Vitamin D is essential, and can be sythesised by the action of sunlight on skin. It can also be taken as a dietary supplement. The question is: where is the balance between the health benefit of sun exposure and the danger of skin cancer from too much.
How much Vitamin D?
Vitamin D does not have a recommended daily intake in the same way as other vitamins and minerals as it is primarily made by the body rather than consumed in food.
If you look at the concentration of the vitamin in the blood then levels below 25 nanomoles per litre are a sign of deficiency.
There is still controversy over the optimal level for health. Around 50 nanomoles per litre is argued for good bones, while some experts champion 75 nanomoles per litre for other conditions such as MS and diabetes, although this remains a source of debate.
Diet is a poor source of the vitamin, although it is in fatty fish, eggs and fortified foods. However, 10 to 20 minutes of sunlight for light-skinned people, or more than half an hour for those with darker skins, should be enough for a week's worth of the vitamin.
The government advises pregnant or breast feeding women and the over-65s to take a daily supplement containing 10 micrograms of vitamin D. Babies and the under fives should have daily vitamin D drops, unless they are taking formula milk.
Half of the UK's white population, and up to 90% of the black and Asian people in the country are thought to be affected by vitamin D deficiency.
A lack of the nutrient is linked to a higher incidence of diabetes, tuberculosis, multiple sclerosis as well as rickets - a disease that causes bones to become soft and deformed.