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Original Article: Shawn Hueglin, PhD, RD, CSSD

Update: By Robert Kunz MS

Pro Triathlete Jeanni Seymour soaking in some vitamin D.

Most athletes have a general understanding of how many grams of carbohydrate, protein, and fat they eat per day. However, few athletes are concerned with the amount of vitamin D they consume and synthesize in their bodies each day. In the past, the importance of this nutrient has often been overlooked. Researchers and sports dietitians have long recognized the important role vitamin D plays in bone health and that a serious deficiency can lead to rickets and osteomalacia.    Though athletes are frequently exposed to the sun, vitamin D insufficiency rates range between 37% – 83%. Emerging evidence shows that vitamin D deficiency can also increase the risk for cardiovascular disease, diabetes, hypertension, autoimmune diseases, compromised immune function, exercise-related inflammation and certain types of cancer. Recent data suggests vitamin D may play a role in athletic performance.

Vitamin D is a unique nutrient in that it can be synthesized in the body when the skin is exposed to UVB radiation. It is very common for people to rely on this process for vitamin D intake. However, because sunlight is required to activate the process, any factor that limits the quality of sun exposure can compromise the status of vitamin D in the body. Some of these factors include sunscreen, aging, skin pigmentation, clothing, cloud cover, time of day and latitude. Vitamin D can also be obtained from a limited number of dietary sources which include fatty fish, cod liver oil, egg yolk and fortified products such as milk, yogurt, orange juice and cereals.

Recently, Vitamin D deficiency and insufficiency (also called marginal deficiency) have received a lot of attention in the media. Some researchers have used the term epidemic due to the high prevalence reported in all age groups (14 – 94% depending on race, geographic region, presence of disease and age). Vitamin D deficiency is typically defined as a blood concentration of 25(OH)D < 20 – 25 nmol/L whereas insufficiency is defined as a concentration of < 37.5 – 50 nmol/L . Recent research indicates blood concentrations of 25(OH)D 75 – 80 nmol/L may be required to support optimal functioning. Athletes are not immune to deficiency or insufficiency, yet less is known about the levels of vitamin D in this population (~37 – 68% based on 3 published studies). An athlete’s training environment (indoor vs outdoor), use of sunscreen and season of assessment may influence these values.

Due to the important role vitamin D plays in bone health, immune function and inflammatory response, vitamin D status may impact an athlete’s ability to adapt to training and improve performance.

  • Stress fractures that prevent optimal training are a common problem in athletes. Evidence suggests an association between decreased serum concentrations of vitamin D and increased risk of stress fractures in males and females. There is also data to support that supplementation with vitamin D can decrease the incidence of stress fractures. Therefore, compromised vitamin D status may increase an athlete’s risk of incurring a stress fracture.
  • Vitamin D has a direct effect on immune cell function. Preliminary self-report data suggest that vitamin D supplementation can decrease the incidence of influenza and the common cold. Athletes who participate in prolonged intense training are typically at increased risk for upper respiratory tract infections (URTI). More data is necessary to show an effect; however, it appears vitamin D intake may influence an athlete’s susceptibility to viruses like the flu and common cold.
  • Currently there is evidence to suggest vitamin D deficiency is related to the inflammatory cycle in animals with autoimmune diseases. More research is needed to understand the impact vitamin D might have on exercise-induced inflammation in humans. An increase in the production of inflammatory factors may be involved in the development of overtraining syndrome which is associated with high volume training and inadequate rest periods. There is data to support that vitamin D increases the production of anti-inflammatory factors. Additionally, studies have found that adequate vitamin D concentrations protect against cartilage loss and progression of osteoarthritis.

Recommended Vitamin D Levels:

Most experts agree that the current recommendations of 200IU – 600IU/day are too low to support optimal health and functioning. However, there is a lack of agreement on optimal intake which varies from 1,000 – 10,000 IU/day (keeping in mind intake depends on synthesis from sun exposure and also storage of vitamin D).  It is estimated that the body requires 3,000-5,000 IU of vitamin D per day to meet the needs of essentially every tissue in the body. Athletes at risk for poor vitamin D status are those with a low intake or limited sun exposure due to use of sunscreen, indoor training, protective clothing, dark skin pigmentation, early morning or late afternoon training sessions or those with minimal or excessive body fat. Vitamin D is stored in subcutaneous (under the skin) body fat and released as needed during winter months or lower exposure times. This process, however, seems to be ineffective in individuals with high or very low amounts of body fat. Extremely high supplemental doses (> 150,000/day) of vitamin D can cause toxicity but doses of 10,000 IU/day for up to five months appears to be safe. This level of dosage may be recommended by a physician when an individual is vitamin D deficient, in order to raise the blood concentration to an optimal level. Excess sun exposure does not lead to vitamin D toxicity, but caution is needed regarding skin cancer.  Recent data suggests that levels higher than 35 nmol/L of Vitamin D are associated with improved VO2 suggesting that a favorable vitamin D status may improve aerobic performance.

Vitamin D and Athletic Performance:

Though the data is limited, Vitamin may play a role in athletic performance.  The data that does exist demonstrates poor vitamin D status is associated with decreased muscle strength, poor physical function, muscle discomfort, and aching bones. Several new studies on athletic populations have shown improved performance, improved VO2 and improved vertical jump on athletes that improved their vitamin D status. These studies showed that athletes with higher vitamin D status >20 – 25 nmol/L had better physical performance and muscle strength.

Researchers have two theories why vitamin D may play a role in improved muscle function.   First, vitamin D receptor (VDR) sites have been identified in every tissue within the body. These VDR sites regulate expression of hundreds of genes that perform dozens of body functions.   The discovery of VDR sites within the muscle suggests vitamin D plays a role as a regulator of skeletal muscle function.   These VDR sites are especially prevalent in fast twitch muscle fibers which have been shown to have better muscle performance.   Secondly, vitamin D has also been shown to modify the transportation of calcium in the sarcoplasmic reticulum which in turn increases the efficiency of calcium binding sites. Since calcium plays a significant role in muscle contraction, the improved efficiency through these binding sights is theorized to be the reason for improved muscle function. In other words, calcium plays a significant role in muscle contraction and therefore along with vitamin D, or more specifically VDR’s, may improve muscle function and strength.

Recommendations:

Vitamin D plays a critical role in bone health, chronic disease prevention, inflammation, immune function and optimal training.   As discussed above vitamin D may also play a role in athletic performance.   The best approach is for athletes to measure vitamin D status and develop a baseline level.   For optimal health a general population should target levels of 20 – 25 nmol/L or higher, whereas athletes seeking additional performance benefits should look for levels at or above 35 nmol/L.

First Endurance MultiV contains 500IU Vitamin D, which may be sufficient for those athletes consuming ample vitamin D from select foods and spending ample time in low latitude states with significant sun exposure year round.   Though the body requires 3,000-5,000 IU’s per day, higher levels do have an ability to be stored in fat for future use.   Athletes seeking an advanced level of vitamin D to ensure optimal health and performance should consider MultiV-PRO which contains 2,500IU vitamin D.