If we've learned anything about vitamin D, it's that getting ample amounts of the important nutrient is a little more complicated than meets the eye. A whole slew of factors affect our ability to produce the sunshine vitamin from the actual sun (ironic, right?), food provides pretty mediocre amounts (unless, of course, you're down to chug bottles of cod liver oil, in which case, good on you), and many of the supplements lining store shelves contain less-than-effective doses.
And that's not all, either. "Every human has different genes, biochemistry, and more," functional nutrition dietitian Whitney Crouch, RDN, CLT, previously told mbg. This means that, in addition to the external factors that influence how much vitamin D we actually get, individual health factors also play a role in determining the amounts different people need in order to reach (and maintain) a level that promotes overall health and well-being for life.
One of these factors—which seems to be scarcely discussed but incredibly important—is the amount of adiposity (i.e., body fat) we have. Here's what we should all know about the relationship between body composition and vitamin D, and what it means in our quest to finally make optimal vitamin D status achievable for all people.
How body fat influences vitamin D status.
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., RDN, 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 health care 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 due 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."
Echoing Ferira's sentiment, Henderson shares, "Though this [adiposity and vitamin D interaction] is well understood in the medical community, I don't see it reported much in resources aimed at patients; however, it is important for clinicians and for patients when considering supplementation."
The reason for this: "Serum vitamin D levels tend to be lower in patients with higher amounts of body fat and can be more difficult to normalize, even with high-dose vitamin D supplementation," Henderson explains.* In fact, leading research suggests that people with obesity, specifically, need up to two to three times more than the standard recommendation in order to satisfy their vitamin D requirement.*
So, while scientific literature and vitamin D experts often tout 5,000 I.U. of vitamin D3 per day as the sweet spot for achieving optimal serum vitamin D status of 50 ng/ml, the truth is that this recommendation falls quite short for many people when body composition is factored in.*
Statistics indicate that 73.6% of adults over the age of 20 in the U.S. have overweight or obesity (specifically, 42.5% meet the criteria for obesity), which means that current standard recommendations for vitamin D don't truly reflect the needs of a significant portion of the population.*
How to support optimal vitamin D status in all people.
If there's a lesson to be learned here, it's reinforcement that "there is no one-size-fits-all approach to optimizing vitamin D levels," says Henderson.
After confirming their vitamin D status with their health care provider, people with higher amounts of body fat may need to supplement with a significant amount of vitamin D3 per day in order to move the needle.* In many cases, Henderson recommends upward of 10,000 I.U. of vitamin D3 per day. (Here's why the D3 form is superior to D2.)* Research similarly suggests sticking with a higher daily maintenance dose once the target vitamin D status is reached.*
Because vitamin D is stored in body fat, those with higher amounts of fat tissue typically have low circulating levels of vitamin D and need as much as two to three times greater than the standard recommendation when supplementing with vitamin D3 in order to meet and maintain their body's needs.* Broader awareness of this important fact will greatly serve the health and well-being of the collective.
As always, it's important to work with your health care provider to regularly test your personal vitamin D status and supplement accordingly to ensure you get the vitamin D you need to achieve healthy status for life.* (Here are some of our favorite vitamin D supplements to get you started.)
Lauren Del Turco, CPT is a freelance health and wellness writer, editor, and content strategist who covers everything from nutrition to mental health to spirituality. Del Turco is also an ACE-certified personal trainer. She graduated from The College of New Jersey with a Bachelor of Arts in English and Creative Writing. When she’s not on deadline, you’ll find Del Turco hiking with her dogs, experimenting with new plant-based recipes, or curled up with a book and tea.