A study conducted by Eneko Fernández from the University of the Basque Country (EHU), in collaboration with the University of Urbino, reveals insights on vitamin D supplementation among runners and non-runners. Published on May 21, 2026, the research indicates that while vitamin D supplements can enhance immune system function, they do not improve athletic performance.
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The study involved participants taking daily vitamin D supplements for eight weeks during the autumn and winter months, while others did not take the supplement. Blood tests revealed that those on supplements exhibited elevated vitamin D levels, but interestingly, the vitamin D levels of outdoor runners who did not take supplements were similar to those of non-runners who did.
Fernández noted that outdoor training contributes to better vitamin D levels due to increased sun exposure. He emphasized that exercising outside is an effective approach to combat vitamin D deficiency, offering additional health benefits not provided by supplementation.
In terms of immune system impact, the study found that vitamin D supplements improved white blood cell counts, enhancing the body’s defense mechanisms against infections. However, the results showed no direct correlation between vitamin D supplementation and physical performance, as no improvements were observed in maximum oxygen consumption or leg power measurements.
Fernández explained that unlike performance-enhancing substances, vitamin D does not boost athletic performance. He advised that the best way to maintain vitamin D levels during winter is to engage in outdoor activities, recommending 20-30 minutes of exercise in fresh air while dressed appropriately for the weather. This approach emphasizes the significant health advantages of even modest physical activity compared to remaining inactive.