Finally, vitamin D can be obtained through sun exposure.  This process happens when ultraviolet rays from sunlight strike the skin and trigger a vitamin D synthesis response. 

Many people will satisfy at least some of their vitamin D needs through sun exposure.  Generally speaking, five to 30 minutes of sun exposure between 10 and 3 p.m. twice a week will provide a good amount of vitamin D synthesis. 

The amount of vitamin D one can obtain from the sun will vary greatly depending on a variety of factors, such as season, time of day, length of day, cloud cover and smog present in the air.