Shawn H. Dolan, PhD, RD, CSSD

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. 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. It is less clear whether vitamin D deficiency can limit 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). However, cut-off values defining deficiency and insufficiency are not always consistent, and sometimes the terms are used interchangeably. Vitamin D deficiency is typically defined as blood concentration of 25(OH)D < 20 ' 25 nmol/L whereas insufficiency is defined as < 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 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 vitamin D increases the production of anti-inflammatory factors. In addition, studies have found adequate vitamin D concentrations protect against cartilage loss and progression of osteoarthritis.
• There is limited data on vitamin D and 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 in the elderly. Vitamin D supplementation can improve muscle function and decrease the risk of falling in older adults. More data is needed in a younger population before recommending vitamin D supplementation to improve performance.

The current recommendations for vitamin D intake include 200 IU/day up to 50 yr old, 400 IU/day 51 ' 70 yrs old, and 600 IU/day for those > 71 yrs old. Most experts agree these recommendations are too low to support optimal health and functioning. However, there is a lack of agreement on optimal intake which varies from 1,000 ' 4,000 IU/day, keeping in mind intake depends on synthesis from sun exposure and also storage of vitamin D. 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, and 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. However this process 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 5 months appear to be safe. This level 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.

If you choose to supplement with vitamin D, be sure to check the label for the form of vitamin (cholecalciferol = D₃ or ergocalciferol = D₂).  D₃ is synthesized from animal tissue and therefore not acceptable to some vegetarians and vegans. D₂ is the plant form used to fortify vegetarian products like soy milk and used in supplements marketed to vegetarians. Unfortunately, D₂ is less effective at increasing blood concentrations of vitamin D. It is not appropriate to simply increase the dose of D₂ to make it as effective as D₃.

Vitamin D plays a critical role in bone health, chronic disease prevention, and optimal training. Take a moment to determine how much vitamin D you consume through food and supplements (see table below for examples) as well as your unprotected sun exposure between 10am and 2pm during summer months. Ensure you are exposing yourself to the benefits of vitamin D!

Are you getting enough vitamin D'

Below are several options to ensure adequate intake including supplements and food sources.