A fat-soluble vitamin, vitamin D is unusual because it can be made in the body, through the action of sunlight on the skin - which is why it's sometimes called the ‘sunshine vitamin'.
Where can I get it?
Most of us get our vitamin D when ultra-violet rays from the sun react with a substance in the skin to form vitamin D. It takes a certain type of sunlight, which we only get in Britain between March and October. However, it doesn't need to be a sunny day for the sunlight to work its magic - even on a cloudy day, at the right time of year, exposed skin will make vitamin D. During the winter months, we use vitamin D that was made in the summer.
Not everyone can make vitamin D in this way. People who are housebound and unable to get outside, or people who completely cover their skin, for example, aren't exposed to the sun enough to make vitamin D. Pregnant or breastfeeding women, and people with naturally brown or black skin, who need more ultra-violet rays to boost their vitamin D levels than people with light-coloured skin, should all eat plenty of vitamin-D rich foods.
There is some concern about vitamin D formation and use of sunscreens. It seems that, by blocking the sun's rays from the skin you prevent the formation of vitamin D. However, you only have to be outside for a brief time (ideally around midday for less than 10 minutes) to make vitamin D, so the advice to use sunscreens above SPF15 when you're in the sun for any longer is still appropriate. Cancer UK warns that heavy sun exposure won't improve levels of vitamin D beyond a maximum threshold, but it may increase the risk of skin cancer.
How much should I eat?
Because it's so difficult to judge how much vitamin D is made by individuals, it's very hard for scientists to figure out just how much we need. If you fall into one of the groups of people who have problems making vitamin D from the sun, then you should try to increase your intake of dietary sources.
Oily fish, such as salmon, mackerel and sardines, are a good source and everyone should try to eat one or two portions a week. Egg yolks and fortified foods such as margarine and breakfast cereals also contain vitamin D. Liver and liver products are good sources but they shouldn't be eaten more than once a week because of the risk of vitamin A toxicity.
If you struggle to increase your intake of these foods and you can't get outside regularly then you might benefit from a supplement of 10 micrograms (0.01 mg) of vitamin D each day. The ability of the skin to form vitamin D decreases as we age, so older people, even those who are outdoors regularly, may benefit from supplements.
Very high levels of vitamin D can cause weakening of bones and because most people get all they need from the sun and a varied, balanced diet, there's usually no need for supplements. However, if you do take a supplement, such as cod liver oil, check that you're taking 25 micrograms (0.025 mg) or less each day.
What does it do?
Vitamin D works with calcium and phosphate to help keep bones and teeth healthy. Without it, bones can become thin and brittle or soft. It's needed for growth so it's especially important for pregnant and breastfeeding women, and growing children.
Children who suffer from deficiency over several months can develop rickets, which causes bone pain and skeletal deformities such as bowed legs and a curved spine.
In adults, the result of vitamin D deficiency is called osteomalacia - a softening of the bones leading to muscle weakness and an increased risk of fractures.
Deficiency in the UK is very common. It's thought that around 2 in 10 adults and 9 in 10 adults of South Asian origin may be deficient but most will be unaware of the problem. If you suffer from unexplained muscle pains or muscle weakness, or bone pains in your back, hips or legs you should ask your doctor to test for vitamin D deficiency. Symptoms to look out for in children are poor growth and delayed or weak teeth.