According to Gram Research analysis of 2,808 vitamin D tests, vitamin D comes in two forms (D2 and D3) that behave very differently in your body. When patients were retested, their D3 levels changed significantly while D2 barely changed, suggesting that standard total vitamin D measurements may hide important compositional details. About 71% of patients had low vitamin D overall, and 16% had meaningful amounts of D2 from fortified foods and supplements.
A Gram Research analysis of over 2,800 vitamin D tests reveals that doctors may be missing important details about how your body gets vitamin D. The study found that vitamin D comes in two main forms: D2 (from fortified foods and supplements) and D3 (from sunlight and animal products). When labs measure total vitamin D, they combine both types, but the two forms behave differently in your body. About 16% of patients had meaningful amounts of D2, and when people retook the test, their D3 levels changed much more than their D2 levels. This suggests that simply looking at total vitamin D numbers might not tell the whole story about your vitamin D status.
Key Statistics
A cross-sectional analysis of 2,808 adults found that 71.3% of routine vitamin D test results fell below the reference cutoff, with a median total vitamin D of 24.6 ng/mL.
Among 446 patients with repeat vitamin D testing, total vitamin D changes correlated almost perfectly with D3 changes (correlation of 0.98) but showed almost no correlation with D2 changes (correlation of 0.04), indicating that D3 drives most variation in total vitamin D.
In a 2026 hospital laboratory study, 16.3% of patients had vitamin D2 contributing at least 10% of their total vitamin D, with composition varying by sex, season, and ordering department.
Repeat testing revealed that median D3 changed by 4.2 ng/mL while D2 changed by only -0.1 ng/mL, demonstrating that the two vitamin D forms respond independently to physiological changes.
The Quick Take
- What they studied: How vitamin D tests measure two different types of vitamin D (D2 and D3) and whether these measurements stay consistent when people get retested.
- Who participated: 2,808 adults who had routine vitamin D blood tests at a hospital between June 2019 and November 2022. A smaller group of 446 people had repeat tests, allowing researchers to see how their vitamin D levels changed over time.
- Key finding: About 71% of patients had low vitamin D levels overall. When patients were retested, their vitamin D3 levels changed significantly, but their vitamin D2 levels barely changed. This means the total vitamin D number can hide important differences between the two types.
- What it means for you: If you get a vitamin D test, your doctor might benefit from knowing whether your vitamin D comes mainly from D2 or D3. This could help explain why some people’s vitamin D levels improve with certain supplements or lifestyle changes while others don’t. However, more research is needed before doctors routinely change how they interpret these tests.
The Research Details
Researchers looked at vitamin D test results from a hospital laboratory over a 3.5-year period. They used a special testing method called liquid chromatography-tandem mass spectrometry, which can measure D2 and D3 separately instead of just giving a total number. For most patients, they looked at their first test result. For 446 patients who came back for repeat testing, they compared the first and last results to see what changed. The researchers defined “vitamin D2-contributory” as when D2 made up at least 10% of the total vitamin D, though they also tested with different cutoff points (5% and 20%) to make sure their findings were solid.
This approach is like looking at the ingredients in a recipe separately rather than just tasting the final dish. By separating D2 and D3, researchers could see patterns that would be invisible if they only looked at the combined total. The study included information about when tests were done (season), who ordered them (which hospital department), and patient characteristics like sex.
Most vitamin D tests only report a single number: total vitamin D. But vitamin D2 and D3 come from different sources and may behave differently in your body. D2 comes from fortified foods and most supplements, while D3 comes from sunlight exposure and animal products. By separating these measurements, researchers can understand whether someone’s vitamin D problem is due to low D2, low D3, or both. This matters because treatment recommendations might differ depending on which type is low.
This study used actual patient data from routine hospital testing, which makes it realistic and relevant to real-world practice. The testing method (liquid chromatography-tandem mass spectrometry) is considered the gold standard for vitamin D measurement. However, this was a single-center study from one hospital, so results might differ in other locations or populations. The study is descriptive rather than experimental, meaning it shows patterns but cannot prove cause and effect. The researchers were careful to note that their findings need further evaluation before changing clinical practice.
What the Results Show
The study found that the median total vitamin D level was 24.6 ng/mL, which is quite low. In fact, 71.3% of all test results fell below the hospital’s reference cutoff for adequate vitamin D. About 16.3% of patients had what researchers called “vitamin D2-contributory composition,” meaning their D2 made up at least 10% of their total vitamin D.
When researchers looked at the 446 patients who had repeat tests, they found interesting patterns. The median change in total vitamin D was 4.3 ng/mL between tests. However, this change was almost entirely driven by changes in D3 (median change of 4.2 ng/mL), while D2 barely changed at all (median change of -0.1 ng/mL). This suggests that D3 is much more variable and responsive to whatever factors affect vitamin D status, while D2 stays relatively stable.
