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?
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:
When the skin is exposed to the ultraviolet B contained in sunlight, it generates the production of vitamin D. Most people generate the majority of the vitamin D in their body from sunlight.
Ultraviolet B does not penetrate glass so you will need to go outside to top up your vitamin D levels.
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, between the months of April to October, for a few minutes in the middle of the day should provide enough exposure to create sufficient vitamin D.
It is difficult to get enough vitamin D from food alone. However dietary sources of vitamin D include:
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.
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:
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.
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, the results were inconclusive and provides little evidence of a need to change behaviour.
Vitamin D deficiency 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.
Unsurprisingly, a significant risk factor for vitamin D deficiency is lack of exposure to the sun.
Other factors include:
Having a darker skin tone means you require a greater amount of sunlight exposure to generate vitamin D.
People with a naturally dark skin tone usually take longer to produce vitamin D compared to a white person.
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.
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.
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).
Aside from bone health, a number of claims have been made about the wider benefits of vitamin D. Here's a summary of some of these health claims and what evidence has been presented to back them up.
Cases of multiple sclerosis are higher in areas further away from the equator. This has led some to conclude that reduced vitamin D levels in countries with less sunlight could be responsible for the increase in multiple sclerosis.
However, the evidence is inconclusive. A 2010 clinical review (PDF, 274.1kb) could only find a single, very small study looking at whether vitamin D could help people with multiple sclerosis. This review highlighted the need for larger studies involving MRI scanners to build a detailed assessment of the effects of vitamin D on the nervous system.
The fact that flu cases tend to peak during the winter has led to speculation that influenza rates, as with multiple sclerosis, may be influenced by exposure to sunshine and, by extension, vitamin D levels.
So could vitamin D supplements help us ward off the flu? A 2010 placebo-controlled trial of vitamin D supplements suggested vitamin D could reduce the chances of having seasonal flu. However, due to the limited study sample these results could have been at least partially down to the effects of chance. The trial also did not compare the effectiveness of vitamin D pills with the seasonal flu vaccine.
A recent study discussed by Behind the Headlines at the beginning of 2013 suggested that vitamin D may help treat severe asthma that fails to respond to conventional treatment.
Researchers found that vitamin D reduced the levels of a molecule called IL-17A produced by cells from people with asthma. IL-17A is thought to be involved in the abnormal immune response that triggers the symptoms of asthma.
However, a positive effect on cells in the lab does not guarantee vitamin D supplements will improve symptoms for people with asthma. Clinical trials in people with asthma are ongoing to test whether this will be the case.
We know that tests have found that vitamin D can help keep bones healthy, but can this help people in the "real world" and keep them healthy? The good news is that it can.
A large 2009 review found good quality evidence that vitamin D supplements, if combined with calcium supplements, reduced the risk of fractures in people aged 65 or over.
It is plausible that vitamin D may help prevent cancer, but it is still far too early to suggest that people take vitamin D supplements to prevent cancer. Laboratory studies have found that vitamin D can slow the growth and spread of cancer cells. Some types of cancer also have a latitude effect, as seen with multiple sclerosis.
A 2009 clinical review into vitamin D and cancer prevention (PDF, 1.25Mb) concluded with the controversial statement that cases of breast and colon cancer could be cut by a quarter if all adults took 50 micrograms (2,000 IU) of vitamin D a day. However, some experts regard this figure as the maximum safe dose for adults, while others argue that this may be potentially risky, especially in the long term. (Current UK guidelines recommend that adults take no more than 25 micrograms per day).
The US National Cancer Institute takes a cautious approach, warning in a 2010 position paper that, "although some evidence suggests that vitamin D may provide some protection against colorectal and possibly other cancers, the evidence of potential benefit is limited and inconsistent.
"Moreover, some studies have suggested the possibility that higher vitamin D levels are associated with increased risk for some cancers, including pancreatic cancer."
The one health outcome that everyone should be interested in is whether an "intervention" (such as taking a vitamin D supplement) can postpone death. Can vitamin D supplements help us avoid an early grave and even extend life expectancy?
A systematic review in 2011 looked at the evidence from more than 50 studies involving more than 90,000 people. The researchers found a very modest benefit in people who took vitamin D3 supplements. No benefits were found for other types of vitamin D.
Importantly, most of the people who took part in the studies were elderly women who were staying in residential care. It is therefore not clear whether these results would apply to other people.
The Department of Health currently recommends that:
You may want to consider taking supplements if:
The optimal daily dosage is a matter of continuing debate.
Some have that adults can safely take 20 micrograms (800 IU). Some have even suggested that 50 micrograms (2,000 IU) would be an optimal dose. Current UK guidelines recommend that adults take no more than 25 micrograms per day.
In the UK, a government committee is currently considering the evidence to see if the UK guidelines need to be amended. A report is expected in 2014.
If you are considering taking vitamin D supplements on a long-term basis, first check with your GP or the doctor in charge of your care that it is safe to do so.
We have seen that vitamin D deficiency could be a health problem in this country, but that doesn't mean everyone should be taking vitamin D supplements.
For most of us not in at risk groups, just making more of an effort to get outdoors during the summer months should be enough. Remember, aim for small and frequent bursts of sun exposure rather than long sessions, as these could damage your skin.
A final point is that if vitamin D does help protect against chronic diseases such as cancer, the level of protection is likely to be modest. Claims that vitamin D is a wonder drug are not supported by the current evidence.
So, if you are smoking 20 cigarettes a day and eating high-fat food, vitamin D isn't going to do much to keep you healthy.
The five most important steps to a healthier life are: