Milk with vitamin D added is called "fortified".

Vitamin D has one main job: it helps the body take in calcium, the mineral that bones are made from. Without enough of it, bones can become soft and weak. In children, a lack of vitamin D can cause a disease called rickets; in adults, it can lead to soft or thin bones that break easily.

Unusually for a vitamin, vitamin D does not have to come from food. The skin makes it when it is exposed directly to sunlight. Very few foods contain much of it naturally. Fatty fish such as tuna, salmon and mackerel are among the best sources, while beef liver, cheese and egg yolks give small amounts. Some mushrooms contain more of it after they have been exposed to ultraviolet light. Because natural sources are so few, the vitamin is often added to foods such as milk, cereals and yoghurt, which are then called fortified.

How much a person needs depends on age. Babies under one year need about 400 international units (IU) a day. Children, teenagers and adults up to the age of 70 need about 600 IU, and people over 70 need about 800 IU.

More is not always better. Too much vitamin D, usually from supplements, raises the level of calcium in the blood. That can cause nausea, vomiting, weakness and confusion, and it can damage the kidneys.