Vitamin D types matter for blood levels

A growing number of patients and doctors are noticing that blood tests show low vitamin D levels even after taking supplements daily. This has sparked new interest in the differences between two types of vitamin D found in supplements.

D2 and D3 have different effects

Vitamin D comes in two forms: D2, derived from plants and fungi, and D3, which the skin produces when exposed to sunlight. A recent meta-analysis from the University of Surrey found that taking D2 can lower the specific form of vitamin D measured in blood tests—25-hydroxyvitamin D3—by up to 18 nanomoles per liter.

Understanding the distinction

While both D2 and D3 aim to increase vitamin D levels in the blood, they behave differently in the body. D2 is often used in vegetarian supplements, while D3 is found in animal-based foods like fatty fish and egg yolks. However, research shows that D2 may not be as effective in maintaining long-term vitamin D levels.

New studies highlight the impact

A meta-analysis published in Nutrition Reviews in September 2025 reviewed 20 clinical trials and found that D2 significantly reduced 25-hydroxyvitamin D3 levels. Eighteen of the 20 studies supported this finding, reinforcing the conclusion that D2 may not be as effective as D3 for maintaining adequate vitamin D levels.

Practical steps for patients

Patients should check the label on their supplements to see if they are taking D2 or D3. In France, for example, Stérogyl contains D2, while ZymaD uses D3. If blood tests still show low levels, consulting a doctor about switching to D3 or adjusting the dosage may be necessary.

Dietary considerations for vegans

For those following a vegan diet, D3 can be harder to obtain without animal products. However, D2 is still a viable option, though it may not raise blood levels of D3 as effectively. The National Institutes of Health in the U.S. recommend D3 for better and longer-lasting vitamin D increases.