Previous research has found a link between lower levels of vitamin D and higher body weight, causing some to speculate that vitamin D deficiency may be a factor in obesity. The problem with this sort of evidence is that while it can show a link between vitamin D deficiency and, say, obesity, we can’t tell if:
1. vitamin D deficiency can cause obesity
2. obesity can cause vitamin D deficiency
3. neither (the two things are merely associated with each other)
An international group of researchers have attempted to discern better the relationship between vitamin D and body weight, and published their findings this week in the on-line journal PLoS Medicine [1].The researchers used genetic studies in their analysis. Here’s an edited version of the journal editor’s account of the research and its findings:
Here, the researchers use bi-directional “Mendelian randomization” to examine the direction and causality of the relationship between BMI and 25(OH)D [vitamin D]. …if a lower vitamin D status leads to obesity, genetic variants associated with lower 25(OH)D concentrations should be associated with higher BMI, and if obesity leads to a lower vitamin D status, then genetic variants associated with higher BMI should be associated with lower 25(OH)D concentrations.
…the researchers showed that the BMI allele score was associated with both BMI and with 25(OH)D levels among the study participants. Based on this information, they calculated that each 10% increase in BMI will lead to a 4.2% decrease in 25(OH)D concentrations.
By contrast, although both 25(OH)D allele scores were strongly associated with 25(OH)D levels, neither score was associated with BMI.
These findings suggest that a higher BMI leads to a lower vitamin D status whereas any effects of low vitamin D status on BMI are likely to be small. That is, these findings provide evidence for obesity as a causal factor in the development of vitamin D deficiency but not for vitamin D deficiency as a causal factor in the development of obesity.
In short, this study found that genes associated with higher body weight were associated with lower vitamin D levels. On the other hand, genes associated with lower vitamin D levels were not associated with a raised risk of obesity. This suggests that obesity drives low vitamin D levels, but that low vitamin D levels do not drive obesity.
Against this, there is some evidence that vitamin D deficiency might have a role in obesity. In one study, vitamin D supplementation was found to speed fat loss in women [2]. While the authors of this latest research acknowledge this study, they also say that other similar (and larger) studies have failed to confirm this effect.
As to why heavier individuals are at increased risk of vitamin D deficiency, the authors mention of variety of potential mechanisms including:
- dilution of vitamin D in a larger body mass
- the sequestration (preferential storage) of vitamin D in the fat tissues (taking it out of the bloodstream)
- more rapid conversion of vitamin D as measured in the bloodstream to a more active form in more obese individuals
- differences in lifestyle factors
It occurs to me that a one potential risk factor relates to the fact that most of our vitamin D requirements come from the action of sunlight on the skin. All other things being equal, lighter individuals will tend to have a higher ‘surface area to volume ratio’, which means that for a given amount of sun exposure, they will achieve higher vitamin D levels compared to bigger individuals.
Another thing worth considering is concerns amount of sunlight exposure. Even in larger people go out in the sun as much as smaller individuals, I suspect that (generally speaking), larger people are likely to expose less of their skin to the sun, and may be at increased risk of vitamin D deficiency because of this.
Anyway, we’ll likely never know the full detail of why obesity might drive vitamin D deficiency, but this research does serve as a reminder that some people are at increased risk of this deficiency.
Increasing numbers of people (and their doctors, it seems) are interested in optimising vitamin D levels. Commenter Ted Hutchinson recently made me aware of a guide to vitamin D dosing based on achieving levels of about 50 ng/ml (125 nmol/l). Thanks to Ted for that.
References:
1. Vimaleswaran KS, et al. (2013) Causal Relationship between Obesity and Vitamin D Status: Bi-Directional Mendelian Randomization Analysis of Multiple Cohorts. PLoS Med 10(2): e1001383. doi:10.1371/journal.pmed.1001383
2. Salehpour A, et al. A 12-week double-blind randomized clinical trial of vitamin D3 supplementation on body fat mass in healthy overweight and obese women. Nutrition Journal 2012;11:78
