Researchers looked at whether taking vitamin D supplements helps people live longer. Two large studies found no benefit in people who already had enough vitamin D. However, when scientists analyzed data from people with low vitamin D levels, they found that supplements might reduce the risk of death by 15-25% over five years. This suggests vitamin D supplements may only help people who actually don’t have enough of this nutrient in their blood, not everyone.

The Quick Take

  • What they studied: Whether taking vitamin D supplements helps people live longer, especially focusing on people with low vitamin D levels
  • Who participated: Over 423,000 adults from the UK Biobank database, analyzed to match the characteristics of two major vitamin D studies (VITAL and D-Health trials)
  • Key finding: Vitamin D supplements showed no benefit in people with normal vitamin D levels, but appeared to reduce death risk by 15-25% in people with low vitamin D levels over a 5-6 year period
  • What it means for you: If you have low vitamin D levels (confirmed by a blood test), taking supplements may help you live longer. If your vitamin D is already normal, supplements probably won’t extend your life. Talk to your doctor about getting your vitamin D level tested before starting supplements.

The Research Details

Scientists used a clever method called ’trial emulation’ to answer a question that two large real-world studies couldn’t fully answer. They took data from over 400,000 people in the UK Biobank (a huge health database) and pretended to run the same vitamin D studies again, but with different groups of people. First, they matched the original studies by using people with similar vitamin D levels to those in the real trials. Then, they ran the same analysis again, but only with people who had low vitamin D levels. This allowed them to see what would have happened if the original studies had included more people with vitamin D deficiency.

The researchers used a statistical method called Cox models to track how many people died over 5-6 years and compared death rates between people who had higher vitamin D levels and those who didn’t. They measured vitamin D using a blood test that checks for a compound called 25-hydroxyvitamin D.

The two original large studies (VITAL and D-Health) tested vitamin D in populations where most people already had enough vitamin D. This is like testing a medicine for anemia in people who don’t have anemia—you probably won’t see benefits. This new analysis helps answer the real question: does vitamin D help the people who actually need it? By looking at people with low vitamin D specifically, scientists can better understand who might actually benefit from supplements.

This study is strong because it uses data from a very large, well-documented health database (UK Biobank) with over 400,000 participants and careful tracking of deaths. However, it’s not a brand-new experiment—it’s an analysis of existing data, which is less definitive than a new randomized trial specifically designed for people with low vitamin D. The researchers did multiple versions of their analysis to check if their findings were consistent, which is a good sign of reliability.

What the Results Show

When researchers analyzed people with similar vitamin D levels to those in the original VITAL and D-Health trials, they found almost no benefit from vitamin D supplements—results that matched what the original studies showed. This confirms that vitamin D supplements don’t help people who already have adequate levels.

However, when they looked only at people with vitamin D insufficiency (low but not severely low), vitamin D supplements appeared to reduce the risk of death by about 15% over 5-6 years. When they looked at people with vitamin D deficiency (severely low), the benefit was even larger—about 19-25% reduction in death risk.

These findings suggest that vitamin D’s life-extending benefits, if they exist, are most likely to appear in people whose bodies are actually lacking this nutrient. The supplements appeared to work by raising vitamin D levels in the blood by about 30-38 nanomoles per liter.

The study showed consistent results across different ways of analyzing the data, which increases confidence in the findings. The benefits appeared similar for both insufficiency and deficiency groups, suggesting a dose-response relationship—the lower someone’s vitamin D, the more potential benefit from supplementation.

This research helps explain why two major studies (VITAL and D-Health) found no benefit from vitamin D supplements. Those studies included mostly people with adequate vitamin D levels, so they were testing supplements in the wrong population. Previous research has suggested vitamin D is important for immune function, bone health, and reducing inflammation—all things that could affect survival. This new analysis supports the idea that vitamin D only helps with survival if you’re actually deficient.

This study analyzes existing data rather than conducting a new experiment, which is less definitive. The researchers couldn’t randomly assign people to take supplements or not—they had to estimate what would have happened based on natural differences in vitamin D levels. The study also only tracked people for 5-6 years, so we don’t know about longer-term effects. Additionally, the UK Biobank participants are mostly of European descent and relatively healthy, so results might differ in other populations. Finally, the study couldn’t prove that vitamin D directly caused the reduction in deaths—other factors could be involved.

The Bottom Line

If a blood test shows you have low vitamin D (insufficiency or deficiency), taking vitamin D supplements appears to be beneficial and may reduce your risk of serious health problems. The evidence suggests a moderate level of confidence in this recommendation. If your vitamin D levels are normal, supplements probably won’t help you live longer. Everyone should get their vitamin D level tested before starting supplements, rather than taking them ‘just in case.’

People with confirmed low vitamin D levels should pay attention to this research. This includes people who don’t get much sun exposure, have darker skin (which makes vitamin D production harder), are older, have certain digestive conditions, or live in northern climates. People with normal vitamin D levels don’t need to worry about this research. Pregnant women, children, and people with kidney or bone disease should talk to their doctor before starting supplements.

Based on this research, benefits from vitamin D supplementation would likely take months to years to appear. The studies tracked people for 5-6 years, so that’s the timeframe where benefits became measurable. Don’t expect to feel different after taking vitamin D supplements—the benefits appear to be about reducing serious health risks over time, not improving how you feel day-to-day.

Want to Apply This Research?

  • Track your vitamin D supplementation daily (dose and type) and log any health markers like energy levels, mood, and illness frequency. Set a reminder to get your vitamin D level retested every 6-12 months to confirm your levels are improving and in the target range.
  • If your doctor confirms low vitamin D, set up a daily reminder to take your vitamin D supplement at the same time each day (ideally with a meal containing fat, since vitamin D is fat-soluble). Use the app to log your supplement intake and track whether you’re taking it consistently.
  • Create a long-term tracking system that records your vitamin D test results over time, your supplement dosage, and any major health events. This helps you and your doctor see if supplementation is working and whether your vitamin D levels are staying in a healthy range. Review this data annually with your healthcare provider.

This research suggests vitamin D supplements may help people with low vitamin D levels, but it is not a substitute for medical advice. Before starting any vitamin D supplementation, consult with your healthcare provider, especially if you have kidney disease, heart disease, or take certain medications. Get your vitamin D level tested with a blood test before starting supplements. This analysis is based on existing data and is not as definitive as a new clinical trial specifically designed for people with low vitamin D. Individual results may vary, and this information should not replace personalized medical guidance from your doctor.