Sun exposure is the second significant source of vitamin D. Ultraviolet light starts a chemical reaction in the skin that produces a usable form of vitamin D.
How much vitamin D your body produces changes with the environment and how well your skin absorbs vitamin D.
Thankfully, restoring vitamin D levels to normal is relatively easy.
This can be done either from eating more foods such as fatty fish, beef liver, cheese, egg yolks and fortified foods or supplements could be used.