The Role of Vitamin D Supplementation for Endurance Athletes: Optimizing Performance During Winter Months

For endurance athletes—runners, cyclists, triathletes—the winter months bring more than just colder weather and fewer daylight hours. The decrease in sunlight directly impacts an essential factor in athletic performance and overall health: Vitamin D. Primarily synthesized in the skin through sunlight exposure, Vitamin D is vital for bone health, muscle function, immune defense, and even athletic performance.

In this article, we’ll delve into the science behind Vitamin D supplementation, its specific benefits for endurance athletes, and why it becomes particularly important during the winter season.

What is Vitamin D?

Vitamin D is a fat-soluble vitamin that acts more like a hormone, regulating calcium and phosphate in the body to maintain healthy bones and muscle function (Holick, 2007). While Vitamin D is found in small amounts in foods like fatty fish and fortified products, the primary source is sunlight exposure, which triggers Vitamin D synthesis in the skin (Holick, 2011).

There are two main forms of Vitamin D:

• Vitamin D2 (ergocalciferol) – Derived from plant sources.

• Vitamin D3 (cholecalciferol) – Produced by the skin from sunlight and found in animal-based sources.

Vitamin D3 is more effective than D2 at raising blood levels of 25-hydroxyvitamin D [25(OH)D], the biomarker used to assess Vitamin D status (Tripkovic et al., 2012).

Why Are Endurance Athletes at Risk of Vitamin D Deficiency?

Vitamin D deficiency is common among endurance athletes, particularly during winter months. Studies show that athletes, especially those training at high latitudes or indoors, are often at risk of Vitamin D deficiency due to limited sun exposure (Ogan & Pritchett, 2013). Research suggests that 56% of athletes training in Northern Europe exhibit suboptimal Vitamin D levels during the winter (Morton et al., 2012).

Endurance athletes are particularly vulnerable due to:

• Early morning or late evening training: Many runners and cyclists train when sunlight is minimal.

• High latitude living: Athletes in northern regions above 35 degrees latitude (e.g., most of Europe and North America) receive insufficient ultraviolet B (UVB) light for Vitamin D synthesis from late fall to early spring (Holick, 2007).

• Indoor training: Cyclists who train on indoor trainers and triathletes who swim in indoor pools miss vital UVB exposure.

Research shows that in Liverpool, England, 36% of Premier League soccer players had low Vitamin D status during the winter months (Morton et al., 2012). Comparable statistics are found among other endurance athletes, underscoring the importance of monitoring and supplementing during these darker months.

The Impact of Vitamin D on Endurance Performance

1. Aerobic Capacity

Vitamin D is critical for muscle function, and emerging evidence suggests that it plays a role in enhancing aerobic capacity—a key factor for endurance athletes. A systematic review conducted by Wyatt et al. (2024) found that Vitamin D supplementation improved VO2max, a measure of aerobic endurance, in athletes with low baseline levels. For instance, Jastrzębska et al. (2018) demonstrated an increase in VO2max among soccer players after eight weeks of Vitamin D3 supplementation (5000 IU/day), with a stronger effect in those who were deficient at baseline.

This increase in VO2max could be particularly beneficial for long-distance runners, cyclists, and triathletes, whose performance relies heavily on efficient oxygen utilization. The precise mechanisms are not fully understood, but studies suggest that Vitamin D may enhance erythropoiesis (red blood cell production) and improve oxygen transport during exercise (Smith & Tangpricha, 2015).

2. Muscle Strength and Recovery

Endurance athletes often focus on aerobic endurance, but muscle strength plays an essential role in supporting sustained performance and preventing injuries. While Vitamin D's role in strength is often studied in power athletes, there are compelling reasons to consider its importance for endurance athletes as well.

Studies indicate that Vitamin D3 supplementation can improve muscle strength by supporting muscle fiber regeneration and reducing muscle damage following intense exercise (Ceglia et al., 2013). A study by Jastrzębska et al. (2022) observed improved sprint performance in young soccer players after Vitamin D3 supplementation (20,000 IU twice weekly for eight weeks), though the impact on endurance athletes like cyclists remains to be fully explored (Wyatt et al., 2024). Nevertheless, the potential benefits for recovery following long endurance training sessions are noteworthy.

Bone Health and Injury Prevention

Maintaining bone health is critical for endurance athletes, especially runners who place repeated stress on their bones during long training sessions. Adequate Vitamin D levels are essential for calcium absorption and maintaining bone mineral density (BMD), which can prevent stress fractures and other injuries common in high-mileage athletes (Holick, 2007).

Several studies suggest that Vitamin D deficiency can lead to bone stress injuries. For example, a review by Dao et al. (2015) highlighted the association between low 25(OH)D levels and an increased risk of stress fractures in military and athletic populations. Moreover, Wyatt et al. (2024) pointed out that serum Vitamin D concentrations below 30 nmol/L are associated with a higher risk of such injuries.

