Vitamin D is the only vitamin our body can make. Vitamin D is produced by our skin naturally when it is exposed to ultraviolet B light. This is because sunlight acts on the substance 7-dehydrocholesterol to produce cholecalciferol (vitamin D3). If you are not getting enough vitamin D this way you can ingest it in your diet or in pill form. Fatty fish, meat and eggs are also good sources of vitamin D.
Olivia Harwood, Science Museum