Tuesday October 4 2011
Most people get enough vitamin D from their diet
“Fair-skinned people who are prone to sunburn may need to take supplements to ensure they get enough vitamin D,” reported the BBC.
This research looked at how vitamin D levels are associated with sun exposure, vitamin supplements, and the presence of certain genetic variations. It found that many participants had suboptimal vitamin D levels, defined in this study as below 60nmol/l. This was particularly the case in sun-sensitive individuals and in those with a particular variation in the gene encoding the vitamin D-binding protein. Meanwhile, higher vitamin D levels were associated with longer sun exposure, taking vitamin D supplements and using lower SPF sunscreen.
However, it is too soon to recommend supplements for fair skinned individuals and more research is needed. Furthermore, as the study was conducted in people with and without melanoma, some caution should be used if applying the results to the general population. Though the study shows some associations between some factors and vitamin D, its design cannot prove causation. It is also possible that many of the associations seen can be explained by differing behaviours; for example, people who are sun-sensitive are likely to cover up more.
Most individuals should get adequate vitamin D from their diet and from casual sun exposure. However, taking 10 micrograms (0.01mg) or less a day of vitamin D supplements is unlikely to cause any harm. If you are concerned about your vitamin D levels see your GP, who may recommend a vitamin D test.
Where did the story come from?
The study was carried out by researchers from the University of Leeds, Leeds Teaching Hospitals Trust, and the University of Pennsylvania. Funding was provided by Cancer Research UK and the NIH.
The study was published in the peer-reviewed journal Cancer Causes and Control.
This story was covered by the BBC and The Daily Telegraph. The BBC article quotes from Cancer Research UK, which says that it is ‘too soon to start recommending supplements’ but that most people could safely take 10 micrograms a day of vitamin D without any side effects.
What kind of research was this?
This was a cross-sectional analysis of a case-control study comparing people who had melanoma with matched people who did not (controls).
The researchers aimed to see if vitamin D levels in the blood were associated with reported sun exposure, dietary supplementation and the presence of certain genetic variations. This study design is appropriate for answering this type of question but is prone to the influence of other differences between the groups (those with cancer and those without) that they could not or did not measure.
What did the research involve?
For the participants who were the ‘cases’, 960 individuals with a new melanoma (an aggressive form of skin cancer) were recruited from a region of the UK between September 2000 and December 2005. Where possible, recruitment to the study and blood sampling took place between three and six months after diagnosis.
For the ‘controls’, 513 people, matched to the cases by sex and age, were randomly invited to take part. In addition, 174 siblings of the cases with melanoma also participated as controls.
All the participants completed questionnaires and interviews to determine weekday, weekend, holiday and overall sun exposure. They also reported their sex, natural hair colour at 18 years, sunburn frequency, propensity to burn, ability to tan, skin colour of inside arm and freckling as a child. Individuals with melanoma (the cases) were asked about their intake of supplements containing vitamin D. Eye colour and freckling scores were determined by research nurses.
A measure of deprivation (the Townsend score) was determined from the participant’s postcode. The researchers also took blood samples to test whether the participants carried certain ‘genetic variations' (single nucleotide polymorphisms).
Levels of vitamin D in the blood were measured through taking blood samples. Samples were taken from 92% of the cases, 74% of the sibling controls and a subset (38%) of the population controls.
For this study, the researchers carried out statistical analyses of this data to determine whether any of the factors they had collected data on correlated with vitamin D levels. The researchers adjusted their analysis to take into account season, body mass index (BMI), sex, age, Townsend score (a deprivation score) and case-control status (where appropriate).
What were the basic results?
Overall, suboptimal levels of vitamin D (<60nmol/l) were common among the participants, being observed in 63% of cases and 55% of controls.
Vitamin D levels were lower in sun-sensitive individuals and those with a particular genetic variations (single nucleotide polymorphism) at the gene coding for the vitamin D-binding protein. However, as the researchers discuss, the lower vitamin D levels in sun-sensitive individuals may predominantly be due to behavioural differences (they avoid sun exposure or cover up when in the sun).
Vitamin D levels were positively associated with total sun exposure and dietary supplementation, with higher levels in individuals with more exposure and in those taking supplements. Vitamin D levels were also positively associated with the use of low SPF sunscreen. The researchers suggest that this is because individuals combine using low SPF sunscreen with sun-seeking behaviour.
How did the researchers interpret the results?
The researchers say that ‘sun exposure was associated with increased vitamin D levels, but levels more than 60nmol/l were reached on average only in individuals reporting lengthy exposure (more than 12 hours a weekend)’.
The researchers also conclude that as sun-sensitive individuals did not achieve optimal levels of vitamin D in the blood, then supplementation ‘should be considered for the majority of populations living in a temperate climate, and melanoma patients in particular’. They suggest that very sun-sensitive individuals are ‘unable to sustain enough sun exposure to synthesise sufficient vitamin D while protecting themselves from sunburn’. They also say that individuals with certain genetic markers may require ‘higher levels of supplementation’.
This study aimed to determine which factors are associated with vitamin D levels in the blood. It found that vitamin D levels were associated with sun exposure and vitamin D supplementation, even after adjusting for season, BMI, sex, age, Townsend score (deprivation) and whether the individual had melanoma.
Vitamin D levels were also positively associated with sunscreen usage, and levels were lower in individuals who were sun-sensitive or who carried a certain genetic marker in the gene coding for the vitamin D binding-protein.
Importantly, the researchers do suggest that some of the associations may be due to behavioural differences (for example, going out into the sun once you have applied sunscreen or avoiding the sun/covering up if you are sun-sensitive). Furthermore, this study can only show associations between these factors and vitamin D levels and cannot determine whether one causes the other.
The study has two main limitations mentioned by the researchers:
- data were not collected about supplementation and serum vitamin D levels for all the controls, which could have led to inaccuracies in recording the average levels
- the data were collected as part of a case-control study involving melanoma patients and controls, which may make it difficult to apply the study’s findings to the general healthy population
According to the Consensus Vitamin D position statement, which was produced by several UK medical organisations and charities, there is no standard definition of optimal vitamin D levels. It does state that levels below 25nmol/l (10ng/ml) can be classified as ‘deficient’. Some scientists suggest that levels above 50nmol/l (20ng/ml) are ‘sufficient’, while 70-80nmol/l, (32ng/ml) is ‘optimal’. However, no randomised controlled trial (RCT) have been performed to determine this.
Vitamin D is essential and is required for good bone health. There is some evidence to suggest that it may protect against cancer, heart disease, diabetes, multiple sclerosis and other chronic diseases. These sorts of studies can be helpful in improving knowledge. Longer, larger studies of levels followed over time in healthy populations will be needed to determine the ideal vitamin D level.
It is too soon to recommend supplements for fair-skinned individuals, as more research is needed. Most individuals should be able to get adequate vitamin D from their diet and from casual sun exposure. However, taking 10 micrograms (0.01mg) or less a day of vitamin D supplements is unlikely to cause any harm. If you are concerned about your vitamin D levels, you should see your doctor who may recommend a vitamin D test.