Vitamin D Protects Immune System During Winter, But Won’t Boost Running Speed

Study: Vitamin D Supplements Help Your Blood Stay Winter-Ready, But Won’t Boost Performance

Every fall, something quiet happens inside your body. Days shorten, sunlight fades, and vitamin D levels begin to slide. A new study published in Scientific Reports tracked recreational runners and non-running adults through an entire autumn and winter supplementation period, then followed them for three months after they stopped. The results offer a clear, practical message for anyone thinking about winter health and a reality check for athletes hoping a daily pill might give them a training edge.

Vitamin D, sometimes called the “sunshine vitamin,” is produced mainly when skin is exposed to sunlight. As fall sets in and people spend less time outside, levels naturally drop. Researchers estimate about one billion people worldwide are deficient in the vitamin, which supports bone health, muscle function, and immune defense. Athletes who train indoors are among the most at risk.

Forty-five healthy adults participated in the trial, conducted between October 2023 and March 2024, a period chosen because it corresponds to months of minimal sunlight at the study location in central Italy. Participants were divided into four groups: runners who took the supplement, runners who did not, non-runners who took the supplement, and non-runners who did not. To be clear on terms: the non-runners were not necessarily couch-bound. The paper’s physical activity data actually classified most of them as “active,” though they had not engaged in structured running or fitness programs in the six months before the study.

Vitamin D Levels Fell Fast Without Supplementation

The most dramatic numbers came from non-running adults who took no supplement. Their vitamin D levels dropped more than 32% over the eight-week autumn period, falling from an average of about 25.9 ng/mL in October to about 17.8 ng/mL in December. Blood levels below 20 ng/mL are considered deficient. Simply doing nothing through fall was enough to push these adults into deficiency territory by midwinter.

Both supplemented groups, runners and non-runners alike, saw their levels rise over that same period. Supplemented runners went from about 30.5 ng/mL up to about 35.4 ng/mL. Supplemented non-runners climbed from about 25.0 ng/mL to about 30.2 ng/mL. Non-supplemented runners held relatively steady, which the researchers suggested may be because they continued training outdoors and getting some sun exposure even in autumn.

Benefits didn’t last long after the supplements stopped, though. By the March follow-up, vitamin D levels had fallen in every group, including those who had been supplementing. Supplemented runners dropped from about 35.4 ng/mL back down to about 23.5 ng/mL once they stopped their daily dose, a reminder that blood vitamin D can fall again within months after supplementation stops, especially during low-sunlight periods.

A Daily Supplement Helped Keep Immune-Related Blood Markers More Stable

Beyond vitamin D levels, the study tracked white blood cells, which the immune system relies on to fight infections and manage inflammation. Adults who took the supplement maintained more stable white blood cell counts across winter, while those who didn’t supplement showed a notable decline.

In the non-supplemented non-runner group, total white blood cell counts dropped about 15% by December and nearly 18% by March. One type of immune cell, neutrophils, which are among the body’s first responders against invading pathogens, fell by more than 31% in that same group over the course of the study. Supplemented participants showed no such dramatic decline.

For anyone thinking about winter health, that finding is worth noting carefully: the study tracked immune-related blood cells, not whether people actually caught fewer colds or infections. Still, it adds to a growing body of research suggesting that adequate vitamin D during winter may help maintain more stable immune markers during the months when respiratory illnesses tend to peak.

No Performance Boost Detected at This Dose

When it came to physical performance, the supplement made no measurable difference. Cardiovascular fitness and vertical jump height stayed flat across all four groups regardless of whether participants supplemented. Leg strength showed a trend toward improvement in supplemented participants, but the finding fell just short of statistical significance, so no firm conclusion can be drawn from it alone. The researchers noted that trend warrants follow-up with a larger group.

“Despite these physiological benefits, no ergogenic effects on aerobic or explosive performance were observed,” the authors concluded, using the word “ergogenic” to mean performance-enhancing.

Prior research on this question is mixed. Some studies using higher doses have found performance gains, while others have not. The dose used here, 2,000 IU per day, falls within the range the paper describes as commonly recommended and cited by many countries as an adult safe upper level.

Whether a runner or someone whose main activity is a daily walk, the immune marker benefit appeared to hold either way. What it won’t do, at least at this dose and duration, is make the legs stronger, the lungs more efficient, or the jumps higher. Athletes chasing a winter training edge will need to look elsewhere.

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Disclaimer: This article is based on a single peer-reviewed study and is intended for informational purposes only. It does not constitute medical advice. Consult a qualified healthcare provider before making any changes to your diet or supplement routine.

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