Rarely a month goes by without the papers reporting at least one health news story related to vitamin D. In recent weeks the media has reported that vitamin D can help relieve the symptoms of asthma and lower blood pressure.
There have been long-standing claims that vitamin D brings a wide range of benefits, from preventing cancer risk to improving mental health, or even reducing your risk of getting multiple sclerosis.
But is there good evidence to back up the claims? And do you need to change your diet or take vitamin D supplements to reduce your risk of disease?
What is vitamin D?
Vitamin D is a group of related molecules that the body needs to help absorb calcium and phosphate. These are substances that help keep the bones healthy and strong.
Vitamin D is somewhat unusual in that we obtain it from two difference sources:
How much sun is needed to get enough vitamin D?
Types of vitamin D
Vitamin D is not a single substance but a "family" of related substances. The two types that are important to humans are:
- vitamin D2 - which is made by plants
- vitamin D3 - which is produced by exposure to the sun
Foods can be fortified with both vitamin D2 and vitamin D3.
When the skin is exposed to the ultraviolet B contained in sunlight, it generates the production of vitamin D. Most people generate around 90% of the vitamin D in their body from sunlight.
Your sunshine requirements differ depending on factors such as your skin tone and your weight. A 2010 consensus statement on vitamin D (PDF, 126.69kb), released by a combination of charities, recommended a "little and often" approach. It says regularly going out with sunscreen for a few minutes in the middle of the day should provide enough exposure to create sufficient vitamin D.
How to get enough vitamin D through your diet
Eating a healthy balanced diet should be sufficient to top up the remaining 10% or so that experts believe we need through our diet. Dietary sources of vitamin D include:
- oily fish, such as salmon, sardines and mackerel
- fortified fat spreads
- fortified breakfast cereals
- powdered milk
What is vitamin D deficiency?
Vitamin D deficiency is when the body does not have enough vitamin D to properly absorb the required levels of calcium and phosphate.
Mild to moderate vitamin D deficiency can lead to bone pain and weakening of the bones (osteoporosis). This could make you more likely to fracture a bone if you had a fall.
More severe levels of deficiency can lead to the development of rickets in children and osteomalacia in adults.
Rickets, osteomalacia and vitamin D
Chronic severe vitamin D deficiency in children can disrupt the normal formation of bones, causing them to become soft and malformed and resulting in the condition known as rickets.
Symptoms of rickets include:
- bone pain
- fragile bones vulnerable to fracture
Previously regarded as a disease of the past associated with Victorian slums, rickets is now making a comeback in some parts of England.
In 2012 the Royal College of Paediatrics and Child Health released a statement highlighting the problems of vitamin D deficiency in children, reporting that rates of rickets have risen fourfold in the last 15 years.
Osteomalacia, like rickets, develops because of softening of the bones. The main symptom of osteomalacia is a dull, throbbing and often severe bone pain that usually affects the lower section of the body. Osteomalacia can also result in muscle weakness.
Other health risks that have been linked with vitamin D deficiency
In a 2010 BMJ clinical review on vitamin D deficiency, researchers presented evidence that vitamin D deficiency may increase the risk of developing a number of chronic conditions, such as:
However, more research is required to prove these associations and provide evidence that people need to change their behaviour or take supplements because of potential health problems.
How common is vitamin D deficiency?
Vitamin D is thought to be much more common than most people realise. A 2007 survey estimated that around 50% of all adults have some degree of vitamin D deficiency.
In 2012 the Chief Medical Officer for the United Kingdom wrote to GPs highlighting the issue of vitamin D deficiency in high-risk groups (see below).
An independent advisory committee is also reviewing current recommendations on vitamin D, but the results of this extensive analysis are not expected until 2014.
What are the risk factors for vitamin D deficiency?
Lack of exposure to sunlight
Unsurprisingly, a significant risk factor for vitamin D deficiency is lack of exposure to the sun.
Possible factors that can result in limited exposure to sunlight include:
- overuse of sunblock
- being housebound or spending long parts of the day inside
- wearing clothes that cover up most of your body, often for cultural or religious regions
There are anecdotal reports that children may be more likely to develop vitamin D deficiency these days, as they are less likely to play outside than children did in the past.
Darker skin tone
Having a darker skin tone means you require a greater amount of sunlight exposure to generate vitamin D.
It is estimated that people with a naturally dark skin tone may require three to five times longer sunlight exposure to make the same amount of vitamin D as a white person.
Obesity could be an overlooked cause of vitamin D deficiency. A recent study published in February 2013 suggested there is a direct relationship between increasing body mass index (BMI) and falling vitamin D levels.
The authors of the study speculated that vitamin D may become "trapped" inside fat tissue, so there is less available to circulate inside the blood.
How is vitamin D deficiency treated?
Mild to moderate vitamin D deficiency can usually be treated by making lifestyle changes such as getting more sun and eating foods rich in vitamin D. In some cases your GP may also recommend you take vitamin D supplements.
In more severe cases where the deficiency has affected bone growth and density, such as rickets, a vitamin D injection may be recommended.
Foods fortified with vitamin D
Unlike in some other countries, in England staple food items such as milk, flour and cereals are not routinely fortified with vitamin D. Fortified versions of goods such as cereals and milk are available from most supermarkets. You can read the food labels to compare the levels of vitamin D between products.
Some argue that people in the UK, especially in the north of England and Scotland, would benefit from fortification. However, the vitamin D we get from dietary sources is thought to stay in the body longer than the vitamin D we get from sunlight. Fortifying staple foods and drinks could potentially lead to dangerously high levels of vitamin D in a small number of people (vitamin D toxicity).
Other benefits of vitamin D
Vitamin D - it's a regular staple of health news stories. But what exactly is it? How much do we need? Are we all getting enough? Can you take too much? Behind the Headlines investigates.
Links to Science
Avenell A et al. Vitamin D and vitamin D analogues for preventing fractures associated with involutional and post-menopausal osteoporosis. Cochrane Database of Systematic Reviews. April 2009
Bjelakovic G et al. Vitamin D supplementation for prevention of mortality in adults. Cochrane Database of Systematic Reviews. July 2011
Garland CF et al. Vitamin D for Cancer Prevention: Global Perspective. Annals of Epidemiology. July 2009
Holick MF et al. Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology and Metabolism. July 2011
Jagannath VA et al. Vitamin D for the management of multiple sclerosis. Cochrane Database of Systematic Reviews. December 2010
Pearce SH, Cheetham TD. Diagnosis and management of vitamin D deficiency. BMJ. January 2010
Urashima M et al. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. American Journal of Clinical Nutrition. March 2010
Vimaleswaran KS et al. Causal Relationship between Obesity and Vitamin D Status: Bi-Directional Mendelian Randomization Analysis of Multiple Cohorts. PLOS Medicine. Published online February 5 2013
Consensus. Vitamin D position statement. December 2010
Royal College of Paediatric and Child Health. Vitamin D - Position Statement. December 2012