D2 vs. D3: Why Form Matters When It Comes To Vitamin D

Everyone seems to be talking about the importance of vitamin D (myself included). In time, it's become the vitamin to rival even the famed vitamin C. 

But when we talk about increasing our vitamin D intake, it's helpful to be clear about the specifics of what we mean, exactly. After all, the body doesn't use all types of vitamin D in the same way; it prefers one kind over another. 

But here's the thing: Most of us aren't getting enough vitamin D. Research indicates anywhere from 931% to 100% of American adults fail to consume just 400 IU of vitamin D every day. (And if you're wondering, that's a meager amount already and ineffective at helping you achieve healthy vitamin D status.) 

Although our bodies can produce some vitamin D on their own with exposure to sunlight, spending too much time inside is not the main reason D deficiency and insufficiency rates are 29% and 42% among U.S. adults2, respectively. With only a handful of foods containing a significant amount of vitamin D (or any at all), getting enough of the essential fat-soluble vitamin from diet alone just isn't realistic.

Truthfully, the most effective way to get your levels up is to take a high-quality daily vitamin D supplement. (Don't worry, we've vetted and compiled the best options for you.)

mindbodygreen recommends an efficacious dose of 5,000 IU of D3 daily for adults to achieve (and more importantly, sustain) optimal levels of 50 ng/ml.* If you're interested in finding out if you're getting enough vitamin D or if your supplement is actually working, you can have your doctor order a serum total 25-hydroxyvitamin D blood test, aka 25(OH)D for short. (And if you have stubborn vitamin D levels, check out these 10 possible contributing factors.)

Vitamin D can be found in two different forms—D2 and D3—based on source and biochemical structure. "D2 (ergocalciferol) is only derived from plant sources and is often in fortified products. D3 (cholecalciferol) is from animal sources and can be synthesized in the skin from UVB light," shares registered dietitian nutritionist Patricia Bannan, M.S., RDN, author of From Burnout to Balance. 

Vitamin D2 and vitamin D3 also differ structurally—i.e., D3 is far more stable and therefore has superior bioactivity and bioefficacy (more on that later).* Although generally found in animal products, there's one very important caveat to mention: D3 can also be found in algae and lichen, which allows for a plant-based D3 supplement (although algal D3 is much more sustainable for the environment than lichen). 

Form matters when it comes to supplementing with vitamin D. According to Bannan, D3 is the body's preferred form of vitamin D because it's produced in the body naturally and is more efficient at increasing blood levels. In fact, D3 is 87% more potent and effective4 at raising and maintaining serum 25(OH)D levels than D2. And since 25(OH)D levels are your status biomarker, that's a majorly impactful difference.

D2 is significantly less effective because of the structural differences that affect the way the body metabolizes5 the vitamin. Vitamin D2 also has increased catabolism3 (i.e., it's broken down, or metabolized, more quickly) and its metabolites have weaker binding affinity3 to the vitamin-D-binding protein (i.e., the major vitamin D transport protein that carries the nutrient to the rest of the body to confer all those wonderful health benefits we highlighted before).

"These two nutrients are not biologically equivalent or interchangeable," Ashley Jordan Ferira, Ph.D., RDN, mbg's vice president of scientific affairs, previously wrote for mindbodygreen when dissecting D3's scientifically proven superiority over D2. "One is effective (vitamin D3), and the other is way less effective (vitamin D2). Any suggestion that vitamin D3 and D2 are both good options falls into the 'bad science' bucket."

When it comes to bioactivity, vitamin D3 has been shown to raise free 25(OH)D levels—an up-and-coming biomarker that may be more sensitive than total 25(OH)D—with greater efficacy than D26. "This is yet one more compelling piece of evidence cementing the superior bioefficacy of D3 over D2," Ferira concludes.

But there is one significant vegan source of this preferred form of D3: algae.

"Unlike some nutrition queries that are somewhat gray and equivocal in nature, when vitamin D3 and vitamin D2 go head-to-head, the winner is clear," concludes Ferira.