption from the small intestine through the actions of 1,25 dihydroxy vitamin D and reduces urinary calcium excretion. Vitamin D also has many other roles in the body, including modulation of cell growth, immune function, and reduction of inflammation. It has been found that vitamin D receptor is widely expressed in almost all nucleated cells in most organs, including the brain, heart, skin, genital organs, prostate, breast, and pancreas. Beside renal cells, the active form of vitamin D is also released in smooth muscle, colon, and immune cells (Figure 1). There are increasing evidence supporting the possible role of vitamin D in protecting against many cancers, heart disease, fractures and falls, autoimmune diseases, influenza, type 2 diabetes, and depression.