This Little-Known Factor Doubles (Or More) Your Vitamin D Needs

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.

As you probably know, vitamin D is a fat-soluble vitamin1, 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 dilution2, 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 calcitriol3, 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.*

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 research4 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.*

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.

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.