Observational studies have consistently reported an increased risk of vitamin D deficiency in those who are obese. However, the underlying explanations and direction of causality are unclear. Does obesity cause vitamin D deficiency, does vitamin D deficiency cause obesity or is it a little of both?
Activated vitamin D (1,25-dihydroxyvitamin D) may influence the mobilization of free fatty acids from the adipose tissue as well as metabolism of fat cells. Rat experiments have shown that large doses of vitamin D lead to increases in energy expenditure, a mechanism of weight loss. However, three randomized controlled trials testing the effect of vitamin D supplementation on weight loss in obese or overweight individuals showed no effect. On the other hand, there have been randomized controlled trials that have shown a very modest weight reduction after vitamin D supplementation, too.
So which is it? A recent new kind of genetic study, called Mendelian randomization, wanted to find out, does obesity more cause vitamin D deficiency, or does vitamin D deficiency more cause obesity?
In Mendelian randomization, cause is assumed when associations exist between genetic variants that mimic the influence of a deficiency and the outcome of interest. Because gene variants do not change over time and are inherited randomly, they are not prone to confounding and are free from the possibility of reverse causation. Thus, if a lower vitamin D status leads to obesity, genetic variants associated with lower 25(OH)D concentrations should be associated with obesity, and if obesity leads to a lower vitamin D status, then genetic variants associated with obesity should be associated with lower 25(OH)D concentrations.
Recently, Doctor Karani Vimaleswaran, working under senior author Professor Elina Hyppönen, both of the UCL Institute of Child Health in London, used Mendelian randomization to find that obesity more causes low vitamin D rather than the other way around.
The researchers created a score based on BMI-related gene variants and 25(OH)D scores based on vitamin D gene variants. Using information on more than 42,000 participants from 21 studies, the researchers showed that the gene variants for obesity 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 about a 4% decrease in 25(OH)D concentrations. By contrast, although 25(OH)D scores were strongly associated with 25(OH)D levels, neither score was associated with BMI.
The authors concluded:
“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. These findings suggest that population-level interventions to reduce obesity should lead to a reduction in the prevalence of vitamin D deficiency and highlight the importance of monitoring and treating vitamin D deficiency as a means of alleviating the adverse influences of obesity on health.”
This makes sense to me. While vitamin D might help you feel and be more active, while vitamin D may play a role in adipose tissue, clearly these effects are minute compared to the heavy hand obesity plays in diluting your vitamin D stores and making you deficient. It’s important to recognize that if you weigh more, you will need more vitamin D than someone who weighs less.