Outdoor Running and Winter Vitamin D Deficiency: Trial Findings

Follow the usual guidance and winter often means taking a vitamin D supplement every day. With less sunshine, your body struggles to produce enough on its own, so a tablet or strip is meant to make up the difference-an established fix for winter vitamin D deficiency, particularly for people who spend most of the season indoors.

However, the picture looks different if you regularly train outside. A new trial followed four groups of adults through the darkest part of the year, and one set of results landed where researchers did not expect it.

Winter vitamin D deficiency

Most vitamin D is produced in the body when ultraviolet light hits uncovered skin. Roughly 80% of our vitamin D supply comes from this route.

During winter at higher latitudes, the narrow range of ultraviolet light that drives vitamin D synthesis scarcely reaches ground level. As a result, production slows to almost nothing.

Current estimates suggest about one billion people worldwide have low vitamin D at any given moment, with the lowest levels typically appearing in the months with the shortest days.

Vitamin D plays a part in controlling calcium within bones, aiding muscle recovery, and keeping the immune system in balance.

Low levels have been associated with respiratory infections and slower wound repair-one reason the supplement market built around vitamin D has become so large.

Setting up the trial

For this study, researchers at the University of the Basque Country (EHU) in Spain partnered with colleagues at the University of Urbino (UniUrb) in central Italy.

Dr Eneko Fernandez-Pena, a sports scientist at EHU, helped to design and deliver the trial.

The research team enrolled 45 adults aged 20 to 45. Around half were amateur runners who trained outdoors several times a week; the remainder had not completed any structured endurance training in the previous six months.

Within both the runner and non-runner groups, half took a daily vitamin D3 supplement provided as a small dissolvable strip, while the others took nothing. The intervention lasted eight weeks starting in October, after which everyone continued through to March without supplements.

Blood tests tell the story

By December, blood tests showed a consistent trend: vitamin D increased in all participants who used the supplement.

Among runners who supplemented, vitamin D rose by about 21% compared with their October starting point. In non-runners who supplemented, the increase was nearly 29 percent.

Non-runners who did not supplement moved in the opposite direction. Over the same eight weeks, their vitamin D readings dropped by roughly a third-an archetypal example of winter vitamin D deficiency.

The runners who took nothing ended up in an unexpected place. From October to December, their vitamin D levels stayed essentially unchanged without supplementation-most likely because consistent outdoor training kept them exposed to whatever daylight December still provided.

Running and vitamin D: exercise rivals the pill

The clearest contrast appeared when researchers compared runners who took nothing with non-runners who took supplements. By December, these two groups recorded almost identical vitamin D levels-matching numbers reached via entirely different routes.

In effect, outdoor exercise substituted for the daily supplement. Sunlight reaching the forearms, calves, and the back of the neck during runs appeared sufficient to keep blood levels stable, even as temperatures fell and daylight narrowed towards the solstice.

This outdoor option also comes with benefits a supplement cannot reproduce.

“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,” said Fernandez-Pena.

Immune system signals

Participants who supplemented also showed more stable white blood cell patterns. Neutrophils-the cells that react most quickly to infection-remained nearer to October baseline values in the supplemented groups.

The sharpest shifts were seen in non-runners who took nothing. Their overall white cell count decreased by almost 18%, and neutrophils fell by nearly a third-the largest drop observed in any group.

Fernandez-Pena avoided overclaiming. Supplements did not make anyone resistant to colds or flu; rather, consistent with related research, they appeared to keep immune defences on a more even keel during the darker months.

No edge in performance

When it came to physical performance, there was no meaningful difference. Across three testing rounds, supplemented and non-supplemented participants produced almost the same results on every measure the team assessed.

Maximal oxygen uptake changed very little in any of the four groups from October through March. An explosive jump test also showed minimal movement.

A static leg-press strength test came closest to improving in the supplemented participants, but the change did not reach statistical significance. The researchers highlighted it as a point to examine more closely in future studies.

Performance-enhancing drugs such as EPO, which increases red blood cell production, operate by fundamentally changing physiology. Vitamin D works through a completely different pathway.

“Vitamin D is not like taking steroids or EPO. It doesn’t improve performance,” said Fernandez-Pena.

Previous trials have reported the same pattern: supplements increase blood markers, but do not translate into better times, higher jumps, or stronger scores.

A simpler prescription

Once supplementation ended, the seasonal pattern became obvious. By March, vitamin D levels had fallen across all groups, including the runners who had remained steady throughout autumn.

Those who had supplemented lost most of their gains within 12 weeks of stopping. Runners who never supplemented also declined as deep winter reduced their outdoor training.

By the end of follow-up, non-supplemented non-runners recorded the lowest vitamin D levels seen at any stage of the study.

Before this trial, the two main approaches to avoiding winter vitamin D deficiency had not been set against each other so directly in healthy adults.

Earlier research indicated that time outdoors could help, but it had not shown so clearly that a runner’s usual routine could match a daily supplement head-to-head.

Fernandez-Pena’s message was straightforward: if you can, leave the bottle alone and get outside. Spending 20 to 30 minutes at an easy pace several days each week, with a small amount of skin exposed, can help maintain levels through winter-and delivers something a supplement cannot.