Does Vitamin D "Deplete" Magnesium? What's Really Going On

With insight from Dr. Bonnie Mallard, immunologist and ImmunoCeutica co-founder

Quick answer: No. Vitamin D doesn't deplete magnesium. It needs magnesium to become active in the body. The enzymes that convert vitamin D in the liver and kidneys depend on magnesium, so people who notice low magnesium after starting vitamin D were most likely already low. More than half of Canadian adults don't get enough magnesium from food. Testing your vitamin D, and retesting after 12 weeks, is the best way to know where you stand.

We recently saw a post on Instagram that had us concerned, not because the person was wrong to ask questions, but because the answers they were getting could steer people away from a nutrient many Canadians need. 

The writer described taking a standard 2,000 IU of vitamin D3 and then finding they were low in magnesium, B vitamins, vitamin A and iron. They concluded that vitamin D had "depleted" those nutrients. The replies went further, calling supplemental vitamin D "rat poison" and saying sunshine is all anyone needs.

It's an understandable reaction. When you start a supplement and then get a surprising lab result, it's natural to blame the supplement. But the science points to a different explanation.

Vitamin D needs helpers to do its job

"Vitamin D requires cofactors to become fully active, and these include magnesium," explains Dr. Mallard. "Vitamin D isn't depleting them. It requires them. So people who notice these deficiencies were likely already low in the nutrients vitamin D depends on."

The vitamin D you take, or make in your skin, isn't active yet. The body converts it in two steps, first in the liver and then in the kidneys. A review in the Journal of the American Osteopathic Association found that all of the enzymes that process vitamin D appear to need magnesium to work, and concluded that getting enough magnesium is essential to get the full benefit of vitamin D.

A good way to picture it is a factory. Vitamin D is the raw material and magnesium is one of the workers on the assembly line. Sending more raw material into the factory doesn't "use up" workers who were never there. It just makes a staffing shortage easier to see.

The clinical evidence

A randomized trial published in The American Journal of Clinical Nutrition tested this directly. Researchers gave 180 adults aged 40 to 85 either magnesium or a placebo and tracked their vitamin D levels. They concluded that optimal magnesium status may be important for optimizing vitamin D status. The trial also found that magnesium had a regulating effect in people with high vitamin D levels, which suggests magnesium helps keep vitamin D balanced in both directions.

Magnesium shortfalls are common in Canada. Statistics Canada's most recent national nutrition survey found that more than half of Canadian men (58%) and women (66%) weren't getting enough magnesium from food, and that the gap grows with age. That means many Canadians starting vitamin D may already be running low on the very nutrient it depends on.

What about B vitamins, iron and vitamin A?

The science on magnesium as a vitamin D cofactor is well established. There's no comparable evidence that a normal vitamin D dose drains B12, folate, iron or vitamin A from the body.

A more likely explanation is that nutrient shortfalls tend to travel together, and Canadian data shows how common they are. The same national survey found inadequate intakes of some B vitamins, as well as vitamin A, vitamin D, magnesium and calcium, among Canadian adults. Nearly 30% of women aged 19 to 50 weren't getting enough iron. Often these gaps are only discovered because someone finally gets bloodwork after starting a supplement. The deficiency was already there; the test just revealed it.

"It's the same ingredient as rat poison"

This claim comes up often. Some rodenticides do use cholecalciferol (vitamin D3). They work by delivering an enormous dose relative to a small animal's body weight, which drives blood calcium to dangerous levels.

The key principle in toxicology is that the dose makes the poison. 

Water, salt and iron are all harmful in large enough amounts. Health Canada sets the Tolerable Upper Intake Level for vitamin D at 4,000 IU per day for adults, and in 2021, after a safety review, it set 2,500 IU per day as the maximum for non-prescription supplements. 

We've covered this in detail, including how vitamin D rodenticides differ from older anticoagulant poisons and what to do if a pet gets into bait, in our article Vitamin D in Rodenticides.

More isn't better, though. Vitamin D is stored in the body's fat, so very high doses taken over time can build up. That's exactly why testing matters.

"Just get it from the sun"

Sunshine is a wonderful source of vitamin D....when it's available. In Canada, for a good part of the year, it isn't.

The landmark study on this measured sunlight in Edmonton, where skin produced no vitamin D from October through March. Even in Boston, which sits south of Toronto, Ottawa and Montreal, skin made none from November through February.

Canadian blood data backs this up. The Canadian Health Measures Survey (2016–2019) found that about two-thirds of Canadians had vitamin D levels above 50 nmol/L, which leaves roughly a third below it. Statistics Canada also found that the chance of having levels below 30 nmol/L doubles during winter. Even summer isn't a guarantee, as we explain in Summer Sun and Vitamin D.

Food isn't the easy fallback it used to be, either. The animals that produce our milk, eggs and meat have largely moved indoors, and it's changed how much vitamin D our food actually contains. Read more in The Hidden Reason Your Food Has Less Vitamin D.

Vitamin D also works alongside vitamin K2, which helps direct calcium to where the body needs it. Learn more in Why Vitamin K2 Deserves More Attention Than It Gets.

Test, don't guess

The online post gets one thing right. People deserve better information about supplements. But the answer isn't to fear vitamin D. It's to understand that nutrients work as a team, and to know your own numbers. Here's what vitamin D levels mean in Canada and the range we recommend aiming for.

If you're taking vitamin D, a few practical steps help:

1. Know your starting level. Test your vitamin D before you supplement so you have a baseline. Ideally test both in the summer and winter, so you have a baseline for both.
2. Look at the whole picture. Make sure your diet includes magnesium-rich foods like leafy greens, nuts, seeds, legumes and whole grains, and talk to your healthcare provider about your overall nutrient status.
3. Retest after 12 weeks. Retesting shows whether your dose is right for you and whether your levels have reached the optimal range. Here's how long vitamin D supplements take to work.

The ImmunoCeutica Vitamin D Test Kit makes that simple to do from home. 

Reviewed by Bonnie A. Mallard, Ph.D., University of Guelph & ImmunoCeutica Inc., and Emily Dutka, MSc. This article is for educational purposes and does not constitute medical advice. Speak with your healthcare provider before starting any supplement.

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References

1. Uwitonze AM, Razzaque MS. Role of Magnesium in Vitamin D Activation and Function. J Am Osteopath Assoc. 2018;118(3):181-189. doi:10.7556/jaoa.2018.037
2. Dai Q, et al. Magnesium status and supplementation influence vitamin D status and metabolism: results from a randomized trial. Am J Clin Nutr. 2018;108(6):1249-1258. doi:10.1093/ajcn/nqy274
3. Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3. J Clin Endocrinol Metab. 1988;67(2):373-378. doi:10.1210/jcem-67-2-373
4. Health Canada. Notice: Prescription Drug List (PDL): Vitamin D. February 22, 2021.
5. Vanderbilt University Medical Center. Study shows magnesium optimizes vitamin D status. December 2018.