A PhD and RD Explains Why Your Vitamin D Level Might Be Stubborn (And What To Do)

If there were ever a nutrient to blow the minds of scientists, clinicians, and consumers over the past several decades because of its wide-reaching health impact, it would have to be the fat-soluble wonder vitamin D. And a la Dirty Dancing, nobody can put this fascinating nutrient in a corner. 

We have covered in detail how scarce vitamin D is in our diet (yes, even if you eat fish), and as a result, how massive the vitamin D gap is in the U.S. and across the world (D deficiency is the other pandemic). We also have explored the complexities and inherent risk of relying on the sun for vitamin D sufficiency. 

And while the case for daily vitamin D supplementation is incredibly strong, don't take our word for it; let the science do the talking: 93% of Americans fail to take in just 400 I.U.1 of vitamin D daily from foods and beverages, and we actually need 12 times more than that (about 5,000 I.U.). Even when you throw sunshine and supplements into the mix, over 40% of U.S. adults still meet the clinical criteria for insufficiency2.

Which leads me to a question my fellow dietitian and respected colleague Whitney Crouch, RDN, CLT, and I were pondering recently: How is it that even with vitamin D supplementation, the beautifully simple solution to a widespread nutrient deficiency, such a large number of folks still don't meet the mark? Crouch and I have both seen family, friends, and clients struggle to get their vitamin D levels to significantly budge or stay up despite supplementing.

The top culprit of what I'll dub "vitamin D resistance" is the absence of a truly daily vitamin D supplementation routine. Listen, we all get busy and miss the occasional day (Saturdays for me), and that's OK because vitamin D levels are resilient. (You can also double up with zero safety concerns if you miss a couple of days, by the way.) 

But, plain and simple: People who prioritize adherence to their daily D ritual consistently achieve and maintain healthy levels of vitamin D (i.e., a 25-hydroxyvitamin D level greater than or equal to 50 ng/ml).* That's assuming their supplement is high-quality and sufficiently potent, of course. 

When it comes to testing vitamin D status in healthy individuals (i.e., folks without medical issues, particularly those related to the liver or kidneys), the go-to status blood biomarker used in clinical practice to assess vitamin D status is total serum 25-hydroxyvitamin D, or 25(OH)D for short. 

What's important here is the word total. You see, total 25(OH)D is the sum of both vitamin D2 (ergocalciferol) and D3 (cholecalciferol) inputs. You definitely want the sum of the parts when measuring vitamin D status because different sources offer different types: natural and fortified foods and beverages can be either D3 or D2, sunshine is D3, medication is D2, and supplements can provide D3 or D2.

Not to mention, I have also seen cases in which people supplement with vitamin D2 (which is a bad idea; we explain why here) but their health care provider inadvertently ordered or only reported the 25(OH)D3 lab test result. This falsely made it seem like the vitamin D supplementation regimen was ineffective. I've seen the opposite mistake happen too.

To detect and treat vitamin D deficiency or insufficiency3, you need cutoffs. A total serum 25(OH)D level less than 20 ng/ml is vitamin D deficiency, while a result greater than 20 but less than 30 ng/ml is vitamin D insufficiency. Both are concerning results and should be calls to action.

So what goal should you aim for, then? Well, first of all, you'll want to avoid lingering around 30 ng/ml. Instead, aim for 50 ng/ml or higher.

Why? Board-certified endocrinologist Brittany Henderson, M.D., puts it this way: "As an endocrinologist, I know that achieving optimal serum 25(OH)D levels in the 50-plus ng/ml range is imperative for immune health, bone health, and more. It's difficult for most standard vitamin D formulations to achieve this."

The next culprit of a stubborn vitamin D level is jumping the gun on testing. Here's a common scenario: A person gets their baseline vitamin D status result, receives the news that they're vitamin D insufficient or deficient, and starts taking a vitamin D supplement.* They are eager to correct the deficiency and thus, complete follow-up testing too soon. This will lead them to discover a (falsely and prematurely) low vitamin D result. 

For all you keto aficionados, passing the fat is a regular mealtime practice. But fats are relevant to supplement consumption, too.

Vitamin D is, by design, a fat-soluble micronutrient, which means it's optimally absorbed in the gut when some fat is consumed with it, or directly before taking it. In fact, the question of how to time your vitamin D supplement is less about a.m. versus p.m. and more about when there's some fat around. Without it, your absorption pays the price!

Just like with basketball free throws, follow-through is everything when it comes to supplementing with vitamin D. And for your supplement to have a shot at scoring (aka increasing your baseline status), you need to follow through.* 

This might sound similar to previous culprits, but I'm actually talking about adhering to your supplement routine and retesting beyond those first three months. 

Adiposity, or the amount of body fat one has, is one key facet of body composition (as are lean mass and bone density). Research has repeatedly demonstrated that fat tissue is inversely correlated with vitamin D status (i.e., higher adiposity, lower vitamin D levels).

The reasons are numerous and still shaking out via emerging research. One major factor is that fat tissue has a tendency to store fat-soluble compounds like vitamin D, making less of this essential nutrient available to circulate and be activated to support our cells, tissues, and organs throughout the body.*

The pragmatic bottom line here: Sub-potent vitamin D dosing that lacks personalized consideration for body composition (specifically moderate to higher adiposity) is very common.

What about other "buddy" nutrients that affect vitamin D status? While vitamin K2 sometimes finds its way into vitamin D3 supplements, it has nothing to do with vitamin D absorption, bioavailability, or status.

Additionally, while vitamin D supports calcium absorption and levels (and both nutrients are hallmark bone essentials),* the converse is not true. Vitamin D does not depend on calcium for absorption.

Interestingly though, another essential macromineral does require our attention. I'm talking about magnesium (which over 40% of American adults1 fail to meet their daily needs through diet).

Suboptimal magnesium status is relevant to vitamin D because the mineral is required for binding 25(OH)D to the vitamin D binding protein (VDP) for circulation around the blood and delivery to tissues throughout the body. Magnesium is also required for the conversion of 25(OH)D to the active 1,25(OH)2D hormone form. Both of these mechanisms will affect vitamin D status. 

Plus, as Crouch sagely warns: "Magnesium deficiency and insufficiency are often missed due to conventional lab testing techniques."

I've said it before, and I'll say it again: Every person's vitamin D response is individualized (hello, genes). And while the above intel and recommendations to solve stubborn D levels are pertinent to most people, they are not to those individuals with specific medical issues. In fact, sometimes vitamin D deficiency can be a clue to such issues before they're even diagnosed.

For example, there could be a gastrointestinal issue that has significantly reduced viable surface area for vitamin D absorption in the gut. Or, perhaps a problem causing suboptimal liver or kidney function. (These two organs are involved in the major steps that convert vitamin D into its circulating form and active, hormone form.)

Interestingly, a compelling body of research has even revealed an inverse relationship between inflammatory pathways in the body5 and vitamin D status. (They likely mess with its normal metabolism and utilization and increasing one's D needs.)