Researchers studied nearly 140,000 people to understand if low vitamin D levels increase the risk of dangerous blood clots. They found that people with very low vitamin D were significantly more likely to develop blood clots in their legs or lungs compared to those with healthy vitamin D levels. The risk was even higher for people with moderately low vitamin D. This large study suggests that maintaining good vitamin D levels might help prevent blood clots, though doctors still need to do more research to prove this connection works the same way in everyone.

The Quick Take

  • What they studied: Whether people with low vitamin D levels are more likely to develop blood clots in their legs (deep vein thrombosis) or lungs (pulmonary embolism)
  • Who participated: 139,690 adults aged 45 and older from a large medical database. Half had very low vitamin D levels (below 20 ng/mL), and half had healthy levels (30 ng/mL or higher). The groups were carefully matched so they were similar in other ways.
  • Key finding: People with very low vitamin D were 62% more likely to develop leg blood clots and 61% more likely to develop lung blood clots within one year compared to people with normal vitamin D levels. This risk stayed elevated even after two years, though it decreased slightly.
  • What it means for you: If you’re over 45, maintaining adequate vitamin D levels may help reduce your risk of dangerous blood clots. However, this study shows an association, not proof of cause-and-effect. Talk to your doctor about your vitamin D levels and whether you should get tested or take supplements, especially if you have other risk factors for blood clots.

The Research Details

This was a retrospective matched cohort study, which means researchers looked back at medical records from 2010 to 2023 and compared two groups of people. One group had very low vitamin D levels (below 20 ng/mL), while the other group had healthy levels (30 ng/mL or higher). The researchers used a special statistical method called propensity score matching to make sure the two groups were as similar as possible in terms of age, sex, and other health conditions. This helps ensure that any differences in blood clot rates were due to vitamin D levels rather than other factors. The final study included 69,845 people in each group, followed for up to two years.

The researchers tracked whether people developed blood clots in their legs or lungs during the 3-12 months after their vitamin D was measured. They also looked at whether people died or needed intensive care during the follow-up period. This approach is stronger than simply comparing people at one point in time because it shows what actually happened to people over time.

This study design is important because it allows researchers to follow people forward in time and see what health problems develop. By matching the two groups carefully, the researchers could be more confident that vitamin D levels—not other differences between the groups—were responsible for the increased blood clot risk. The large number of participants (nearly 140,000) makes the findings more reliable than smaller studies.

This study has several strengths: it included a very large number of people, used real medical records rather than relying on people’s memories, and carefully matched the comparison groups. However, because it looked back at existing records rather than randomly assigning people to take vitamin D or a placebo, we can’t be completely certain that low vitamin D actually causes blood clots. Other unmeasured factors could play a role. The study also only included people aged 45 and older, so results may not apply to younger adults.

What the Results Show

During the first year of follow-up, people with very low vitamin D were significantly more likely to develop blood clots. Specifically, they had a 62% higher risk of developing deep vein thrombosis (blood clots in the legs) and a 61% higher risk of developing pulmonary embolism (blood clots in the lungs) compared to people with normal vitamin D levels. These differences were statistically significant, meaning they were very unlikely to have happened by chance.

The study also found a dose-response relationship, which means the risk increased as vitamin D levels decreased. People with moderately low vitamin D (20-30 ng/mL) had an intermediate risk level—higher than normal but lower than those with very low vitamin D. This pattern suggests that vitamin D levels matter in a gradual way, not just as a simple yes-or-no threshold.

When researchers followed people for two years, the increased risk persisted but decreased slightly. People with very low vitamin D still had a 49% higher risk of leg blood clots and a 61% higher risk of lung blood clots. This suggests the effect of vitamin D deficiency on blood clot risk is real but may be strongest in the first year.

Beyond blood clots, the study found that people with very low vitamin D had a 120% higher risk of dying from any cause during the follow-up period and a 47% higher risk of needing intensive care. These findings suggest that vitamin D deficiency may affect overall health and survival, not just blood clot risk. The increased mortality risk was particularly striking and suggests vitamin D plays a broader role in health than just preventing blood clots.