While more research is needed to confirm the extent of Vitamin D’s role in directly enhancing bone mineral density, supplementation appears to reduce bone turnover markers and may protect against injury during high-impact activities like running (Wyatt et al., 2024).

Immune Function and Recovery

Endurance athletes often face increased susceptibility to illnesses, especially following long training sessions or races. Vitamin D plays an important role in immune modulation, reducing the risk of infections and aiding recovery.

Studies show that Vitamin D supplementation may reduce inflammatory markers such as C-reactive protein (CRP)and interleukin-6 (IL-6), which are elevated during periods of intense exercise (Wyatt et al., 2024). By modulating immune response, Vitamin D could help athletes recover faster from hard training sessions and prevent illness, a crucial factor during peak training periods.

Why Supplementation is Key During Winter

During the winter, UVB light is insufficient for Vitamin D synthesis at latitudes above 35 degrees north, which includes most of the U.S., Europe, and Canada (Holick, 2007). As a result, endurance athletes training in these regions are at a heightened risk of Vitamin D deficiency.

Testing and Supplementation Guidelines:

Athletes should get their serum Vitamin D levels tested to determine if supplementation is necessary. Blood levels of 25(OH)D are the gold standard for assessing Vitamin D status, with optimal levels typically between 50-75 nmol/L. Supplementation becomes important if levels fall below this range, and doses of 2000-5000 IU/day are often recommended for maintaining sufficiency (Wyatt et al., 2024).

In cases of deficiency, higher doses may be necessary under medical supervision, as some studies recommend doses of 10,000 IU/week or more for athletes with severe deficiency (Holick, 2011).

Ensuring Safe Supplementation

For endurance athletes, ensuring the quality and safety of supplements is essential, particularly when competing in events governed by anti-doping regulations. Athletes should choose supplements that are batch-tested for banned substances. The Informed-Sport program provides an excellent resource for verifying that supplements are safe and free of contamination.

Conclusion: Optimizing Performance with Vitamin D

Endurance athletes, especially those living in northern regions or training indoors during the winter, should be aware of their Vitamin D levels and consider supplementation. Vitamin D plays a crucial role in aerobic performance, muscle strength, bone health, and immune function—all key components of endurance sports. By maintaining adequate Vitamin D levels, athletes can optimize their training, improve recovery, and reduce the risk of injury.

Getting tested, supplementing as needed, and choosing certified supplements are vital steps in ensuring that you’re ready to perform at your best, even when the sun isn’t shining.

References

Ceglia, L., Niramitmahapanya, S., Morais, M., Rivas, D. A., Harris, S. S., Bischoff-Ferrari, H. A., & Fielding, R. A. (2013). A randomized study on the effect of vitamin D₃ supplementation on skeletal muscle morphology and vitamin D receptor concentration in older women. Journal of Clinical Endocrinology & Metabolism, 98(12), E1927-E1935.

Dao, D., Sodhi, S., Tabasinejad, R., Farrokhyar, F., et al. (2015). Serum 25-hydroxyvitamin D levels and stress fractures in military personnel: A systematic review and meta-analysis. The American Journal of Sports Medicine, 43(8), 2064-2072.

Holick, M. F. (2007). Vitamin D deficiency. The New England Journal of Medicine, 357(3), 266-281.

Holick, M. F. (2011). Vitamin D: Evolutionary, physiological and health perspectives. Current Drug Targets, 12(1), 4-18.

Jastrzębska, M., Kaczmarczyk, M., Michalczyk, M., & Jastrzębski, Z. (2018). Can supplementation of vitamin D improve aerobic capacity in well-trained youth soccer players? Journal of Human Kinetics, 61, 63-72.

Morton, J. P., Iqbal, Z., Drust, B., Burgess, D., Close, G. L., & Brukner, P. D. (2012). Seasonal variation in vitamin D status in professional soccer players of the English Premier League. Applied Physiology, Nutrition, and Metabolism, 37(4), 798-802.

Ogan, D., & Pritchett, K. (2013). Vitamin D and the athlete: Risks, recommendations, and benefits. Nutrients, 5(6), 1856-1868.

Smith, E. M., & Tangpricha, V. (2015). Vitamin D and anemia: Insights into an emerging association. Current Opinion in Endocrinology, Diabetes & Obesity, 22(6), 432-438.

Tripkovic, L., Lambert, H., Hart, K., Smith, C. P., Bucca, G., Penson, S., Chope, G., Hyppönen, E., Berry, J., Vieth, R., & Lanham-New, S. A. (2012). Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: A systematic review and meta-analysis. The American Journal of Clinical Nutrition, 95(6), 1357-1364.

Wyatt, P. B., Reiter, C. R., Satalich, J. R., et al. (2024). Effects of vitamin D supplementation in elite athletes: A systematic review. Orthopaedic Journal of Sports Medicine, 12(1).