New study finds outdoor running helps maintain vitamin D levels

Vitamin D dips in winter as sunlight weakens, but a study of runners and non-runners found outdoor exercise helped protect levels naturally. Supplements supported immune balance, yet they did not boost performance, leaving a practical question about what really helps most.

As winter settles in and daylight fades earlier each evening, your body quietly loses one of its most important natural resources. Vitamin D, often called the “sunshine vitamin,” depends heavily on sunlight exposure. During colder months, weaker ultraviolet B rays reach the Earth’s surface, and people spend more time indoors. For many, vitamin D levels begin to drop without warning.

Researchers from the EHU-University of the Basque Country and the University of Urbino in Italy wanted to understand what this seasonal decline means for health and physical performance. Their study focused on recreational runners and non-runners during autumn and winter. Some participants took vitamin D supplements daily for eight weeks, while others did not.

The findings revealed something both simple and powerful. Daily vitamin D supplementation helped maintain healthier blood levels and supported parts of the immune system. However, the supplements did not improve athletic performance. The study also highlighted another important point: spending time outdoors while exercising may naturally protect against vitamin D deficiency better than many people realize.

Vitamin D plays a major role in helping the body function properly. It supports bone health by regulating calcium. It also contributes to muscle recovery and immune balance. Researchers estimate that around 1 billion people worldwide have low vitamin D levels.

About 80% of the vitamin comes from sunlight exposure. The remaining amount comes from food. During winter, this balance becomes harder to maintain because sunlight weakens and skin exposure decreases.

Scientists measure vitamin D using blood concentrations known as serum 25(OH)D₃. Levels below 20 nanograms per milliliter indicate deficiency. Levels between 20 and 29 are considered insufficient. Concentrations above 29 are generally considered sufficient.

Athletes and physically active people can also experience low levels, especially if they train indoors. Previous research suggested vitamin D might improve muscle strength, endurance and recovery. Other studies, however, found little evidence of performance benefits.

The researchers behind this study wanted clearer answers.

The research followed 45 healthy adults between 20 and 45 years old from October 2023 through March 2024. The participants included 23 recreational runners and 22 non-runners.

The runners were not elite athletes. They were amateurs with at least three years of endurance training experience. They regularly logged workouts and trained outdoors.

Participants were divided into four groups. One group of runners received vitamin D supplements. Another group of runners received no supplements. The same arrangement applied to non-runners.

Those receiving treatment took 2,000 international units of vitamin D₃ daily for eight weeks. Researchers measured blood markers, physical performance and immune indicators at the beginning of the study, after eight weeks and again after a 12-week follow-up period without supplementation.

Weather conditions also changed dramatically during the study. In October, average temperatures measured 18.1 degrees Celsius. By December, temperatures had fallen by roughly 10 degrees. Solar radiation also dropped significantly during that time.

One of the study’s most surprising findings involved the runners who did not take supplements.

“At the beginning and end of the study, we carried out blood tests and physical tests on all of them. Naturally, the vitamin D levels rose in those who had taken the supplement. Beforehand we knew that would happen, but there is a significant finding: after the two-month period, the vitamin D levels of the runners who did not take the supplement and the non-runners who did were very similar,” explained researcher Eneko Fernández.

That finding suggests outdoor physical activity itself may help protect vitamin D levels during colder months. According to Fernández, runners trained outdoors with more skin exposed to sunlight. Even reduced winter sun appeared to provide meaningful benefits.

“So, one of the main conclusions is that performing physical activity outdoors is a very effective strategy for combating vitamin D deficiency, and it also offers other health benefits that supplementation does not offer,” he added.

The blood data supported this observation. Non-runners without supplementation experienced the sharpest decline in vitamin D during winter. Their levels dropped by more than 32%.

Meanwhile, runners who trained outdoors maintained stronger vitamin D levels despite not taking supplements.

Researchers also examined white blood cells, which help defend the body against infections and viruses. The results suggested vitamin D supplementation helped support immune balance during winter.

The study focused especially on leukocytes and neutrophils, two important categories of white blood cells. Participants taking supplements showed more stable levels during the colder months.

“In other words, the supplements have a positive effect on white blood cells, which are the body’s defence cells. That does not mean that taking vitamin D supplements will prevent you from getting ill. What it does mean is that it helps to maintain a more balanced immune system and strengthens the body’s defences against infections and viruses,” Fernández said.

Non-supplemented non-runners showed the steepest declines in immune markers over time. Their leukocyte and neutrophil counts gradually decreased between October and March.

Researchers did not observe major changes in lymphocytes or monocytes. Calcium levels also remained within healthy ranges throughout the study.

Despite improved vitamin D levels and immune stability, the supplements did not enhance physical performance.

Researchers tested maximum oxygen consumption, explosive leg power and isometric leg strength. None of these measurements improved significantly after supplementation.

The runners consistently demonstrated stronger fitness levels than the non-runners, but the supplements themselves did not provide extra performance gains.

“Vitamin D is not like taking steroids or EPO. It doesn’t improve performance. That was no surprise either, but the study served to confirm this,” Fernández said.

Measurements of aerobic fitness remained stable across all groups. Jump performance and leg force also showed no meaningful improvement tied directly to supplementation.

The researchers concluded that vitamin D may help support health during winter, but it should not be viewed as a shortcut to improved athletic ability.

One of the study’s strongest messages centered on everyday lifestyle choices rather than pills alone.

Fernández emphasized that outdoor activity may provide benefits that supplements cannot fully replace. Sunlight exposure, fresh air and physical movement work together in ways that support overall health.

“The recommendation is to be outside more in the winter and, if possible, with less clothing on. The way to do this without getting cold is to exercise in the fresh air. For example, just going outside and running at your own pace for 20-30 minutes is enough. There is a huge difference between doing nothing and doing very little. It has major benefits,” he stressed.

That advice may feel especially meaningful during darker winter months, when many people naturally become less active and spend more time indoors.

The study also showed that supplementation effects fade over time once intake stops. After the 12-week follow-up period, vitamin D levels declined again across all groups.

This study suggests that moderate outdoor exercise during autumn and winter may help people naturally maintain healthier vitamin D levels. Even recreational activity, such as short outdoor runs or walks, appears beneficial. That finding could encourage more people to stay active during colder months rather than remaining indoors.

The research also shows that vitamin D supplementation may help stabilize parts of the immune system during seasons when infections tend to rise. For people at risk of deficiency, especially non-runners or individuals with limited sunlight exposure, supplements may provide additional support.

At the same time, the findings challenge the idea that vitamin D acts as a performance-enhancing supplement. Instead, the results reinforce that consistent exercise and healthy outdoor habits remain more important for physical fitness.

Future research may explore longer supplementation periods, higher doses and how vitamin D affects different age groups or athletic populations. For now, the study highlights a simple but valuable lesson: sunlight, movement and fresh air still matter deeply for human health.

Research findings are available online in the journal Scientific Reports.

The original story "New study finds outdoor running helps maintain vitamin D levels" is published in The Brighter Side of News.

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