Scientists reviewed 20 studies comparing two types of vitamin D supplements: D2 and D3. They found something surprising: when people took vitamin D2 supplements, their levels of vitamin D3 actually went down compared to people who didn’t take D2. This happened in a consistent pattern across multiple studies. The researchers aren’t sure exactly why this happens, but they think the body might be getting rid of D3 faster when D2 is present. This discovery could change how doctors recommend which type of vitamin D supplement people should take.

The Quick Take

  • What they studied: Whether taking vitamin D2 supplements affects how much vitamin D3 stays in your blood
  • Who participated: Data from 20 different research studies involving people who took either vitamin D2 supplements or no supplements, with measurements of their vitamin D3 levels before and after
  • Key finding: People who took vitamin D2 supplements had about 18 units less vitamin D3 in their blood compared to people who didn’t take D2, and this difference was statistically significant (very unlikely to happen by chance)
  • What it means for you: If you’re taking vitamin D2 supplements, your body might be clearing out D3 faster than normal. This suggests vitamin D3 supplements might be more effective for some people, but you should talk to your doctor about which type is best for you

The Research Details

Researchers searched a major medical database for all studies published between 1975 and 2023 that tested vitamin D2 supplements in people. They found 202 articles but only included 20 that met their strict quality standards. Of those 20, they were able to combine data from 11 studies in their final analysis. All the studies they included were randomized controlled trials, which means people were randomly assigned to either take vitamin D2 or receive no supplement (the control group). The researchers measured vitamin D3 levels in blood samples taken at the start and end of each study.

The researchers used a special statistical method called meta-analysis to combine results from multiple studies. This approach is powerful because it looks at patterns across many different studies rather than relying on just one. They calculated the average difference in vitamin D3 levels between people who took D2 and those who didn’t, and they reported how confident they were in these numbers.

This type of study design is considered very reliable because randomized controlled trials are the gold standard in medical research. By combining results from multiple trials, the researchers could see if the effect was consistent and real.

Understanding how different types of vitamin D interact in your body is important because vitamin D affects bone health, immune function, and many other systems. If D2 supplements reduce D3 levels, this could mean that D2 might not be the best choice for everyone. This research helps doctors make better recommendations about which supplement to prescribe.

This study is a meta-analysis, which is one of the highest-quality types of research. The researchers were careful to only include studies that met strict quality standards. They searched thoroughly for all relevant studies to avoid bias. However, the individual studies they combined had different designs and measured things slightly differently, which can affect the overall results. The researchers were transparent about this variation in their analysis.

What the Results Show

The main finding was clear and consistent: when people took vitamin D2 supplements, their vitamin D3 levels dropped. On average, people taking D2 had about 18 units less vitamin D3 in their blood compared to people who didn’t take D2. This difference was statistically significant, meaning it’s very unlikely to have happened by chance.

When researchers looked at how much vitamin D3 changed over the course of the studies, they found a smaller but still significant drop of about 9 units. This suggests that D2 supplementation causes the body to get rid of D3 more quickly than it normally would.

The researchers found this pattern in both types of comparisons they made: when they looked at final vitamin D3 levels and when they looked at how much the levels changed during the study. This consistency across different ways of measuring the effect makes the finding more trustworthy.

The researchers think this happens because the body has a regulatory system that speeds up the disposal of vitamin D when vitamin D levels get too high. When D2 is added, the body might treat it as extra vitamin D and start clearing out D3 faster to maintain balance.

While the main focus was on D3 levels, the research also suggests that vitamins D2 and D3 may work differently in the body’s metabolism. The studies showed that this wasn’t just a random variation but a consistent pattern across multiple trials. This indicates there’s a real biological mechanism at work, not just chance.

Scientists have suspected for a while that D2 and D3 might interact in the body, but this is one of the first times researchers have measured exactly how much D3 levels drop when D2 is added. Previous studies hinted at this relationship, but this meta-analysis provides stronger evidence by combining data from many trials. The findings support the idea that the two types of vitamin D aren’t completely interchangeable in the body.

The researchers couldn’t find enough studies to look at some important questions, like whether this effect is different in men versus women, or in younger versus older people. The studies they included used different doses of vitamin D2 and measured vitamin D3 at different time points, which made it harder to compare results directly. Some studies were small or had other quality issues. The researchers also couldn’t determine the exact biological mechanism explaining why this happens. Finally, most studies were relatively short-term, so it’s unclear if this effect continues over months or years.

The Bottom Line

Based on this research, vitamin D3 supplements may be more effective than D2 for maintaining vitamin D3 levels in your blood (moderate confidence). If you’re currently taking D2, talk to your doctor about whether switching to D3 might be better for you. This is especially important if you have conditions related to vitamin D deficiency. However, this research doesn’t mean D2 is bad—it just suggests D3 might work better for some people (low to moderate confidence for general population recommendations).

This research is most relevant for people taking vitamin D supplements, especially those taking D2. It’s particularly important for people with bone health concerns, those living in areas with limited sunlight, or people with conditions affecting vitamin D absorption. People who are healthy and get enough sun exposure may not need to worry about this. Pregnant women, children, and people with kidney or liver disease should definitely discuss this with their doctor before making any changes.

If you switch from D2 to D3, you might see changes in your vitamin D3 blood levels within a few weeks, but it typically takes 2-3 months to reach stable levels. The full benefits for bone health and immune function may take several months to become noticeable.

Want to Apply This Research?

  • Track your vitamin D supplement type (D2 or D3) and dose daily, and note any symptoms related to vitamin D deficiency like fatigue or muscle weakness. If possible, track your vitamin D blood test results every 3-6 months to see how your levels change.
  • If your doctor recommends it, switch from D2 to D3 supplements and log the change in the app. Set a reminder to take your supplement at the same time each day. Record any changes you notice in energy levels, mood, or bone/muscle health over the following weeks.
  • Create a long-term tracking system that records your supplement type, dose, and timing. Set quarterly reminders to check your vitamin D blood levels with your doctor. Compare your results over time to see if switching from D2 to D3 (or vice versa) makes a difference in your vitamin D status.

This research suggests that vitamin D2 and D3 may have different effects on vitamin D3 blood levels, but it does not provide medical advice. Before making any changes to your vitamin D supplementation, consult with your healthcare provider, especially if you have kidney disease, liver disease, heart conditions, or are taking medications that interact with vitamin D. This study does not replace professional medical diagnosis or treatment. Individual responses to supplements vary, and what works best depends on your personal health situation.