New Research: The Two Reasons Your Vitamin D Supplement May Not Be Working

You can’t walk through a supplement aisle without knocking over a Vitamin D-related section. Vitamin D is everywhere. And for good reason, because it’s one of the most-supplemented nutrients in the United States. So what’s the issue? Well, it doesn’t perform as well as it should for most people. And the reason comes down to two factors: body weight and magnesium. In a recent study, 42% of Americans were deficient or borderline in Vitamin D. Why? Probably because we don’t go outside as much as we used to. Between the office cubicle life and social media, we’re much more content to sit inside and watch the world from inside there. But there are other reasons we may be deficient. And this one is pretty sad. Vitamin D is fat-soluble, which means that your body keeps it out of the bloodstream. If you’re obese, this means that you get about 40% less Vitamin D from the same dose as someone who is not obese. And that translates to needing 2-3x the dosage size for obese people. And in case you hadn’t noticed, there is an obesity epidemic in the US.

“Mg [magnesium] deficiency is an overlooked cause of vitamin D deficiency, and vitamin D aids in Mg absorption.” — Deeptha Sukumar and Nathalie May, Drexel University, writing in Dietetics, August 2026

So with the information above, the knee-jerk reaction is to just “take more Vitamin D.” But that doesn’t have that great of a track record, either. Because jacking up your Vitamin D intake can have unintended consequences in a wide array of other areas. In one example, older adults were falling more when their Vitamin D passed 40-45 ng/ml in their blood. Another trial found postmenopausal women’s reaction time slowed when they were taking 4,000 IU/day instead of 600 IU/day. And a third trial of people taking 10,000 IU/day for three years had lower bone density when compared to the group taking 400 IU/day. 

This is where Magnesium comes in

In short adding magnesium to your diet had a better impact than just blindly raising your dose. Why? Because it helps convert Vitamin D into the form your body measures and uses in a much smoother way. Without hitting you over the head with stats and clinical trial data, let me just give you two examples. In one trial 360mg of magnesium + 1,000 IU of Vitamin D raised the amount of Vitamin D in the blood by 6.2 ng/ml vs 2.1 ng/ml for those just taking the 1,000 IU of Vitamin D without the magnesium. That’s a huge absorption jump! There’s another trial of 83 Vitamin D-deficient women aged 40-55. Over the course of 8 weeks they took 50,000 IU of Vitamin D per week and 250 mg of magnesium per day. And they saw stronger handgrips, quicker “get-up-and-go” (I guess they timed them getting off the floor) and lower hs-CRP inflammation vs a placebo. So there’s a lot of evidence here that magnesium is Vitamin D’s best friend.

“Physicians treating overweight or obese patients for vitamin D deficiency should consider co-supplementation with Magnesium and/or encourage intake of Magnesium-rich foods to improve vitamin D status and supplementation outcomes in this population.” — from the review’s conclusions

As always we want to put a caveat with all this data. Some of these clinical trials we mentioned are small. We need more data, and you should always talk to your doctor before taking any supplement. But the good news here is that this is not a megadose story. This is just letting two supplements work together so you can actually take less. Sounds like pretty good news to us. Sources: Role of Magnesium in Vitamin D Supplementation Outcomes in Adults Living with Obesity — Dietetics, August 2026 · Vitamin D and Magnesium Co-Supplementation in Vitamin D-Deficient Middle-Aged Women — Biological Trace Element Research, 2021

Cory Jones

Cory Jones has spent 20 years in media and publishing and now leads the data and editorial operation behind The Greenest. He built and maintains the site’s Neighborhood Wellness Score — a proprietary dataset scoring food access, walkability, air quality, healthcare access, and health outcomes across 18,356 U.S. ZIP codes, drawing on CDC PLACES data, EPA air quality readings, and verified location intelligence. He is a huge fan of Rancho Gordo beans and tries to work out more than he actually works out.