Our main source of vitamin D is the sun, however synthesis of vitamin D is a function of skin pigmentation and of the solar zenith angle which depends on latitude, season, and time of day. Between October and March the angle of the sun to the earth means that in the UK we can not make vitamin D – even on the brightest sunniest winter day. A good measure of the angle of the sun to the earth is the length of our shadow. If our shadow is shorter than our body we can make vitamin D, if it is longer we can not. Good food sources of vitamin D are oily fish, egg and milk however eating these foods in a balanced diet will not provide enough vitamin D, therefore supplementing over winter is necessary.
Vitamin D is essential for good bone health as it regulates calcium absorption. It is also important for the immune system and researchers are discovering that vitamin D may play an important part in mental health and depression.