Researchers in Japan tracked vitamin D levels in thousands of people over 10 years and found something encouraging: fewer people have dangerously low vitamin D levels now than they did a decade ago. The study looked at blood samples from over 1,800 people in 2005-2007 and compared them to nearly 1,900 people in 2015-2016. They discovered that vitamin D deficiency dropped from 29.5% to 21.6%, while average vitamin D levels increased slightly. This improvement is good news because vitamin D helps keep bones strong and prevents fractures, especially as people age.

The Quick Take

  • What they studied: Whether vitamin D levels in Japanese people’s blood have gotten better or worse over a 10-year period, and what this means for bone health
  • Who participated: About 1,800 Japanese adults (mostly women) in 2005-2007, and about 1,900 adults (also mostly women) in 2015-2016. Some people participated in both surveys, and some were new participants
  • Key finding: The percentage of people with dangerously low vitamin D dropped from about 3 in 10 people to about 2 in 10 people. Average vitamin D levels went up from 23.3 to 25.1 ng/mL, which is a small but meaningful increase
  • What it means for you: If you live in Japan or a similar climate, vitamin D levels in the general population are improving, which may mean fewer broken bones from weak bones in the future. However, many people still don’t have enough vitamin D, so it’s still worth checking your levels and getting enough sun exposure and vitamin D-rich foods

The Research Details

This study followed the same group of people over 10 years, comparing their vitamin D levels at the start and at the end. Researchers collected blood samples from participants and measured how much vitamin D was in their blood using a standard test called 25-hydroxyvitamin D. They also asked people questions about their health and lifestyle, measured their bone density, and took X-rays to check for bone problems.

The researchers used the same methods and measurements both times, which makes the comparison fair and reliable. They defined vitamin D deficiency as levels below 20 ng/mL (very low) and insufficiency as levels between 20-30 ng/mL (not quite enough). This is important because it lets them track whether people are moving from deficient to insufficient to healthy levels.

By comparing the same measurements 10 years apart, researchers can see real trends in a population rather than just taking a snapshot at one moment. This type of study is valuable because it shows whether public health efforts (like encouraging sun exposure or vitamin D supplementation) are actually working. The long time period also allows researchers to see if improvements lead to fewer bone problems down the road.

This study is fairly reliable because it used the same measurement methods both times, had a large number of participants, and was conducted in a real-world population rather than a controlled lab setting. However, some people from the first survey didn’t participate in the second survey, which could have affected the results slightly. The study is published in a peer-reviewed medical journal, meaning other experts reviewed it before publication.

What the Results Show

The main finding is that vitamin D deficiency (the most serious form of low vitamin D) decreased significantly over the 10-year period. In 2005-2007, about 29.5% of people had deficient vitamin D levels. By 2015-2016, this had dropped to 21.6%—meaning about 8 out of every 100 fewer people had dangerously low vitamin D.

At the same time, average vitamin D levels in the blood increased from 23.3 ng/mL to 25.1 ng/mL. While this might sound like a small increase, it’s statistically significant, meaning it’s unlikely to have happened by chance. This improvement happened across the population, not just in a few people.

Interestingly, the percentage of people with insufficient vitamin D (not quite enough, but not deficient) actually stayed about the same, going from 52.9% to 54.8%. This means that while fewer people are severely deficient, more than half the population still doesn’t have optimal vitamin D levels.

The study also measured parathyroid hormone levels, which increase when vitamin D is low. The researchers noted that the improvements in vitamin D levels could potentially lead to fewer cases of osteoporosis (weak bones) and osteoporotic fractures (broken bones from weak bones) in the future, though they didn’t directly measure this in the study. The fact that improvements were seen across both men and women suggests this is a population-wide trend rather than affecting just one group.

This study adds to existing research showing that vitamin D deficiency is common in many parts of the world, including Japan. The good news is that unlike some countries where vitamin D deficiency is staying the same or getting worse, Japan appears to be making progress. This could be due to increased awareness about vitamin D, more people taking supplements, or changes in sun exposure habits. The findings suggest that public health efforts may be working.

The study has some limitations worth knowing about. First, some people who participated in the first survey didn’t participate in the second one, which could have skewed the results if those people had different vitamin D levels than those who stayed in the study. Second, the study only looked at Japanese people, so the results might not apply to other populations with different genetics, diets, or sun exposure. Third, while the study shows that vitamin D levels improved, it doesn’t prove that this will actually prevent more fractures in the future—that would require additional research. Finally, the study didn’t measure vitamin D from food and supplements separately from sun exposure, so we don’t know which factor contributed most to the improvement.

The Bottom Line

If you live in Japan or a similar climate: (1) Have your vitamin D levels checked if you haven’t recently, especially if you’re over 50 or have risk factors for bone disease—this is a moderate-confidence recommendation based on this research. (2) Aim for regular sun exposure (about 10-30 minutes several times per week, depending on skin tone and season), which is a low-to-moderate confidence recommendation since this study didn’t directly measure sun exposure. (3) Eat vitamin D-rich foods like fatty fish, egg yolks, and fortified milk—moderate confidence. (4) Consider a vitamin D supplement if your doctor recommends it, especially in winter months—moderate confidence based on this and other research.

This research matters most for: older adults (especially women over 50), people with osteoporosis or a family history of it, people who don’t get much sun exposure, and people living in climates with long winters. The findings are less directly relevant to young, healthy people with good sun exposure, though vitamin D is still important for everyone. If you live outside Japan, the trends might be different in your country, so check local research.

If you start improving your vitamin D levels now through sun exposure, diet, or supplements, you might see improvements in blood tests within 2-3 months. However, the real benefits for bone health—like reduced fracture risk—typically take years to become apparent. This is why the 10-year timeframe in this study is so valuable: it shows long-term trends rather than quick fixes.

Want to Apply This Research?

  • Track your vitamin D supplementation (if taking it) and sun exposure minutes per week. Set a goal of 150+ minutes of moderate sun exposure weekly (adjusted for your skin tone and season), and log it in your app. If you’ve had a blood test, enter your vitamin D level (in ng/mL) and track changes over time.
  • Use the app to set a reminder for daily sun exposure during appropriate times (typically 10-30 minutes, 3-4 times per week depending on season and skin type). Create a checklist for vitamin D-rich foods to eat each week (salmon, egg yolks, fortified milk). If your doctor recommends supplements, set a daily reminder to take them at the same time each day.
  • Check your vitamin D levels annually through your doctor and log the results in the app to track trends over time. Monitor any bone-related symptoms or fractures. Track seasonal patterns—vitamin D levels often drop in winter, so you might notice your supplementation needs to increase during darker months. Share your data with your healthcare provider to adjust recommendations as needed.

This research shows trends in vitamin D levels in a Japanese population and suggests potential benefits for bone health, but it does not prove that improving vitamin D levels will prevent fractures or osteoporosis in any individual. Vitamin D needs vary by age, skin tone, location, and health status. Before making changes to sun exposure, diet, or starting supplements, consult with your healthcare provider, especially if you have bone disease, take medications, or have other health conditions. This summary is for educational purposes and should not replace professional medical advice. Always discuss vitamin D testing and supplementation with your doctor to determine what’s right for your individual situation.