Previous research on vitamin D and blood clots has shown mixed results, with some studies finding a connection and others not. This large study provides stronger evidence for an association because of its size and careful design. The findings align with biological research showing that vitamin D affects how blood clots form and how blood vessel walls function. However, most previous studies were smaller or used different methods, so this research adds important new information to the discussion.

This study has several important limitations. First, it shows an association between low vitamin D and blood clots but doesn’t prove that low vitamin D actually causes blood clots. Other factors not measured in the study could explain the connection. Second, the study only included people aged 45 and older, so we don’t know if the same relationship exists in younger adults. Third, the study used medical records, which may not capture all cases of blood clots or all vitamin D measurements. Finally, the study couldn’t account for all possible differences between groups, such as physical activity levels or diet quality, which could influence both vitamin D levels and blood clot risk.

The Bottom Line

Based on this research, maintaining adequate vitamin D levels (generally 30 ng/mL or higher) appears to be associated with lower blood clot risk. However, this is not yet a strong enough recommendation to change treatment guidelines. If you’re over 45, especially if you have risk factors for blood clots (such as recent surgery, cancer, or a family history of blood clots), ask your doctor about checking your vitamin D level. If your level is low, your doctor may recommend vitamin D supplements. The evidence suggests this may help reduce blood clot risk, though more research is needed to be certain. Confidence level: Moderate—this is good evidence of an association, but we need randomized trials to confirm cause-and-effect.

This research is most relevant to adults over 45, particularly those with risk factors for blood clots. People with diabetes may be especially affected, as the study found diabetes modified the relationship between vitamin D and leg blood clots. If you have a personal or family history of blood clots, recent surgery, cancer, or are immobilized for long periods, maintaining good vitamin D levels seems particularly important. However, this study doesn’t change recommendations for younger adults, and it shouldn’t replace other proven blood clot prevention strategies like compression stockings, blood thinners, or movement after surgery.

If you start taking vitamin D supplements or increase your vitamin D levels, it may take several weeks to months to reach optimal levels. The protective effect against blood clots appears strongest within the first year, based on this study. However, you shouldn’t expect immediate results—blood clot prevention is a long-term health strategy. If you’re at high risk for blood clots, work with your doctor on a comprehensive prevention plan that may include vitamin D optimization along with other proven strategies.

Want to Apply This Research?

  • Track your vitamin D supplementation daily (dose and type) and log any symptoms that might indicate blood clots, such as leg swelling, calf pain, chest pain, or shortness of breath. Record your vitamin D blood test results when available, aiming to monitor levels every 6-12 months.
  • If your doctor recommends vitamin D supplementation, set a daily reminder to take your supplement at the same time each day. Log your intake in the app to build consistency. Additionally, track outdoor time and sun exposure, as sunlight helps your body produce vitamin D naturally. Aim for 10-30 minutes of midday sun exposure several times per week, depending on your skin type and location.
  • Create a long-term tracking system that includes: (1) daily vitamin D supplement intake, (2) quarterly notes on any leg swelling or pain, (3) annual vitamin D blood test results, and (4) any changes in blood clot risk factors (surgery, immobilization, new medications). Share this information with your doctor during annual check-ups to assess whether your vitamin D optimization strategy is working and whether additional blood clot prevention measures are needed.

This research shows an association between low vitamin D and increased blood clot risk, but does not prove that vitamin D deficiency causes blood clots. This information is for educational purposes and should not replace professional medical advice. If you have symptoms of blood clots (leg swelling, calf pain, chest pain, or shortness of breath), seek immediate medical attention. Before starting vitamin D supplements or making changes to blood clot prevention strategies, consult with your healthcare provider, especially if you take blood thinners or have a history of blood clots. Individual vitamin D needs vary based on age, health conditions, medications, and other factors that only your doctor can assess.