What is vitamin D?

When ultraviolet B rays hit the skin, they convert a form of cholesterol into vitamin D, which the liver and kidneys then turn into an active hormone. Its main job is helping the gut absorb calcium and phosphate, and a severe lack causes rickets in children and soft, painful bones in adults. Because winter sunlight at high latitudes is too weak, many people in places such as the UK and Canada run low from October to March. The US National Institutes of Health recommends 600 international units (15 micrograms) a day for most adults and 800 IU for those over 70. Good food sources include oily fish, egg yolks and fortified milk and cereals.