Vitamin D

Vitamin D is a fat soluble vitamin needed to allow absorption of calcium from the diet. Without it as little as 10% of dietary calcium may be absorbed and with it up to 70%.


This is important, especially to allow mineralisation of bones, and also to maintain calcium levels in the blood which is essential for normal function of the nerves, muscles including the heart and other organs. Vitamin D also acts directly in the bone and is probably important for the normal functioning of the immune system. Roles have been suggested in preventing heart disease and some cancers.


Deficiency of vitamin D can cause rickets in children, where bones are soft and deformed, and if severe, this can be fatal. In adults, vitamin D deficiency can also weaken bones, causing them to fracture abnormally and can cause bone pain and deformity and muscle weakness.


Humans can make all the vitamin D they require by exposure of the skin to adequate sunshine, whereby the ultra-violet (UV) B rays from the sun convert a fatty molecule in skin to active vitamin D. In Northern Europe including the UK, this is only possible during the spring and summer months (roughly between the March and September equinox) when there is enough UV B in the sunlight we receive. During these months, in young adults only 30 minutes of exposure of the hands and face to the midday sun allows production of the daily vitamin D requirement. It is important that this is without sun block, as even low factors block most of the UVB from reaching the skin. Sunburn should not occur in so short a time. This skin production is less efficient in the elderly and in those with pigmented skin.


In the winter, in the UK, we are dependent first on good summer vitamin D stores, which may last a month or so, then on vitamin D in our diets.


The best natural dietary sources of vitamin D are oily fish such as salmon, sardines, mackerel and even pilchards. 100g of any of these would provide most of your daily requirement. Meat and eggs also contain vitamin D, but in lesser amounts – you would need 10-20 eggs or 2kg of beef a day to provide enough! Dairy products such as milk, cheese and yoghourt are excellent sources of calcium but are low in vitamin D. Vegetarian diets are often low in vitamin D unless supplements are taken. The best vegetarian sources include mushrooms, whose vitamin D content can be raised 10 times by exposure to the sun for 30 minutes! Sun dried mushrooms are even better. In the UK, much of our dietary vitamin D comes from food fortification of products such as breakfast cereals and margarine. Flour and bread are not generally fortified.


In the UK, population surveys have shown that many people have suboptimal levels of vitamin D especially in winter. This may contribute to bone diseases such as osteoporosis and fractures and general poor health.


Some people are particularly at risk. These include:

  • Babies and children including adolescents
  • Pregnant and breastfeeding mothers
  • People who are unable to get outside for 30 minutes a day.
  • People with illnesses which may prevent absorption of Vitamin D eg coeliac and Crohns disease, stomach or bypass surgery, liver disease, chronic pancreatitis etc
  • People on some medications especially anticonvulsant drugs for epilepsy, and steroids
  • People who avoid skin exposure for cultural, religious or medical reasons.


People in these groups may be advised to take vitamin D supplements.


In people with osteoporosis, suboptimal vitamin D levels may further weaken the bones and increase the risk of fracture. Adequate intake of both calcium and vitamin D either from diet, supplements or sunlight are important. This is especially needed to allow osteoporosis treatments such as Alendronic acid to be effective and to reduce the risk of side effects. These people may be advised to take a supplement of vitamin D and or calcium as part of their treatment. Also their vitamin D levels may be measured.


There are many vitamin D supplements available over the counter. ‘Multivitamins’ usually contain only around 200IU (5mcg) which is a good start, but only doses of 800IU (20mcg) have been shown to reduce fracture risk. Cod liver oil capsules are another good source.


It is important not to exceed the recommended dose of either of these, preparations as they also contain vitamin A which can be toxic in overdose. It is actually quite hard to take toxic doses of vitamin D itself with the commercial preparations available. Many others available contain only vitamin D, often with some calcium.


If you have bone disease and take or are considering taking these, it is worth checking this with your doctor. You may also be formally prescribed a supplement as part of your treatment.


Sometimes, we find that people with bone disease are severely deficient in vitamin D in which case very high dose loading course of vitamin D may be prescribed. These very high doses can produce side effects if too much is taken so please follow the advice of your doctor carefully.


There are rare groups of people who should avoid excessive vitamin D (eg active sarcoidosis) but this is very rare.


Other information

If you wish to access further information about vitamin D, The National Osteoporosis Society is a good source of advice and has produced an excellent guide.