Vitamin D in winter: what the sun on your runs already gives you

As soon as the clocks go back, the sun starts clocking in late and knocking off early. And with it goes much of the vitamin D you make without noticing. So is it time to dash to the chemist for a tub? A recent trial in runners and non-runners helps separate what the pill does from what it does not.

The experiment

Researchers from the University of Urbino (Italy), with the University of the Basque Country taking part, followed 45 healthy young adults between October 2023 and March 2024, precisely the months when the sun is barely any use for making vitamin D. Half were genuine recreational runners: at least three years of running, three sessions a week and a good helping of kilometres outdoors. The other half did not run.

Each group was randomly split in two: some took 2,000 IU of vitamin D3 a day for eight weeks, from October to December, and the others took nothing. They were then followed for three more months, until March, to see what happened after stopping. The work was published in 2026 in Scientific Reports.

What happened to their levels

For reference, the study treats anything below 20 ng/ml as deficiency, 20 to 29 as insufficiency and 30 or more as sufficient. In October, the runners were slightly below 30 but above the non-runners. The supplement worked as expected: runners who took it went from about 30.5 to 35.4 ng/ml, and non-runners from 25 to 30.2.

The most striking figure is in the groups without the supplement. Non-runners who took nothing fell from 25.9 to 17.8 ng/ml in just two months, in other words into deficiency territory. Runners without the supplement, by contrast, kept their levels fairly steady until December, which the authors put down, most likely, to carrying on training outdoors. That said, from December to March levels dropped in both groups of runners: the sunshine buffer runs out too.

And performance… did not budge

Here comes the part that matters if you are after an edge. Vitamin D did not improve aerobic capacity (VO₂max) or jumping. Maximal isometric strength showed a trend towards improvement in those who took it, but it did not reach significance. The authors did see steadier white blood cell counts in the supplemented groups, a curious finding that, for now, does not translate into running faster. If it is a boost you are after, well-dosed caffeine has far more evidence behind it.

So, pill or no pill?

The authors suggest that non-runners start supplementing in October and runners by December at the latest. We would read that calmly: this is 45 young people, in a town in central Italy, over a single winter. It is an interesting lead, not a universal prescription. Vitamin D also causes problems in excess, and the right dose for you depends on your starting point, your age and your health. That is why the sensible decision goes through your doctor, who can order a 25-hydroxyvitamin D blood test if it makes sense in your case.

The moral, if you want one: your outdoor runs do more good than you log in the app. One of those things, it seems, is keeping your vitamin D afloat until winter really bites.