Vitamin D Supplements Protect Runners' Immunity — But Won't Make Them Faster

Every autumn, as the days shorten and temperature drops, something quiet happens inside recreational runners: their vitamin D levels start to slip. They keep training. They feel fine. And then, somewhere between November and March, their immune cells — the neutrophils that form the first line against infection — begin to fall away too. A new study published in Scientific Reports by researchers at the University of Urbino Carlo Bo and collaborating Italian institutions followed 45 adults from October through March to see whether 2,000 IU of daily vitamin D₃ could change that picture. It could — but only partially, and not in the way most runners were probably hoping.

The Vitamin D Problem Athletes Don't Know They Have

Ask most recreational runners if they worry about vitamin D, and the honest answer is probably no. They're outdoors every day. Sunlight should handle it. Except that above roughly 43° latitude — which includes most of northern Italy, France, Germany, and nearly all of the UK and Canada — UVB radiation from October to March is too weak to meaningfully trigger the skin's vitamin D synthesis. The participants in this study trained at latitudes between 43.6°N and 43.9°N, right on that borderline. Runners in the study started October with slightly deficient levels (just below 30 ng/mL), while their sedentary counterparts were already in the insufficient range. By December, the non-runners who took no supplement had dropped a full 32% from their October baseline. That's a steep fall over just eight weeks.

What Previous Studies Got Wrong About the Follow-Up

There's a methodological gap that most vitamin D trials quietly ignore: they stop measuring participants as soon as the supplements run out. This study didn't. After the 8-week supplementation phase ended in December, the team kept tracking everyone for a further 12 weeks — all the way through to March. That decision turned out to matter quite a lot. By March, vitamin D levels in the supplemented runners had collapsed from their December peak of 35.35 ng/mL back down to just 23.52 ng/mL. Still above baseline, but heading in the wrong direction fast. The supplemented non-runners followed the same arc. The body, it turns out, doesn't stockpile vitamin D from a two-month course in the way you might hope. Without continued supplementation — or meaningful sun exposure — the gains erode surprisingly quickly.

Why Does Vitamin D Stabilise Immunity But Not Performance?

Here's what's strange. The supplement did something real to participants' immune profiles. In the unsupplemented group — particularly the non-runners — total white blood cell counts and neutrophil levels fell steadily through winter. Neutrophils dropped 19% by December and 32% by March. Among those who took the supplement, that decline was blunted. The time-by-supplementation interaction for both total leukocytes (p = 0.026) and neutrophils (p = 0.026) was statistically significant. Vitamin D receptors are found throughout immune tissue — monocytes, macrophages, neutrophils all carry them — and research suggests vitamin D acts on those receptors to support granulopoiesis, the production of white cells. So the immune signal is plausible. Performance, though? VO₂max barely moved in any group across the entire 20-week period. Runners clocked roughly 60 mL/kg/min at T0, T1, and T2 regardless of whether they took the pill. Jump height didn't change. Maximal isometric leg press force edged toward significance (p = 0.056) in supplemented subjects, suggesting a possible strength signal, but it didn't clear the threshold. At 2,000 IU daily, vitamin D appears to act more like a maintenance micronutrient than a performance lever.

What This Actually Means for Runners This Winter

The practical takeaway is specific, and the researchers are fairly direct about it. Non-runners — meaning anyone without a structured outdoor training programme — should probably start 2,000 IU of vitamin D₃ daily in October, before levels begin their seasonal slide. Runners, who get more sun exposure through training and tend to start the season with higher baseline levels, can likely wait until December. After that, both groups need to keep going. The study's follow-up data make a compelling case that stopping supplementation in December, which is when many people quit after an 8-week course, is exactly the wrong moment — the coldest, darkest stretch of winter still lies ahead. It's also worth flagging what didn't change: carbohydrate intake was significantly higher in runners than non-runners (about 250 g/day vs. 200 g/day), but dietary vitamin D from food was nearly identical across all four groups, sitting around 2.9 µg per day. Neither running nor diet explained the differences in vitamin D status. Sun exposure did.

The Questions the Study Leaves Open

The trial was small — 10 to 13 people per group — which is enough to detect the large vitamin D effect, but probably underpowered for the subtler performance signals. The maximal isometric force trend (p = 0.056) is exactly the kind of finding that a larger study might resolve. Sex-specific analysis wasn't done here; the male-to-female split varied across groups, and there's good reason to think vitamin D response differs between sexes. The study also used 2,000 IU — the European safe upper limit — but some prior trials with 6,000 IU/day reported aerobic and anaerobic gains, which raises the question of whether dose matters more than the trials at 2,000 IU can show. Future work, the authors suggest, should also look at ionised calcium, gut microbiota composition, and genetic variants in vitamin D metabolism — all of which could explain why some people respond robustly to supplementation and others barely move. The answers are coming, just not yet.

• Start earlier than you think — Non-runners should begin daily 2,000 IU vitamin D₃ in October; waiting until symptoms appear means immune deficits have already set in.
• Don't bank on outdoor training — Even runners averaging 50+ km a week outdoors saw significant vitamin D drops after stopping supplements, suggesting sun exposure in winter months isn't enough at northern latitudes.
• Performance gains need a higher bar — At 2,000 IU, vitamin D protects immune function but doesn't appear to improve VO₂max, jump height, or isometric strength in already-active adults.

"While non-runners should begin a daily intake of 2,000 IU vitamin D in October, runners ought to initiate supplementation by December to prevent insufficiency or deficiency that may result in immune alterations and associated pathologies." — Gervasi et al., Scientific Reports, 2026.