The statistical analysis confirmed this pattern: total vitamin D changes correlated very strongly with D3 changes (correlation of 0.98), but showed almost no correlation with D2 changes (correlation of 0.04). In other words, if you know how much someone’s D3 changed, you can predict their total vitamin D change very accurately. But knowing their D2 change tells you almost nothing about their total change.
The study found that vitamin D2-contributory composition varied by patient sex, season of testing, and which hospital department ordered the test. This suggests that different groups of patients have different patterns of D2 versus D3. For example, certain seasons or certain types of patients may be more likely to have higher D2 contributions. The researchers also noted that within any given total vitamin D category, there was significant variation in how much was D2 versus D3. This means two patients with identical total vitamin D numbers could have very different compositions.
Most previous vitamin D research has focused on total vitamin D levels without separating D2 and D3. This study adds to a growing body of evidence suggesting that the two forms should be considered separately. Some earlier research has suggested that D2 and D3 may have different effects on health, though this remains debated. By showing that D2 and D3 change independently on repeat testing, this study supports the idea that they should be measured and interpreted separately rather than combined into a single number.
This study looked at only one hospital’s data, so results might not apply everywhere. The researchers couldn’t determine why some patients had high D2 versus D3, only that it happened. The study is descriptive, showing patterns but not proving that separating D2 and D3 measurements would actually improve patient care. The researchers didn’t have information about patients’ diets, supplement use, or sun exposure, which would help explain the patterns they found. Finally, the clinical importance of these compositional differences remains unclear and needs further research.
The Bottom Line
Based on this research, there is moderate evidence that vitamin D testing could become more informative if labs routinely separated D2 and D3 measurements. However, current clinical guidelines still focus on total vitamin D, and this study does not provide enough evidence to change that practice yet. If you have low vitamin D, work with your doctor on standard treatment recommendations (supplementation, sun exposure, dietary changes) rather than worrying about the D2 versus D3 split. Future research may show that knowing the composition matters for treatment decisions.
This research is most relevant to laboratory professionals, doctors who order vitamin D tests, and researchers studying vitamin D metabolism. Patients with low vitamin D should be aware that their total number might hide important details, but this shouldn’t change their treatment approach right now. People taking vitamin D supplements (which are usually D2 or D3) might find it interesting that the two forms behave differently, but again, standard treatment recommendations remain the same.
If you start vitamin D supplementation or increase sun exposure, you might see changes in your D3 levels within weeks to months. D2 levels appear to change more slowly and less predictably. If you get retested, expect to see more change in D3 than D2. However, it typically takes several months of consistent supplementation or lifestyle changes to see meaningful improvements in total vitamin D levels.
Frequently Asked Questions
What is the difference between vitamin D2 and D3 in blood tests?
Vitamin D2 comes from fortified foods and most supplements, while D3 comes from sunlight and animal products. Most labs combine them into one total number, but this study shows they change independently. D3 levels fluctuate much more than D2 levels on repeat testing.
Why do 71% of people have low vitamin D according to this study?
The study found that 71.3% of test results fell below the hospital’s reference cutoff of 24.6 ng/mL. This likely reflects limited sun exposure, dietary gaps, and the fact that vitamin D deficiency is common in many populations, especially in winter months.
Should I ask my doctor to measure D2 and D3 separately?
This research suggests it could be useful, but current clinical guidelines still focus on total vitamin D. Talk to your doctor about whether separate measurements would change your treatment. For now, standard vitamin D supplementation and sun exposure recommendations remain the same regardless of composition.
Does it matter whether my vitamin D supplement is D2 or D3?
Both D2 and D3 raise total vitamin D levels, but this study suggests D3 may be more responsive to changes. Most doctors recommend either form, though some research suggests D3 may be slightly more effective. Your doctor can recommend the best option for your situation.
How often should I get my vitamin D tested if it’s low?
This study doesn’t specify testing frequency, but typical practice is to retest 8 to 12 weeks after starting supplementation to see if levels improved. After that, annual testing is often recommended. Work with your doctor on the best schedule for your situation.
Want to Apply This Research?
- Track your vitamin D test results over time, noting the date, total vitamin D level, and if available, the separate D2 and D3 numbers. Record any changes in sun exposure, supplement use, or diet around the time of testing to identify patterns.
- If your vitamin D is low, use the app to log daily sun exposure (15-30 minutes), vitamin D supplement doses, and vitamin D-rich foods (fatty fish, egg yolks, fortified milk). Set reminders for consistent supplementation and track how these changes correlate with your next test result.
- Set up quarterly or semi-annual vitamin D test reminders in your app. When you get new results, log them and compare to previous tests. Note whether changes align with your behavior changes. If your doctor eventually has access to separated D2 and D3 numbers, track those separately to see which form responds to your interventions.
This research describes patterns in vitamin D test results but does not establish cause and effect or provide medical advice. Vitamin D testing and supplementation decisions should be made in consultation with your healthcare provider based on your individual health status, medical history, and local clinical guidelines. This study was conducted at a single hospital and may not apply to all populations or testing settings. Do not change your vitamin D supplementation or sun exposure habits based solely on this research without discussing with your doctor.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.