Functions of Vitamin D

Regulating Serum Calcium Levels

The most significant function of vitamin D is its role in regulating blood calcium levels. Serum calcium (regulated by vitamin D) is essential for muscle contraction, and nerve conduction, which are certainly important functions for athletic performance!  Active vitamin D regulates blood calcium levels through multiple mechanisms- increasing calcium reabsorption in the kidneys, resorption in bone, and promoting calcium absorption in the intestine.  

Low blood calcium signals the parathyroid hormone to release parathyroid hormone (PTH). The resulting increase in PTH signals the kidneys to convert calcidiol (inactive form) to calcitriol (active form). An increase in calcitriol stimulates the kidneys to increase calcium reabsorption.The increased PTH and resulting increase in active vitamin D from the kidneys signals the bone to release more calcium. Both of these mechanisms increase blood calcium levels. 

Additionally, the increase in calcitriol (active form of vitamin D) as a response to low blood calcium levels, helps with calcium’s absorption in the intestine through multiple mechanisms, including calcium transporter synthesis. 

The activation of vitamin D is influenced by serum calcium levels through an additional mechanism. When blood calcium levels are low, it increases the activation of the enzyme 1-alpha-hydroxylase, a rate limiting enzyme in the second/ final step of vitamin D’s activation. When calcium levels increase, the enzyme activity is suppressed. 

Other Functions

There has been some evidence to suggest that vitamin D plays a role in cardiovascular health (regulating blood pressure and cholesterol), diabetes, cancer, certain autoimmune diseases, and infections since observational studies have demonstrated a correlation to these ailments with low serum vitamin D levels. Vitamin D is believed to play a role in muscle cell synthesis and function since vitamin D receptors are present on and within muscle cells. 

Vitamin D and Exercise

Vitamin D plays a role in muscle function, both directly and indirectly. As discussed in the functions section, vitamin D receptors are present on and in muscle cells, and therefore believed to play a role in their synthesis. Active vitamin D (calcitriol) indirectly influences muscle function by increasing calcium uptake into cells. 

While the research is lacking that directly examines vitamin D metabolism for athletes, indirect research suggests that vitamin D needs for athletes are comparable to non-athletes. There is no evidence suggesting that vitamin D supplementation will have any performance benefits when the athlete has adequate vitamin D status. However, if the athlete is vitamin D deficient, their overall health and performance will be compromised. It is debatable if insufficient (but not deficient status) has negative impacts on performance. The research on this is lacking, and there is no consensus as to what is considered ‘insufficient’, which adds more complexity to the topic.

Athletes who train outside will have more opportunity to synthesize vitamin D from the sun, compared to athletes who spend a majority of their training indoors. Luckily, distance runners generally spend a majority of their training outdoors, increasing their opportunity to get more vitamin D. 

Requesting a vitamin D lab test from you doctor would be recommended for athletes who live above the 37th parallel and/ or consume little or no dairy in their diet,  Vitamin D deficiency or insufficiency may not manifest in noticeable symptoms, and is a common vitamin to fall short on since it is not widely distributed in foods. Supplementation would only be recommended for individuals who do not have sufficient serum vitamin D levels. Supplementing unnecessarily will not have any performance or health benefits. 

Vitamin D Needs- Synthesis, Food sources, and Daily Requirements 

The body gets multiple forms of vitamin D, which is usually a combination of dietary and endogenous synthesis of the vitamin. Ergocalciferol (vitamin D2) comes from plants, fortified milk, and some supplements. Cholecalciferol (vitamin D3) comes from  synthesis in the skin, which requires both cholesterol (7-dehydrocholesterol) and sun exposure. Both forms of vitamin D make their way to the liver for step 1 (of 2) in the activation of vitamin D. The liver converts both vitamin D2 and D3 into 25-hydroxy cholecalciferol (calcidiol). Most of the vitamin D in the body is in this inactive form, which is the form that is checked on serum vitamin D lab tests. 

The second and last step of vitamin D activation occurs in the liver, where it is converted to 1,25 dihydroxy cholecalciferol (calcitriol); this is the only form of vitamin D that has physiological activity. There is some evidence suggesting that vitamin D2 is less efficient in the conversion to calcitriol, making it less bioavailable than vitamin D3. 

Vitamin D is not widely found in foods. Some food sources include: cod liver oil, swordfish, salmon, sardines, tuna, herring, beef liver, eggs yolk (provide a very small amount), and fortified milk. The absorption efficiency is ~50%, and mostly depends on the amount of fat consumed with it since fat is required for its absorption (as it is a fat soluble vitamin).

The RDA is 600 international units (IU)/ 15 micrograms (ug) for ages 1-70 years of age, and 800 IU/ 20 ug for individuals >70 years of age. These RDAs were based on the amount of vitamin D needed for optimal bone health of the majority of the population. 

Some of the risk factors for inadequate vitamin D include inadequate sun exposure, living above the 37th parallel, use of sunscreen, dark skin, being elderly and inadequate intake of food sources of vitamin D. Most of the risk factors interfere with the endogenous synthesis of vitamin D, highlighting the importance of the endogenously synthesized contribution of the vitamin. ~30 minutes per day of midday sun exposure may be sufficient to meet vitamin D needs for individuals with light skin, and more for individuals with darker skin. Remember that sunscreen will impair the ability to synthesize  vitamin D; an SPF of just 15 decreases vitamin D synthesis by ~95%! Additionally, keep in mind, where you live, the season, and your age will all impact the sun exposure needed for adequate vitamin D synthesis. 

Recommendations for Staying in Good Status 

To help get adequate vitamin D, try to train outdoors during midday, and consume food sources of vitamin D, as discussed above. Considering the high prevalence of vitamin D insufficiency and deficiency, it may be a good idea to request biannual vitamin D lab tests from your doctor, and annual checks if you have risk factors of having insufficient levels, or if your level lean toward the lower end of the ‘normal’ range. By understanding your vitamin D levels, you may  prevent the development of a deficiency or correcting one that may be negatively impacting your health and performance. 

Reference: 

Forrest KY, Stuhldreher WL. Prevalence and correlates of vitamin D deficiency in US adults. Nutr Res. 2011 Jan;31(1):48-54. doi: 10.1016/j.nutres.2010.12.001. PMID: 21310306.

Dunford M & Doyle JA. Nutrition for Sport and Exercise 4th ed. Cengage. 2019.