Tuesday June 18 2013
The majority of vitamin D comes from sun exposure
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 toxicity
Dangerously high levels of vitamin D can result in a build-up of vitamin D in the blood, resulting in symptoms such as:
- feeling and being sick
- loss of appetite
- frequent urination
Vitamin D toxicity is uncommon and usually only affects people who have been taking vitamin D supplements well above the recommended dosage for several months.
However, some medical conditions can make you more sensitive to the effects of vitamin D, such as liver or kidney disease. This means that sometimes vitamin D toxicity can happen at a lower level. This is why it is a good idea to check with your GP if you do have a history of chronic health conditions before beginning to take supplements.
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.
Can vitamin D help in multiple sclerosis?
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.
Does lack of vitamin D leave us susceptible to flu?
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.
Can vitamin D help treat uncontrolled asthma?
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.
Does taking vitamin D prevent bone fractures?
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.
Can vitamin D prevent cancer?
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.
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."
Vitamin D and mortality
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 major 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.
Who should take vitamin D supplements?
How is vitamin D dosage measured?
Confusingly, there are two measuring systems for vitamin D – international units (IU) and micrograms (µg or mcg). One microgram is equivalent to 40 international units.
Taking 10 micrograms of vitamin D a day would be the same as 400 IU.
The Department of Health currently recommends that:
- all pregnant and breastfeeding women should take a daily supplement containing 10 micrograms (400 IU) of vitamin D to ensure the mother's requirements for vitamin D are met and to build adequate foetal stores for early infancy
- all babies and young children aged six months to five years should take a daily supplement containing vitamin D in the form of vitamin drops to help them meet the requirement for this age group of 7-8.5 micrograms (280-340mg) of vitamin D a day
- babies fed infant formula will not need vitamin drops until they are receiving less than 500ml (about a pint) of infant formula a day, as these products are already fortified with vitamin D
- breastfed infants may need to receive drops containing vitamin D from one month of age if their mother has not taken vitamin D supplements throughout pregnancy
- people aged 65 years and over and people not exposed to much sun should also take a daily supplement containing 10 micrograms (400 IU) of vitamin D
What about the rest of us?
You may want to consider taking supplements if:
- your skin tone is naturally dark
- you are obese
- your lifestyle means your exposure to the sun is limited – for example you are housebound, you work a nightshift or you live in a part of the world where there is little sunlight
The optimal daily dosage is a matter of continuing debate. Current UK guidelines place the recommended daily amount for adults at 10 micrograms (400 IU).
Some have argued that this dosage has been set too low for people to receive the full benefits, and that adults can safely take 20 micrograms (800 IU). Some have even suggested that 50 micrograms (2,000 IU) would be an optimal dose.
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 high-risk groups, just making more of an effort to get outdoors should be enough. Remember, aim for small and frequent bursts of sun exposure rather than long sessions, as these could damage your skin.
If you are overweight or obese, you may want to lose weight as this will allow your body to make more efficient use of vitamin D. There are many other benefits of achieving a healthier weight – the NHS Choices 12-week weight loss guide can help you lose weight safely.
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:
- quitting smoking
- taking plenty of exercise
- eating a healthy diet
- trying to maintain a healthy weight
- moderating your consumption of alcohol
Links to the 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
Edited by NHS Choices