When it comes to vitamin supplements, you want to use what works. It doesn’t make sense to spend money and time on a vitamin D supplement that doesn’t meet your needs. But how do you navigate the medical terminology to figure out the best option?

You can find vitamin D in two main forms on the market today: D2 and D3 (D1 was named before it was later discovered to actually be a mixture of compounds, and is no longer used). Here’s what you need to know about the difference between vitamin D2 and D3.

What does vitamin D do?

Whether vitamin D is supplemented in the D2 or D3 form, it performs several critical functions in the human body. Most famously, vitamin D increases intestinal absorption of calcium and supports healthy bone growth and maintenance. Lesser known roles of vitamin D include reducing inflammation as well as modulating cell growth, muscle and immune function, and glucose metabolism.

For a full review of what we know about vitamin D and its role in human nutrition, check out the NIH Fact Sheet.

What form is most effective?

Answer: it’s complicated. Most studies have shown that both D2 and D3 increase serum levels of 25-hydroxyvitamin D (25D is the metabolite that is measured in the body as your “vitamin D level”). A controlled study of 38 adults showed that D3 supplementation increased total and free 25D levels to a greater extent than D2.

However, a more recent study with a larger sample size and a lower dose rate of vitamin D2 and D3 (respectively) showed they are equally effective at raising serum levels of 25D. Yet another study found that the effects of either D2 or D3 were dependent on dosage level, sex, and when the supplement was given.

While this evidence obviously varies, supplementation with either D2 or D3 has been shown to be safe and effective at appropriate doses. Whichever form you choose to use, identifying your general vitamin D levels is crucial to the success of your supplementation.

What are my next steps?

Knowledge is key. Get tested with an at-home vitamin D test to find out if you have a vitamin D deficiency, and consider getting tested at two different times of the year. Vitamin D levels have been shown to decline in winter months when our bodies don’t get as much exposure to sunlight.

Before making any drastic changes, be sure to talk to your health care provider and consult a local health authority (such as the FDA’s Reference Daily Intakes) to ensure you are making the best-informed decision.

Note: This is a brief overview of emerging evidence and should not be taken as treatment advice or treatment recommendations for any individual or specific medical condition. The strategies reviewed may not be appropriate for you. For any treatment advice or consideration we strongly suggest discussions with your personal healthcare professionals.