A large study of over 375,000 people in the UK looked at whether vitamin D levels affect heart disease and bone fracture risk. Researchers found that people with very low vitamin D (deficiency) had higher risks of both heart problems and broken bones. However, people with moderately low vitamin D (insufficiency) didn’t show increased risk compared to those with normal levels. This suggests that the worry about ‘insufficient’ vitamin D might be overstated, and the real concern should focus on people with severely low levels.

The Quick Take

  • What they studied: Whether different levels of vitamin D in the blood affect the chances of having heart attacks, strokes, heart-related deaths, or broken bones
  • Who participated: 375,044 adults aged 40-69 years from the UK Biobank, a large health database tracking British people’s health over time
  • Key finding: People with very low vitamin D (below 30 nmol/L) had about 29% higher risk of heart problems and 20% higher risk of fractures. People with moderately low vitamin D (50-75 nmol/L) had no increased risk compared to those with normal levels.
  • What it means for you: If your vitamin D is very low, it may be worth addressing. However, if it’s just slightly below the ’normal’ range, this study suggests it may not be a major health concern. Talk to your doctor about your specific vitamin D level and what it means for you.

The Research Details

This was a cohort study, meaning researchers followed a large group of people over time and tracked what health problems they developed. The study used data from the UK Biobank, which is like a giant health record system for hundreds of thousands of British people. Researchers measured vitamin D levels in participants’ blood and then sorted them into three groups: deficient (very low), insufficient (moderately low), and sufficient (normal). They then tracked hospital records, death certificates, and health registries to see who had heart attacks, strokes, died from heart disease, or broke bones.

The researchers were careful to account for other factors that might affect the results, like age, sex, smoking, exercise, diet, and other health conditions. This helps them figure out whether vitamin D itself was causing the differences in risk, or whether other factors were responsible. They used a special statistical method called ‘spline curve analysis’ to look at the exact point where vitamin D levels started to matter for health risks.

This research approach is important because it looks at real-world health outcomes in a huge number of people over a long time period. Rather than just measuring vitamin D in a lab, the researchers tracked actual heart attacks, strokes, and broken bones that happened to real people. This makes the findings more meaningful than studies that only look at blood markers. The study also specifically looked at the ‘insufficient’ range, which is often talked about in health advice but hadn’t been carefully studied before.

This is a strong study because it included a very large number of people (375,044), which makes the results more reliable. The researchers used official medical records and death certificates rather than relying on people to remember their own health problems, which reduces errors. However, the study is observational, meaning it shows associations but can’t prove that low vitamin D directly causes health problems—other unmeasured factors could be involved. The study was conducted in the UK, so results might be slightly different in other countries with different populations or climates.

What the Results Show

The study found a clear difference between very low vitamin D and moderately low vitamin D. People with very low vitamin D (below 30 nmol/L) had significantly higher risks: about 29% higher risk of heart attacks, strokes, or heart-related death, and about 20% higher risk of major bone fractures. People with low vitamin D (30-50 nmol/L) also showed increased risks, though not as high as the severely deficient group.

The surprising finding was about the ‘insufficient’ range (50-75 nmol/L). This group showed no significant increase in heart disease or fracture risk compared to people with normal vitamin D levels (75 nmol/L or higher). This means that having vitamin D in this moderately low range didn’t appear to harm health outcomes in this study.

When researchers looked at the relationship between vitamin D levels and health risk, they found that risk only increased when vitamin D dropped into the deficient range. There was no additional benefit or harm seen as vitamin D levels moved through the insufficient range. This suggests there may be a threshold below which vitamin D becomes problematic, rather than a gradual increase in risk as levels drop.

The study also examined whether the relationship between vitamin D and health risks was different for men versus women, or for different age groups. While the main findings held true across these groups, the exact strength of the associations varied slightly. The research confirmed that the most severe vitamin D deficiency (below 30 nmol/L) was particularly concerning, with the strongest associations to both heart disease and fracture risk.

Previous research has suggested that vitamin D deficiency increases disease risk, and this study confirms that finding for severe deficiency. However, many earlier studies and health guidelines have warned about ‘vitamin D insufficiency’ as a health concern. This new research challenges that idea by showing that the insufficient range (50-75 nmol/L) doesn’t appear to increase health risks. This doesn’t mean those earlier studies were wrong, but it suggests the focus should be on severe deficiency rather than mild deficiency.

This study has several important limitations. First, it’s observational, meaning it shows that low vitamin D and health problems happen together, but it doesn’t prove that low vitamin D causes the problems. People with very low vitamin D might have other health issues or lifestyle factors that increase their risk. Second, vitamin D was measured only once at the start of the study, so researchers didn’t know if levels changed over time. Third, the study was done in the UK where people have less sun exposure, so results might be different in sunnier countries. Finally, the study included mostly white British people, so the findings might not apply equally to all ethnic groups.

The Bottom Line

Based on this research, if your vitamin D level is very low (below 30 nmol/L), it’s worth discussing with your doctor about ways to increase it through sunlight exposure, diet, or supplements. If your vitamin D is in the ‘insufficient’ range (50-75 nmol/L), this study suggests it may not be a major health concern, though your doctor might still recommend increasing it for other reasons. If your vitamin D is normal (75 nmol/L or higher), this study doesn’t suggest you need to do anything special. Confidence level: Moderate to high for severe deficiency, lower for the insufficient range.

People with very low vitamin D levels should pay attention to this research and consider ways to improve their levels. People who are at high risk for heart disease or osteoporosis should be especially interested. However, people with vitamin D in the insufficient range might worry less based on this study. People living in very sunny climates or those who get regular sun exposure may have less reason to worry about vitamin D levels. Pregnant women, young children, and people with certain medical conditions should still follow their doctor’s specific recommendations.

If you decide to increase your vitamin D levels, it typically takes several weeks to months to see changes in blood levels. Health benefits from improved vitamin D levels would likely take months to years to become apparent, since heart disease and fractures develop over long periods. Don’t expect immediate changes in how you feel.

Want to Apply This Research?

  • Track your vitamin D blood test results when you get them (measured in nmol/L or ng/mL), and note the date. Set a reminder to retest every 6-12 months if your doctor recommends it. Record any changes in sun exposure, supplement use, or diet changes that might affect vitamin D.
  • If your vitamin D is very low, use the app to track: (1) minutes of sun exposure daily (aim for 10-30 minutes several times per week), (2) vitamin D supplement doses if you take them, and (3) vitamin D-rich foods eaten (fatty fish, egg yolks, fortified milk). Set weekly goals for one of these behaviors.
  • Create a simple monthly check-in where you review your vitamin D-related behaviors and note any changes in energy, bone health, or cardiovascular symptoms. If you have a follow-up blood test, log the result in the app and compare it to your previous level to see if your behavior changes are working.

This research shows associations between vitamin D levels and health outcomes, but does not prove that vitamin D deficiency directly causes heart disease or fractures. Individual vitamin D needs vary based on age, skin tone, location, and health conditions. Before making changes to your vitamin D intake or supplementation, consult with your healthcare provider, especially if you have existing heart disease, bone problems, or take medications. This information is for educational purposes and should not replace professional medical advice. Your doctor can determine your appropriate vitamin D level based on your individual health situation.