According to Gram Research analysis of a 2026 Australian study, adults may need a summer vitamin D level of 84 to 98 nmol/L to avoid deficiency through winter without supplements. Even so, 41% of participants were deficient by the end of winter, and only 8% in Victoria and Tasmania met the targets.
Australia’s current guidelines say a summer vitamin D level of at least 60 nmol/L should protect people from winter deficiency. According to research reviewed by Gram, that target may not be high enough. In a 2026 study of 251 adults in eastern Australia who did not take vitamin D supplements, 41% were deficient by the end of winter, including 31% of people whose summer levels met the 60 nmol/L guideline. The researchers estimate that levels of 84 to 98 nmol/L, depending on location, would be needed to keep most people adequate through winter without supplements. Only 8% of participants in Victoria and Tasmania reached these levels, and fewer than half of those in other regions did. The authors suggest that taking a vitamin D supplement during winter may be the most practical option for many people, especially those living farther from the equator.
Key Statistics
A 2026 cohort study of 251 Australian adults who did not take vitamin D supplements found that 41% were vitamin D deficient at the end of winter.
The same 2026 Australian study found that 31% of participants with end-of-summer vitamin D levels of at least 60 nmol/L were still deficient at the end of winter.
A 2026 Australian study estimated that summer vitamin D levels of 84 to 98 nmol/L, depending on location, are needed to keep about 90% of people adequate through winter without supplements.
The Quick Take
- What they studied: The researchers wanted to know how high people’s vitamin D levels need to be at the end of summer to avoid running low by the end of winter in Australia.
- Who participated: The study included 251 adults from Australia’s eastern states who were part of a larger sun and vitamin D trial. They were recruited through the national electoral roll, media, and email lists. Only people who did not take vitamin D supplements were included in this analysis.
- Key finding: Even among people whose summer vitamin D level was at least 60 nmol/L, 31% were deficient by winter’s end. Overall, 41% of participants were deficient. To keep about 90% of people adequate through winter without supplements, summer levels of 84 to 98 nmol/L were needed, depending on location.
- What it means for you: The current summer target may not be enough to protect you through winter, particularly if you live in southern parts of Australia. If your summer level is below the target, talking with your doctor about a winter supplement may be a reasonable step. Anyone considering supplements should first check with a healthcare professional.
The Research Details
This study followed people over time to see how their vitamin D levels changed from summer to winter. The researchers measured blood levels of 25-hydroxyvitamin D, the main form of vitamin D used to judge whether someone has enough. Participants were tested at the end of summer and again at the end of winter.
The study was a cohort study, which means it tracked the same group of people over a set period. It was built into a larger randomized controlled trial called the Sun-D Trial. The analysis used only the trial’s control group, the people who did not take vitamin D supplements. This allowed the researchers to see what happens to vitamin D levels naturally, without the influence of pills.
To estimate the summer levels needed to avoid winter deficiency, the researchers used statistical models. These models looked at how summer levels related to winter levels across different regions of Australia, which vary in sunlight and temperature.
Following the same people from summer to winter shows what really happens to their vitamin D levels over time. This is more useful than a single snapshot, because it links a person’s summer level directly to their later winter level. Because participants did not take supplements, the results reflect what sunlight and diet alone provide, which helps answer the question of how much summer buildup is needed.
The study drew on a randomized trial, a design that is generally considered strong, and its participants were tracked with blood tests at two points in time. However, the people in the study were recruited through volunteer methods, so they may not perfectly represent all Australians. The study also did not follow a large number of people, and results from a single cohort should be confirmed by other studies. It was peer reviewed and published in a respected endocrinology journal.
What the Results Show
The overall rate of vitamin D deficiency at the end of winter was 41%. Deficiency was defined as a blood level below 50 nmol/L, which is a common cutoff used by health experts.
Even people who met the current Australian summer guideline did not always stay in a healthy range. Among those with summer levels of at least 60 nmol/L, 31% were deficient by the end of winter. This suggests the current target may leave a meaningful share of people at risk.
To keep about 90% of people adequate through winter without supplements, the researchers estimated that summer levels would need to be 84 to 98 nmol/L. The exact number depended on where people lived, with the higher end likely applying to regions farther from the equator where winter sunlight is weaker.
Only 8% of participants in Victoria and Tasmania reached these higher targets. In other states and territories, fewer than half did. This suggests that most people are not building up enough vitamin D during summer to carry them through winter.
