It’s no surprise that vitamin D is one of the most popular health topics these days or that its supplement sales have soared in recent years. Numerous studies have linked low vitamin D levels to a variety of health conditions and diseases in the general public, including cardiovascular disease, hypertension, diabetes, cancer, arthritis (both osteo and rheumatoid), inflammation, acute illness, infections, and stress fractures. Beyond general health concerns, an insufficient level of vitamin D can also impact exercisers and athletes, resulting in issues such as diminished skeletal muscle function, poor immunity, and inflammation (Larson-Meyer, 2010).
The scientific understanding of vitamin D and how it affects the body is constantly evolving. As a health and fitness professional, it is important to stay abreast of the current research, both in regard to the overall health implications of vitamin D as well as how it impacts the fitness performance of the athletes and clients you train.
Vitamin D Basics
The primary natural source of vitamin D is sunlight. Our bodies synthesize vitamin D when sunlight hits the skin, however various factors can affect how much vitamin D is actually produced from sun exposure. Darker skin, sunscreen use, advanced age, limited outdoor activity, and even the presence of smog and clouds are just some of the factors that can reduce an individual's the endogenous synthesis of vitamin D. Most people are not able to get sufficient vitamin D through sunlight exposure alone.
Dietary sources of vitamin D are available, but limited. Fatty fish (tuna, salmon, sardines, mackerel), fortified dairy products and juices, egg yolks, and cereals are all good sources of vitamin D (Larson-Meyer, 2010), but without supplementation and/or adequate exposure to sunlight, one is still likely to be deficient in this vitamin.
Vitamin D levels can be determined through a simple blood test, which categorizes vitamin D status in one of the following areas:
- Deficiency: <20 ng/mL
- Insufficiency: <32 ng/mL
- Sufficiency: ≥32 ng/mL
- Optimal: 40-70 ng/mL
- Toxicity: >150 ng/mL plus hypercalcemia
In 2010, the U.S. Institute of Medicine increased the recommended daily allowance (RDA) of vitamin D to 600 IU for individuals up to 70 years old and 800 IU for those older than 70. It is expected that individuals can obtain these levels of vitamin D intake through dietary sources and/or supplementation, assuming minimal sunlight exposure.
Not everyone is in agreement with these guidelines, however. Many practicing physicians and researchers believe they are too low, and recommend a range of between 800 and 5,000 IU per day, especially throughout the winter months and for those with other risk factors for deficiency.
Body Weight's Impact on Vitamin D Absorption
Body fat and vitamin D status are inversely related, so obesity is an important factor to consider among the causes of vitamin D deficiency. Excess body fat can make vitamin D that has been stored in the body unavailable for use. Since many personal training clients are overweight, this is an important factor for fitness professionals to understand.
A recent study published in the American Journal of Clinical Nutrition (Barclay, 2011) showed that higher circulating levels of vitamin D are directly associated with weight loss (through calorie restriction or increased exercise) in postmenopausal women. The greater the weight loss, the greater the increase in vitamin D levels, up to 7.7 ng/mL for those who lost 15% or more of their baseline weight over a 12-month period.
The Link between Vitamin D and Overall Health
According to the most recent U.S. Health and Nutrition Examination Survey (Looker et. al, 2011), current vitamin D levels are insufficient in 25 to 57% of adults. Other studies report this number as being even higher.
In research conducted on 10,899 adults at the University of Kansas Hospital and Medical Center, 70% of were found to have insufficient vitamin D. Those with vitamin D deficiency were found to be twice as likely to develop diabetes, 40% more likely to have hypertension, and 30% more susceptible to cardiomyopathy. The good news in the study was that participants taking vitamin D supplements were 60% less likely to die of any cause, especially if they started out being vitamin D deficient (Hayes Taylor, 2011).
Another study found a 70% higher chance of a cardiovascular event in those with vitamin D levels below 37.5 nmol/L. Older men and women with a vitamin D level less than 37.5 nmol/L were also more likely to be diagnosed with hypertension compared with individuals with a level of >75 nmol/L (Jonnalagadda, Culp, Sharma & Campbell, 2010).
Vitamin D and Athletic Performance
Vitamin D insufficiency is common in athletes and fitness enthusiasts, just as it is in the general public.
Factors affecting the vitamin D status of athletes include skin color, sport, duration of sun exposure, location and season. Groups of athletes with the most vitamin D deficiency include Middle Eastern sportsmen in Qatar and gymnasts in East Germany and Finland. Athletes who train outdoors are likely to maintain optimal levels of vitamin D (Larson-Meyer, 2010), and it makes sense that those with the highest levels of vitamin D include athletes training outside during the summer months in mountainous regions.
