An EHU study says that vitamin D supplements can help to strengthen the immune system, but not to improve athletic performance

Eneko Fernández has studied the effect of taking vitamin D as a supplement in runners and non-runners. The conclusions indicate that it may benefit the body's defence mechanisms, but it has no direct effect on maximum oxygen consumption, explosive leg power or isometric strength. The research also shows that runners who train outdoors have better vitamin D levels.

Vitamin D is important for the body to function properly: it balances the immune system, helps to keep bones healthy and benefits muscle regeneration. Yet, 1 billion people worldwide are vitamin D deficient. This is the case in winter, particularly, as UVB rays barely reach the Earth's surface. In fact, the body produces 80% of this type of vitamin through solar radiation (the remainder comes from the diet). Given this, and considering the physiological functions of vitamin D, the effect it might have on both health and physical performance when taken as a supplement has kindled interest.

In this context, in collaboration with the University of Urbino in Italy, the researcher Eneko Fernández from the EHU-University of the Basque Country conducted a study on runners and non-runners. Some of the participants took daily vitamin D supplements for eight weeks during the autumn and winter, while the others did not. “At the beginning and end of the study, we carried out blood tests and physical tests on all of them. Naturally, the vitamin D levels rose in those who had taken the supplement. Beforehand we knew that would happen, but there is a significant finding: after the two-month period, the vitamin D levels of the runners who did not take the supplement and the non-runners who did were very similar," explained Fernandez.

According to the researcher, this is because the runners do their training outdoors, in the sun, and with a bigger surface area of their skin exposed. “So, one of the main conclusions is that 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,” he added.

The effect of vitamin D supplements on the immune system and physical performance

By examining the participants’ blood analyses, the EHU study concluded that vitamin D supplements help to strengthen the immune system. Indeed, the white blood cell counts of those who took the supplement improved. “In other words, the supplements have a positive effect on white blood cells, which are the body's defence cells. That does not mean that taking vitamin D supplements will prevent you from getting ill. What it does mean is that it helps to maintain a more balanced immune system and strengthens the body's defences against infections and viruses,” said Fernández.

Regarding physical performance, the results are clear: the study confirmed that vitamin D supplements have no direct effect. These conclusions were drawn from the physical tests done by the participants. Maximum oxygen consumption and leg power (explosive, jumping, and isometric static leg press) were measured. No differences were observed in the three tests conducted before and after the treatment.

“Vitamin D is not like taking steroids or EPO. It doesn't improve performance. That was no surprise either, but the study served to confirm this. As did seeing, once again, that the runners had the best physical condition. And don’t think they were marathon runners. They were amateurs. So, while vitamin D supplements may offer some benefits, the recommendation is not to take them. The recommendation is to be outside more in the winter and, if possible, with less clothing on. The way to do this without getting cold is to exercise in the fresh air. For example, just going outside and running at your own pace for 20-30 minutes is enough. There is a huge difference between doing nothing and doing very little. It has major benefits,” the EHU researcher stressed.

Additional information

Dr Eneko Fernández belongs to the Faculty of Education and Sport. He teaches on the Degree in Physical Activity and Sport Sciences and on the Master's degree of the same name.

Bibliographic reference

Marco Gervasi, Eneko Fernández-Peña, Sabrina Donati Zeppa, Giosuè Annibalini, Alessia Bartolacci, Eugenio Formiglio, Deborah Agostini, Claudia Barbato, Guido Fiaccarini, Iacopo Spaccazocchi, Antonino Patti, Piero Sestili, Rosa Grazia Bellomo & Francesco Pegreffi

Effects of vitamin D supplementation during autumn and winter on blood biomarkers and physical performance in runners and non-runners

Scientific Reports

DOI: 10.1038/s41598-026-38528-1