Vitamin D is a hormone mainly produced by the skin when exposed to sunlight.
Certain foods such as egg yolk, oily fish, milk and fortified foods such as breakfast cereals also contain small levels of vitamin D.
Vitamin D is important for bone health via managing calcium release/absorption to promote bone remodelling (breakdown/build new bone cells).
Vitamin D is also important to support the immune system but researchers believe there are more, yet unknown, roles for vitamin D.
Vitamin D deficiency leads to rickets in children and osteamalacia or osteoporosis in adults.
People at risk of vitamin D deficiency are those whose exposure to sunlight is limited and/or have darker skin.