Levels of vitamin D can be linked to many factors that may affect athletic performance. For instance, vitamin D plays a role in skeletal muscle function, and deficiency can manifest in muscle pain and weakness. Research suggests that athletes may be impacted by illness, inflammation, and stress fractures if they have a suboptimal status. One of the inflammatory cytokines that can be elevated with low vitamin D levels – TNF-alpha – has been linked to multiple diseases, including over-training syndrome (Larson-Meyer, 2010).
According to Dr. Michael K. Shindle, vitamin D can increase fast twitch muscle fibers, contributing to better performance. Additionally, when Dr. Shindle tested the vitamin D status of New York Giants NFL team, he observed that all the players who had suffered with a muscle injury the previous season had significantly lower levels than those who had not suffered a muscle injury. There were no other significant differences between the two groups (Melville, 2011).
According to Larson-Meyer (2010), a number of studies have shown improved athletic and functional performance with increased vitamin D status:
- A study conducted over eight weeks demonstrated that female Naval recruits reduced their stress fractures when supplementing with 800 IU of vitamin D and 2000mg of calcium compared with a placebo.
- In Germany, chronic pain from sports injuries was reduced in athletes who underwent UVB irradiation for six weeks.
- Positive correlations were seen between vitamin D status and aerobic fitness, power, velocity, and height of jump in two studies on non-athlete young women.
A 2011 study evaluated older women with vitamin D insufficiency to see whether supplementation would improve muscle strength and mobility over the course of a year. After daily supplementation with 1000 IU of vitamin D2 for one year, the study group’s mobility, knee flexor strength, and hip muscle strength all improved significantly. It is believed that the improvements in mobility were due to neuromuscular function as well as enhanced muscle strength, since another study showed that vitamin D supplementation improved neuromuscular function in older people with a history of falling and low vitamin D levels (Zhu, Austin, Devine, Bruce & Prince, 2010).
Since personal trainers are often looked to as a trusted source of health information and guidance, it is important to have reliable information on nutrition and supplementation. Vitamin D is being widely discussed in media as well as in research circles. While trainers should not prescribe supplementation or dosages, they can still explain why maintaining optimal vitamin D levels is important to both health and athletic performance. They can also look for possible reasons why a client may be deficient (darker skin, overweight, limited sun exposure, etc.) and should advise their clients to keep track of their vitamin D status. Clients with suboptimal blood levels of vitamin D should be encouraged to discuss supplementation of vitamin D3 with their physician.
- Barclay, L. (2011). Weight Loss Linked to Increased Circulating 25(OH)D Levels. American Journal of Clinical Nutrition. Retrieved from http://www.medscape.com/viewarticle/744361
- Hayes Taylor, K. (2011). Low vitamin D linked to heart disease, death. The American Journal of Cardiology. Retrieved from http://www.reuters.com/article/2011/11/26/us-vitamind-heartdisease-idUSTRE7AO1UM20111126.
- Institute of Medicine, Food and Nutrition Board. (2010). Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academy Press.
- Jonnalagadda, S.S., Culp, J., Sharma, B., & Campbell, J. (2010). Impact of Vitamin D and Calcium on Health Outcomes: Reviewing the Evidence. SCAN’s Pulse 29(2), 11-13.
- Larson-Meyer, D.E. (2010). What Sport Dietitians Should Know About Vitamin D. SCAN’s Pulse 29(4): 6-9.
- Looker, A.C., Johnson, C.L., Lacher, D.A., Johnson, C.L., Pfeiffer, C.M., Schleicher, R.L. & Sempos, C.T. (2011, March). Vitamin D Status: United States, 2001–2006. NHS Data Brief. Retrieved from http://www.cdc.gov/nchs/data/databriefs/db59.htm.
- Melville, N. (2011). ‘Alarming’ Vitamin D Deficiencies in NFL Football Players. American Orthopaedic Society for Sports Medicine (AOSSM) 2011 Annual Meeting. Retrieved from http://www.medscape.com/viewarticle/746310.
- O’Riordan, M. (2011). Vitamin D does not improve endothelial function or arterial stiffness, but it remains the vitamin du jour. Theheart.org. Retrieved from http://www.theheart.org/article/1315347/print.do
- Zhu, K., Austin, N., Devine, A., Bruce, D., & Prince, R. (2010). A Randomized Controlled Trial of the Effects of Vitamin D on Muscle Strength and Mobility in Older Women with Vitamin D Insufficiency. Journal of the American Geriatrics Society, 58(11): 2063-2068.