And it may not be bringing you the benefits you think.
In the wintertime study, researchers split 335 people into five groups: a control group given non-fortified juice and biscuits, one given vitamin D2 (ergocalciferol) juice, one given vitamin D2 biscuits, one given vitamin D3 (cholecalciferol) juice, and the last given vitamin D3 biscuits. The vitamin D dosage was 600 IU daily.
Over the course of 12 weeks, the people in the control group experienced a 25 percent drop in their total vitamin D levels—not unexpected, since D levels tend to decline in the winter. On the other hand, those taking 600 IU of D a day didn’t show any drop. In fact, their D levels rose.
But not nearly to the same extent. People taking D3 experienced about a 75 percent increase in their vitamin D levels, compared to just around 34 percent for those taking D2. That shows that vitamin D3 was more than twice as effective.
That’s major: Current guidelines given by the National Institutes of Health (NIH) state that the two forms of vitamin D are equivalent, and can be used to equal effect at nutritional doses.
As these findings suggest, though, that might not be the case: People who take supplements containing vitamin D3, or consume it through fish or eggs may be more effectively raising their D levels than those taking vitamin D2 supplements, or eating D2-heavy foods like mushrooms or fortified bread, study author Laura Tripkovic, Ph.D., said in a statement.
The differences between the two forms of vitamin D may come down to how they bind to molecules in your body, the study authors believe. It may also be because D2 has a shorter half-life than D3, meaning it may not stay as potent for as long.
Now, the study was done in women, so it’s not clear yet if the same results would hold true in men.
Still, the findings suggest that you may be selling yourself short if you’re relying on D2 supplements or foods to boost your levels. And that’s a problem, since low levels of vitamin D can cause a bunch of health issues—it’s been linked to conditions like depression, dementia, and more severe heart disease.
While more research needs to be done, you can boost your D3 levels now by choosing a supplement that contains that form of the vitamin (like Solgar Vitamin D3 Capsules, which contain the same amount of the vitamin used in the study.) And shoot for D3-heavy foods like fish and eggs.