Thursday February 7 2013
Researchers speculate vitamin D may get 'trapped' inside fat
"Study finds obesity can lead to lack of vitamin D," BBC News has reported.
This fascinating, insightful and accurate BBC story highlights a new danger to add to the list of problems caused by obesity.
The headline is based on a large, complex and broad study investigating the link between obesity and vitamin D levels in the body.
Previous research has suggested a link between vitamin D and obesity. Until now it has been unclear whether obesity caused vitamin D deficiency or whether low levels of vitamin D made people more likely to put on weight.
This research found that people with genetic variations that are known to be associated with obesity had lower levels of vitamin D. Conversely, people with genetic variations linked with lower levels of vitamin D were no more likely to be obese.
This would strongly suggest that obesity causes lower levels of vitamin D, rather than the other way round. The researchers speculate that vitamin D may become ‘trapped’ inside fat tissue so that less of it is available to circulate inside the blood.
Before firm conclusions can be drawn more evidence is needed from different sources that have looked at the effects of BMI on vitamin D levels. A convincing explanation for why this might be the case is also needed.
Where did the story come from?
The study was carried out by a large collaboration of researchers from US and European institutions. It was funded by the British Heart Foundation and the UK Medical Research Council. The study was published in the peer-reviewed medical journal PLOS Medicine.
The BBC News coverage was factually accurate and was particularly helpful because it included a concise summary of complex research. It put the research in context with a quote from Professor David Haslam, from the National Obesity Forum. He said that "food intake and genetics all play a part in obesity – but this research is a reminder that physical activity, like walking the dog or going for a run out in the sunshine, shouldn't be forgotten and can help correct both weight and lack of vitamin D”.
What kind of research was this?
This study combined existing data from genetic studies to investigate the link between vitamin D levels in the body and body mass index (BMI). People with a BMI of 30 or more are considered obese.
The researchers in this study used an approach (known as bi-directional Mendelian randomisation analysis) that can help to establish whether an exposure is causally linked to an outcome of interest. This means it determines whether one thing causes another or whether the association occurs by chance.
This approach aimed to establish whether vitamin D levels caused or were caused by high BMI. It used both physical and genetic measures. The physical measures were BMI and vitamin D levels and the genetic measures were genetic variations associated with vitamin D levels and BMI.
The researchers hypothesised that if lower vitamin D levels in some way ‘caused’ obesity, a genetic variant associated with lower vitamin concentrations should be associated with BMI. Alternatively, if obesity leads to lower vitamin D status, then genetic variants associated with higher BMI should be related to lower vitamin D concentrations.
Although this type of study can give information about possible causal links, a large body of evidence of different types needs to be accumulated before a firm causal link can be established.
What did the research involve?
The researchers primarily used information from 21 studies (42,024 adult participants of European ancestry) to establish genetic links between:
- 12 BMI-related genetic variations and BMI
- four vitamin D-related genetic variations and vitamin D levels
For each individual a genetic “score” was generated which indicated the strength of their genetic tendency towards higher BMI or lower vitamin D levels.
Associations between the vitamin D-related genetic variations and BMI were further tested in a group of 123,864 people taking part in the Genetic Investigation of Anthropometric Traits (GIANT) study.
The researchers then pieced the two elements of the study together and performed statistical analysis to test whether the genetic variations associated with BMI and vitamin D levels were linked to either BMI or vitamin D levels in the body.
The statistical analysis made adjustments for some factors that could influence results (potential confounding variables).
What were the basic results?
The researchers found that:
- Every unit increase in BMI (1kg/m2) was associated with a 1.15% reduction in the level of vitamin D in the blood. This finding was confirmed in a different analysis that showed each 10% increase in BMI score was associated with a 4.2% lower level of vitamin D. The evidence also showed that every point increase in BMI genetic variation score was associated with a small but statistically significant 0.06% decrease in vitamin D concentration.
- The BMI-associated genetic variations were associated with both higher BMI and lower vitamin D levels.
- As the researchers expected, the genetic variations known to be associated with vitamin D levels were strongly associated with vitamin D levels in the body but, crucially, not with BMI.
- No association was seen between vitamin D genetic variation scores and BMI, a finding that was confirmed in the large GIANT study.
How did the researchers interpret the results?
Piecing together the complex pieces of the jigsaw above, the authors concluded that their findings suggest that higher BMI could lead to lower vitamin D levels, but that any corresponding effects of vitamin D levels on BMI are likely to be small.
From a public health perspective, they noted that, “population-level interventions to reduce BMI are expected to decrease the prevalence of vitamin D deficiency”.
This complex study used physical and genetic measures to attempt to establish whether there could be a causal link between obesity and lower vitamin D levels in Caucasian individuals.
The study results suggested that it was higher BMI that caused lower vitamin D levels rather than the other way round.
This interesting finding highlights a potential additional benefit of reducing obesity in that it may also reduce the prevalence of vitamin D deficiency.
However, by itself this research cannot prove that higher BMI directly causes lower vitamin D levels. A larger body of different types of evidence, including evidence showing whether reducing BMI can affect vitamin D levels, is needed before firm conclusions can be drawn.
The authors note that vitamin D is stored in fatty tissue, and that “the most likely explanation” for the association is that obese people store more vitamin D in their fat, and have less vitamin D circulating in their blood.
It is important to remember than the main modifiable factors known to influence vitamin D levels are exposure to sunlight and dietary vitamin D intake.
Public health advice remains unchanged – maintaining a healthy weight is beneficial to physical and mental health.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.