Vitamin D is a fat soluble vitamin and can therefore be stored in the human body. Main sources are milk and fatty fish, plants only contain a preliminary stage. The human body is also able to synthesize Vitamin D when the skin is exposed to the sun (UVB light). This highly depends on latitude and intensity of insolation and this can lead to a vitamin D deficiency in northern countries in winter. New studies even indicated the skin of people over an age of 65 years can’t synthesize Vitamin D anymore.
Vitamin D is especially import for healthy bones.
To supplement vitamin D on one’s own initiative is absolutely not recommended! As the vitamin can be stored in the body an overdose is possible. Always contact a doctor, have a blood test and discuss the necessity of a supplementation.