Vitamin D3 is an essential fat-soluble vitamin made naturally in the skin from exposure to the sun’s ultraviolet rays. The sunlight converts cholesterol within the skin into a form of vitamin D3 called calciol. This is converted by the liver into calcidiol (25-hydroxyvitamin D3), which is the inactive form of vitamin D3 circulated and stored in the body. The kidneys convert calcidiol into the active form of vitamin D3 called calcitriol (1,25-dihydroxy-vitamin D3).
Vitamin D3 increases calcium absorption in the gut to sustain steady blood calcium and phosphate concentrations that enable normal bone mineralization. As a hormone, it regulates cellular production of important proteins and peptides.
Vitamin D3 helps increase the number of receptors on immune system cells to help them fight pathogenic bacteria and viruses. It helps white blood cells recognize foreign invaders and may also influence the release of protective proteins when there is an infection.
Vitamin D3 may benefit cognition by protecting neurons and supporting their development. Its receptors are found on many different tissues in the body, including those in muscles, the prostate, and the heart, indicating that vitamin D3 has more effects than we currently know.