In endurance training, you know nutrition is key to managing athletic performance. You focus on strategically incorporating balanced nutrition into your training regimen by monitoring your calories, carbohydrate, lipid, protein, and fluid intakes. But there is one micronutrient that you may have overlooked, that is vitamin D. Not only is vitamin D associated to bone health as a regulator of calcium and phosphorus homeostasis, but has also been the focus of several scientific studies investigating its effects on athletic performance.
Before we dive into what the studies are saying, let’s first cover some basics about vitamin D.
Vitamin D comes in different forms depending on the source and is metabolized through different pathways in your body. Vitamin D3 originates from dietary animal sources such as beef, eggs, and fatty fish, while vitamin D2 can be found in plant sources like shitake mushrooms. Nowadays, you can also find vitamin D in fortified foods such as cereals, dairy, or orange juice. Keep in mind that dietary sources provide minimal amounts of vitamin D.
The best source is exposure to sunlight because of your skin’s ability to synthesize vitamin D. The resulting pre-vitamin D3 diffuses into the blood and is transported to organs such as the liver, kidney, skeletal muscle, or adipose (fat) tissue. Vitamin D synthesized in the skin provides about 80% to 100% of your body’s needs1.
So, why is vitamin D so important to your health and performance?
Once vitamin D is metabolized into its active form, Calcitriol, it has been found to be an integral part of several functions in the body, targeting many tissues such as bone, intestines, kidneys, heart, muscle, pancreas (b cells), central nervous system, immune system, and more. It is no surprise that vitamin D deficiency has been linked to a higher risk for chronic diseases such as cardiovascular disease, hypertension, diabetes, arthritis, and certain cancers<sup>1,2,3</sup>. What you may find most interesting is that several studies have also demonstrated the relationship between vitamin D and athletic performance, muscle function, and inflammation (as summarized below):
- Performance. A correlation between seasonal training (amount of sun exposure) and performance found peak performance during the summer months and declining performance in the winter<sup>3</sup>. Although a correlation was found, further research is necessary to rule out other contributing factors. On the other hand, well-known German and Russian studies in the early 20th century demonstrated how UVB radiation improved some components of athletic performance on physically active/athletic subjects, especially those who were found with vitamin D insufficiency or deficiency<sup>4</sup>.
- Muscle function. Muscle weakness and impaired muscle strength are commonly known symptoms of vitamin D deficiency<sup>2</sup>. Several studies also suggested a direct relationship of vitamin D to muscle strength, mass, and function that are important to athletic performance.
- Inflammation. A recent study on 19 healthy, endurance-trained, male and female runners showed a possible link between low vitamin D status and increased levels of an inflammatory marker, cytokine TNF-α<sup>5</sup>. This study suggested further investigation on vitamin D’s immunity functions on the athletic population.
You may have noticed that most of these studies focused on the impact of vitamin D insufficiency or deficiency. This is because vitamin D deficiency has been a growing concern with the general population and even in healthy adults engaged in athletics<sup>4</sup>. Inadequate dietary intake of vitamin D is just a small part of the problem.
As discussed earlier, sun exposure is the main source of this nutrient. Some contributing factors to vitamin D deficiency include training mostly indoors, early- or late-day training2, wearing too much sunscreen, living in higher latitude areas of the country<sup>1,2</sup> and/or being dark-skinned<sup>3</sup>.
But before you go out soaking in the sun or loading up on vitamin D supplements after reading this post, consider going to your doctor first to get tested on your vitamin D status. The physician may use circulating 25-OH D as a marker for your vitamin D status. According to the Endocrine Society’s clinical practice guidelines of vitamin D, the desirable serum concentration of 25-OH D is above 30 ng/mL (or > 75 nmol/L) to maximize good health. In general, it has also recommended at least 1,500 to 2,000 IU/day of supplemental vitamin D for adults or at least 1,000 IU/day for children and adolescents in order to consistently increase vitamin D status to normal serum levels.
Depending on your specific status (i.e., if you have insufficiency or deficiency) and health condition, it would be best to consult your doctor for the appropriate prescription of vitamin D supplementation, if necessary. Vitamin D toxicity is rare, but there is not enough evidence either to support if high amounts of vitamin D enhances or improves athletic performance.
Zen Huynh, B.S. Nutrition Sciences
Fuelixir Content Creator
- Hamilton, B. (2010), Vitamin D and Human Skeletal Muscle. Scandinavian Journal of Medicine & Science in Sports, 20: 182– 190. doi:10.1111/j.1600-0838.2009.01016.x
- Larson-Meyer, D. E., & Willis, K. S. (2010). Vitamin D and athletes. American College of Sports Medicine, 9(4), 220-226. doi:10.1249/JSR.0b013e3181e7dd45
- Angeline, M. E., Gee, A. O., Shindle, M., Warren, R. F., & Rodeo, S. A. (2013). The effects of vitamin D deficiency in athletes. The American Journal of Sports Medicine, 41(2), 461-464. doi:10.1177/0363546513475787
- Cannell, J. J., Hollis, B. W., Sorenson, M. B., Taft, T. N., & Anderson, J. J. (2009). Athletic performance and vitamin D. American College of Sports Medicine, 1102-1110. doi: 10.1249/MSS.0b013e3181930c2b
- Willis, K. S., Smith, D. T., Broughton, K. S., & Larson-Meyer, D. E. (2012). Vitamin D status and biomarkers of inflammation in runners. Open Access Journal of Sports Medicine, 3, 35–42. http://doi.org/10.2147/OAJSM.S31022