The results point to latitude, or how far a place is from the equator, as an important factor. People in southern regions, where winter days are shorter and sunlight is weaker, appear to need higher summer levels. The authors also noted that supplements taken in winter may be the most practical way to lower the risk of deficiency for people who fall short of the summer target. These findings may help clinicians decide who should be considered for winter supplementation.
Previous research has generally agreed that vitamin D levels drop during winter, especially at higher latitudes, and many guidelines have set summer targets to prevent this. This study adds to that work by estimating the specific summer levels needed in different Australian regions. It also suggests that the current guideline of 60 nmol/L may be too low to prevent winter deficiency for many people, which is an important update to how the guideline is understood.
The study included only 251 people, and they were volunteers rather than a random sample of the whole population, so the results may not apply perfectly to everyone. Only participants who did not take supplements were included, which makes the findings most relevant to people without supplements but does not directly show what happens when supplements are used. The estimated targets depend on statistical models, so they are best seen as estimates rather than exact figures. Factors such as diet, skin type, time spent outdoors, and clothing were not explored in depth here.
The Bottom Line
Research shows that the current summer target of 60 nmol/L may not be enough to prevent winter deficiency for many Australians. Without supplements, the study suggests summer levels of 84 to 98 nmol/L may be needed. Given the high rate of winter deficiency even among people who met the guideline, a winter vitamin D supplement may be a practical option for people whose summer levels fall short. Confidence in the overall pattern is moderate, while the exact target numbers are estimates that need further confirmation.
This research is most relevant to adults in Australia, especially those living in southern regions such as Victoria and Tasmania, and to people who spend little time outdoors. Clinicians, dietitians, and public health planners may find it useful. People who already take a vitamin D supplement, or who have conditions that affect vitamin D absorption or metabolism, should talk with a doctor before changing their routine. Children, pregnant people, and people with kidney or other health conditions need individual medical advice.
Vitamin D levels change gradually over the seasons, so the effects of changing summer habits or starting a winter supplement would appear over several months. A blood test can show where a person’s level stands, and many doctors recheck levels after a few months of a new routine. Results will vary from person to person, so it is best to plan for ongoing checks rather than expecting a quick change.
Frequently Asked Questions
What vitamin D level do I need in summer to avoid winter deficiency in Australia?
A 2026 Australian study estimated that summer levels of 84 to 98 nmol/L, depending on location, are needed to keep about 90% of people adequate through winter without supplements. The current guideline of 60 nmol/L may not be high enough for many people.
How common is vitamin D deficiency in Australian winters?
In a 2026 study of 251 adults who did not take supplements, 41% were vitamin D deficient at the end of winter. Even among people who met the 60 nmol/L summer guideline, 31% were still deficient by winter’s end.
Should I take a vitamin D supplement in winter if I live in southern Australia?
Researchers suggest that winter supplementation may be the most practical option for people who do not reach end-of-summer targets, especially at higher latitudes. Only 8% of participants in Victoria and Tasmania met the higher summer targets, so talking with a doctor about your level is a sensible next step.
Why does vitamin D drop in winter in Australia?
Winter days are shorter and the sun is weaker, especially in southern regions, so the skin makes less vitamin D from sunlight. This study found that people living farther from the equator appear to need higher summer levels to stay adequate through the colder months.
Want to Apply This Research?
- Log a vitamin D blood test result at the end of summer and again at the end of winter, noting the date and the number in nmol/L. Also track whether you took a supplement and its daily dose. Comparing the two numbers over the year can show how your personal level changes with the seasons.
- Set a reminder in late summer to check whether your most recent vitamin D level meets the research-based target range for your region. If it falls below, ask your doctor whether a winter supplement is right for you, and log the dose once you start. Log time spent outdoors in midday sun on a weekly basis to see how it lines up with your levels.
- Build a yearly cycle into the app: a blood test check at the end of summer, a second check at the end of winter, and a note on any supplement changes. Over several years, the data can show whether your summer levels are holding up and whether a winter supplement is needed. Review the trend with your healthcare provider at least once a year.
This article summarizes a single cohort study and is for general information only. It is not medical advice. Vitamin D needs vary by person, and the target levels described are research estimates. Talk with a doctor or other qualified healthcare professional before starting, stopping, or changing any vitamin D supplement, especially if you are pregnant, have kidney or bone conditions, or take other medications.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.