I'm A PhD & RD: These Are The 4 Vitamin D Myths I See All The Time

If mbg director of scientific affairs Ashley Jordan Ferira, Ph.D., RDN, had a favorite micronutrient (a great ice-breaker question, if you ask us), it would be vitamin D. No surprise there: She did her doctoral research on the essential fat-soluble nutrient (in addition to weight management and bone health), so it's safe to say she knows a thing or two about the science behind the vitamin. 

See, vitamin D is naturally found in small amounts in a handful of foods4—which are helpful for preventing extreme vitamin D deficiency and its related (serious) consequences for bone mineralization. (For example, 1 cup of milk contains just 100 I.U. of vitamin D.) But to reach sufficient, healthy levels of vitamin D daily for life? Those amounts do not make a dent. 

To get 5,000 IU per day [a science-backed dose to reach a serum 25(OH)D goal of 50 ng/ml], Ferira says you would need to consume 294 cubes of cheddar cheese, 50 glasses of milk or orange juice, 62 cups of cereal, seven cups of irradiated mushrooms, nine servings of salmon, and 113 eggs. You get the point. 

Another pet peeve of Ferira's? When people declare that vitamin D is toxic simply because it's fat-soluble. "That is like a 1982 science textbook. We're in 2021—we need to stop making these sweeping statements," she says. While vitamin D toxicity is possible, it's much more challenging to reach than you might think, and it's not even relevant at useful, science-backed supplement doses (like 5,000 IU per day). 

We explain the reasoning in full here, but let's go over the highlights: Unless you're taking mega, mega amounts of vitamin D, toxicity isn't actually a concern. Clinical trials have found that toxicity considerations can happen with a serum 25(OH)D concentration greater than 150 ng/ml. That is three times higher than the range clinicians typically recommend (50 ng/ml) for vitamin D sufficiency. 

"The actual clinical science shows that you could even take 20,000 to 50,000 I.U. of D3 a day5, and not only is it not toxic, but these amounts helped the participants actually raise their blood levels to sufficiency,"* says Ferira. "I'm not saying that's your level, but it was successful—not scary." 

On to myth No. 3: Vitamin D supplementation is a temporary fix. Not only is this simply untrue, but according to Ferira, this notion is a major reason why so much of the nation is vitamin D insufficient or deficient. 

"It's a really common scenario," she explains. People will go get their vitamin D levels checked, find out it's less than 30 ng/ml (which means you're clinically insufficient), and their doctor might recommend a supplement for eight to 12 weeks or so to bump them back up to the sufficient range.* Then when they get tested again and their levels are above 30 ng/ml (even just slightly above), they'll stop the regimen—and fluctuate in and out of sufficiency as a result. 

Ferira has received a few questions regarding vitamin K2, and she wishes to address the issue head-on: "It is a myth that vitamin K is a required nutrient for vitamin D absorption or activity," she says. It's a fine combination, she continues, and brands have historically paired the two because they're both fat-soluble. But it's not like vitamin D3 relies on K2 in the gut. 

If vitamin D had a nutrient buddy, anyway, it would be magnesium: "Magnesium is an essential mineral," Ferira says. You want to make sure you're filling that gap because that's at the level of vitamin D conversion and activities."