Outdoor Running Can Counter Winter Vitamin D Deficiency

Follow the usual advice and winter means taking vitamin D every day. With less sunlight available, the body cannot produce enough independently, so a supplement is meant to make up the difference – the familiar response to winter vitamin D deficiency for people who spend the season indoors.

That calculation appears to be different for those who exercise outside. A recent trial followed four groups of adults during the darkest part of the year, with one group producing blood-test results that researchers did not anticipate.

Winter vitamin D deficiency

Most vitamin D is made by the body when ultraviolet light reaches uncovered skin. This route supplies roughly 80% of the body’s vitamin D.

During winter at more northerly latitudes, very little of the particular ultraviolet wavelength needed to start vitamin D production reaches the surface. As a result, production drops to almost nothing.

Current estimates suggest that about one billion people globally have insufficient levels at any one time. Concentrations are at their lowest during the months with the shortest daylight hours.

Vitamin D plays a role in regulating calcium in bones, aiding muscle recovery and maintaining a balanced immune system.

Lower levels have been associated with respiratory infections and delayed healing – one reason the supplement market built around it has become so substantial.

How the trial was arranged

For the trial, researchers from the University of the Basque Country (EHU) in Spain worked with colleagues from the University of Urbino (UniUrb) in central Italy.

Dr. Eneko Fernandez-Pena, a sports scientist at EHU, contributed to designing and conducting the research.

The researchers enrolled 45 adults between 20 to 45 years old. Around half were amateur runners who exercised outdoors several times weekly, while the remainder had completed no organised endurance training during the preceding six months.

Within both groups, half took a daily vitamin D3 supplement in the form of a small dissolvable strip, and the other half received no supplement. The eight-week programme started in October, after which all participants were monitored without supplements until March.

Blood tests reveal the pattern

The December blood tests showed a distinct trend: vitamin D increased in every participant using the supplement.

Runners taking pills rose by around 21% from their October baseline. Among non-runners taking pills, levels increased by nearly 29 percent.

Non-runners who received nothing experienced the reverse. Their readings dropped by about a third during the same eight weeks – a classic example of winter vitamin D deficiency.

The unsupplemented runners produced the unexpected outcome. Between October and December, their vitamin D levels remained stable without supplementation – probably because frequent outdoor training maintained their exposure to the limited December sunlight.

Outdoor running and vitamin D supplements

The clearest contrast emerged when the researchers compared runners taking nothing with non-runners taking pills. In December, their vitamin D readings were almost identical – the same outcome reached by different means.

Exercising outdoors had, in effect, matched the role of a daily capsule. Sunlight reaching runners’ forearms, calves and the backs of their necks was sufficient to maintain blood levels as temperatures fell and daylight shortened towards the solstice.

The outdoor option also provides benefits that a capsule 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.

Signals from the immune system

Participants who used supplements also had more consistent white blood cell counts. Neutrophils – the cells that react most quickly to infection – remained nearer to October levels among the supplemented groups.

The largest changes occurred in non-runners who received nothing. Their overall white blood cell count declined by almost 18%, while their neutrophil count dropped by nearly a third – the greatest reduction in any group.

Fernandez-Pena stressed that the result should not be overstated. Supplements did not make participants immune to colds or flu, but they did – consistently with a wider body of related research – help keep the body’s defences on a steadier level during darker months.

No performance advantage

There was no meaningful shift in physical performance. Across all measures assessed during three testing rounds, supplemented and unsupplemented participants recorded almost identical results.

Maximum oxygen uptake changed very little in any of the four groups between October and March. The explosive jump test showed similarly minimal movement.

A static leg-press strength assessment came nearer to improving among supplemented participants, but the difference did not reach statistical significance. The researchers identified it as an area for closer examination in future research.

Performance-enhancing substances such as EPO – a hormone that increases red blood cell production – act by changing the body’s physiology at a fundamental level. Vitamin D works through a wholly different process.

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

Previous trials found the same pattern: supplements increased blood markers but did not improve times, jumps or strength scores.

A more straightforward prescription

When supplementation ended, the seasonal nature of the system became evident. By March, vitamin D levels had declined in every group, including the runners whose levels had remained steady throughout autumn.

Within 12 weeks of stopping, supplemented participants had lost most of their gains. The runners who received nothing also declined as deep winter reduced their outdoor training.

The non-supplemented non-runners ended the study with the lowest vitamin D readings measured at any stage.

Before this trial, the two approaches to avoiding winter vitamin D deficiency had not been compared so directly in healthy adults.

Earlier research indicated that outdoor exposure could help, but no study had demonstrated directly that a runner’s usual routine could equal a daily supplement.

Fernandez-Pena’s conclusion was simple: avoid the bottle where possible and go outdoors. Spending 20 to 30 minutes moving at an easy pace on several days each week, with some skin exposed, may maintain levels through winter while offering something a pill cannot provide.