As you probably know, vitamin D is a fat-soluble vitamin, which means that it is absorbed, transported, metabolized, and stored similarly to fats. And while you probably know that this means you should take your vitamin D supplement with a source of fat so that it’s absorbed properly, what you likely don’t know is that it influences your vitamin D status in a perhaps surprising way.*
Since vitamin D is fat-soluble, it is stored in body fat, explains board-certified endocrinologist Brittany Henderson, M.D. So, simply put, the more body fat a person has, the more vitamin D they store in that body fat, leaving less vitamin D in circulation throughout the body (and meaning that people with higher levels of body fat are more likely to come up short when their serum D levels are tested).
As mbg’s nutrition scientist and director of scientific affairs Ashley Jordan Ferira, Ph.D., R.D.N. explains, “less serum vitamin D [aka, 25(OH)D], circulating in our blood means less D is available to our kidneys and other target tissues for conversion to its active, hormone form for its pleiotropic actions throughout our body.”
Ferira goes on to say, “And mind you, that’s just one way that adiposity can compromise vitamin D status and its ability to do its widespread job in our bones muscles, immune cells, brain, liver, etc.”* What she means is that research now suggests that there are other mechanisms at play in the relationship between body composition (how much fat versus lean tissue someone has) and their vitamin D status and needs.
The other mechanism with the strongest science behind it: Volumetric dilution, which simply means that the vitamin D people with obesity consume (from diet, beverages, supplements) or produce from the sun is just more diluted within the body, which then contributes to lower serum vitamin D status overall. Based on this, researchers suggest that vitamin D intake recommendations should really be based on body size instead of “one-size-fits-all.”*
Another mechanism, albeit one still being full worked out via research: That those with obesity experience increasing circulating levels of calcitriol, the active hormone form of vitamin D in the body, which then “turns off” the production of 25-hydroxyvitamin D, or 25(OH)D, which is the form of vitamin D healthcare providers measure when performing a blood test to assess vitamin D levels. As a result, people with higher amounts of body fat have lower vitamin D status when tested.*
As Ferira concludes, “whether sequestration into fat tissue, a dilution factor to body size, or wonky feedback loop metabolism of circulating vitamin D levels—or all three—adiposity inserts nuance into one’s vitamin D status equation, so it’s time to talk more about it.”