Researchers in the Netherlands checked vitamin D levels in over 1,000 adults and found that while things have improved since 2011, many people—especially older men—still don’t have enough vitamin D in their blood. The study showed that vitamin D levels are lowest during winter months, which makes sense because our bodies make vitamin D from sunlight. Men had lower vitamin D levels than women overall, and people over 70 were most likely to be deficient. The good news is that people taking vitamin D supplements had much better levels, suggesting that supplements really do help.

The Quick Take

  • What they studied: Whether Dutch adults have healthy levels of vitamin D in their blood and how this has changed over the past 12 years
  • Who participated: 1,221 adults living in the northern Netherlands, ranging from age 18 to over 70 years old, with blood samples taken in March 2023
  • Key finding: About 13-26% of older adults (age 70+) had low vitamin D levels, with men affected more than women. However, vitamin D levels improved significantly compared to 2011, likely due to increased supplement use
  • What it means for you: If you live in a northern climate with limited winter sunlight, you may benefit from a vitamin D supplement, especially if you’re over 50 or don’t spend much time outdoors. Talk to your doctor about whether you need one

The Research Details

This was a snapshot study where researchers measured vitamin D levels in blood samples from over 1,000 Dutch adults collected in March 2023. They compared these results to similar measurements taken in March 2011 to see if things had changed over 12 years. The researchers also looked at how vitamin D levels varied throughout 2011 to understand seasonal patterns.

Vitamin D levels were measured using a specific blood test that looks for a form of vitamin D called 25(OH)D₃. The researchers then sorted people into age groups (18-49, 50-69, and 70+) and compared men to women to see if there were differences. They used standard statistical tests to determine if the differences they found were real or just due to chance.

This type of study is important because it shows us the real-world vitamin D status of regular people in a specific region. By comparing 2023 results to 2011, researchers can track whether public health efforts (like vitamin D supplement recommendations) are actually working. Understanding seasonal patterns helps explain why people in northern countries struggle with vitamin D in winter.

This study has good strengths: it used a large sample size (over 1,000 people), measured actual blood levels rather than relying on what people remember eating, and compared results over time. However, because it’s a snapshot study, we can’t prove that vitamin D supplements directly caused the improvement—other factors like increased awareness or dietary changes could have helped. The study only included people from northern Netherlands, so results might be different in other regions or countries with more sunlight.

What the Results Show

The study found that vitamin D deficiency remains a problem in the Netherlands, though it has improved since 2011. In March 2023, about 7% of women aged 50-69 had low vitamin D, compared to 26% of men aged 70+ and 13% of women aged 70+. Men overall had lower vitamin D levels than women at the same age.

When comparing 2023 to 2011, vitamin D levels were significantly higher in 2023 (median levels increased from 45-49 nmol/L to 54-75 nmol/L). This improvement suggests that more people are either taking supplements or getting more vitamin D from their diet and sun exposure.

The study also revealed a clear seasonal pattern: vitamin D levels were lowest during winter months in 2011, which makes sense because people get less sunlight during winter and can’t produce as much vitamin D naturally. Interestingly, people who were already taking vitamin D supplements had the healthiest vitamin D levels, confirming that supplements work.

Vitamin D levels increased with age in the study population, which was somewhat unexpected but may reflect that older adults are more likely to take supplements or be more aware of vitamin D needs. The seasonal variation data from 2011 showed that winter deficiency was a significant issue, with the lowest levels occurring in the coldest, darkest months.

This research fits with other studies showing that vitamin D deficiency is common in northern European countries due to limited winter sunlight. The improvement between 2011 and 2023 aligns with increased public health recommendations for vitamin D supplementation in the Netherlands. However, the fact that deficiency still affects up to 26% of some groups suggests that current recommendations may not be reaching everyone who needs them.

The study only measured vitamin D at one point in time (March 2023), so we don’t know individual patterns throughout the year. It only included people from northern Netherlands, so results may not apply to southern regions with more sun. The study can’t prove that supplements caused the improvement—other lifestyle changes could have contributed. Additionally, the study didn’t collect detailed information about sun exposure, diet, or supplement use for all participants, which could have explained some differences.

The Bottom Line

If you live in a northern climate (like the Netherlands), especially if you’re over 50, consider talking to your doctor about vitamin D supplementation, particularly during winter months. The evidence suggests supplements effectively raise vitamin D levels. For younger adults (18-49), vitamin D deficiency was less common in this study, but supplementation may still be beneficial if you have limited sun exposure or dietary sources. Confidence level: Moderate—this study shows supplements work, but individual needs vary.

This research is most relevant to people living in northern climates with limited winter sunlight. It’s especially important for adults over 50, men (who showed lower levels), and people who spend little time outdoors. Older adults (70+) should definitely discuss vitamin D with their doctor. People living in sunny climates may not need supplementation unless they have other risk factors.

Vitamin D levels in blood can increase within weeks of starting supplementation, but it typically takes 4-8 weeks to see meaningful improvements. Most people notice benefits like improved energy and mood within a few months, though individual responses vary. Winter deficiency is a seasonal issue, so supplementation is especially important from October through March in northern regions.

Want to Apply This Research?

  • Track daily vitamin D supplement intake (if taking one) and note outdoor time in minutes per day. Record energy levels and mood weekly to correlate with supplementation patterns, especially comparing winter vs. summer months
  • Set a daily reminder to take a vitamin D supplement during winter months (October-March) if recommended by your doctor. Log outdoor time during sunny days to track natural vitamin D exposure and identify patterns in your personal vitamin D needs
  • Create a seasonal tracking system that increases monitoring during winter months. Record supplement adherence, outdoor activity time, and subjective wellness measures. Compare patterns year-over-year to identify your personal vitamin D needs and optimal supplementation timing

This research describes vitamin D status in a Dutch population and should not be interpreted as personal medical advice. Vitamin D needs vary based on age, location, sun exposure, diet, and individual health conditions. Before starting any supplement regimen, consult with your healthcare provider to determine if vitamin D supplementation is appropriate for you and at what dose. This study was conducted in a specific geographic region and may not apply to all populations. If you have concerns about your vitamin D levels, ask your doctor about blood testing to determine your individual status.