Vitamin D deficiency and risk of fracture in football players

By Mark A. Duca, MD; Christina M. Mathyssek, PhD; Jeffrey W. Bost, PA-C; and Joseph C. Maroon, MD

Many US football players have lower than adequate serum vitamin D levels, but questions remain about the potential benefits of proactive treatment with supplementation to address vitamin D deficiency and reduce the risk of fracture in this patient population.

Vitamin D deficiency is an epidemic, with more than 77% of the general population considered to have inadequate vitamin D levels.¹ The general adverse health effects of low vitamin D levels involve every organ system. The most noted effects involve bone health, but low vitamin D levels have also been associated with increased rates of depression,² suicide,³ autoimmune diseases, including type 1 diabetes,⁴ multiple sclerosis,^5-7 rheumatoid arthritis,⁸ and greater rates of mortality and morbidity related to cardiac disease and colon, prostate, and breast cancers.

The consequences of vitamin D deficiency with respect to bone health and muscle function are widely recognized in the elderly. However studies involving younger adults—including military personnel, athletes, and certain ethnic populations—have recently identified other at-risk groups. In 2006, Ruohola et al⁹ reported that male Finnish military recruits (n = 756, mean age 19 years) with vitamin D levels below 30.3 ng/mL had a 3.6 times higher risk of stress fracture than those with higher vitamin D levels. In a double-blinded randomized controlled trial involving female US Navy recruits (n = 3700, median age 19 years), the group that took 2000 mg calcium and 800 IU vitamin D per day had a significantly lower incidence of daily stress fractures than the placebo supplementation group.¹⁰

As in the military, the relevance of vitamin D for bone and muscle health is also of particular importance to athletes who require optimal musculoskeletal functioning due to the increased demands of sports on the musculoskeletal system. Despite this, vitamin D level and its associations with bone health and performance have only recently been researched in athletes who participate in high-impact sports, such as US football players.

Mechanism of vitamin D in bone health

Aside from exposure to ultraviolet B (UVB) rays, humans obtain vitamin D from their diet, including dietary supplementation. Few foods naturally contain vitamin D, and the vitamin is not abundant even in those that do. One exception to this is fish liver oils, which naturally contain relatively high levels of vitamin D. Several food items are commonly fortified with vitamin D, including milk, margarine, and ready-to-eat cereal.^11,12

After the conversion of vitamin D to 25-hydroxyvitamin D (25(OH)D) in the liver, it will further convert to its active form, 1,25(OH)₂D, in the kidneys or in other tissues such as the breasts, colon, prostate gland, and immune cells. While the production of 1,25(OH)₂D in local tissue regulates cell growth, controls immune function, and affects gene expression, the production of 1,25(OH)₂D in the kidneys is used for calcium and phosphorus metabolism. Both calcium and phosphorus are of particular importance for optimal bone and muscle health.¹²

The mechanism underlying the effect of vitamin D on bone health is relatively well understood: Adequate calcium and phosphorus levels promote bone mineralization, which is necessary for strong bones. The interaction of 1,25(OH)₂D with vitamin D receptors (VDR) markedly increases intestinal absorption of calcium and phosphorus. Without vitamin D, only up to 15% of the dietary calcium intake and 60% of phosphorus intake is absorbed in the intestines; with adequate vitamin D levels (at least 30 ng/mL), calcium absorption increases to 30% and phosphorus absorption is also markedly increased.¹³

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A parallel mechanism has to do with the association between vitamin D and parathyroid hormone (PTH), and is based on the body’s efforts to maintain optimal blood serum calcium levels. Lower vitamin D levels are associated with increased levels of PTH. PTH employs several mechanisms to regulate and achieve optimal blood calcium levels, including the activation of osteoblasts, which stimulate transformation of preosteoclasts to osteoclasts. Osteoclasts dissolve the mineralized collagen matrix in the bone (bone resorption), mobilizing calcium from the bone and into the blood, and thereby preventing hypocalcemia but weakening the bone structure and increasing risk for fractures.

Mechanism of vitamin D in muscle health

The mechanism through which vitamin D affects skeletal muscle performance is not well understood,¹⁴ but several pathways seem to contribute. Multiple studies^15,16 showed that low vitamin D negatively affects handling, binding, and storage of calcium in the muscle sarcoplasmic reticulum. Therefore, one proposed action of vitamin D is to increase calcium accumulation in the muscle fiber’s sarcoplasmic reticulum, which plays a key role in muscle contraction and, hence, muscle performance. Furthermore, phosphate deficiency, which can be induced by vitamin D deficiency, was shown to cause muscle weakness that can be reversed with vitamin D supplementation.¹⁷ Lastly, there is evidence of a direct effect of vitamin D on the muscle cells via the membrane-bound VDR that induces new protein synthesis and subsequent muscle growth.¹⁴ In adults with low vitamin D levels, biopsies of skeletal muscles showed signs of muscle atrophy, especially in fast-twitch fibers.^18,19

Optimal serum vitamin D levels

There is currently no consensus on the ideal level of vitamin D in humans as measured by serum level of 25(OH)D,²⁰ neither for the general public nor for athletes specifically. A scientifically supported and commonly used categorization defines vitamin D deficiency as 25(OH)D levels below 20 ng/mL, vitamin D insufficiency as 25(OH)D levels between 20 and 32 ng/mL, and adequate vitamin D levels as greater than 32 ng/mL.^20,21 Studies show that 25(OH)D is inversely associated with levels of PTH, which promotes bone loss until 25(OH)D reaches levels between 30 and 40 ng/mL. Furthermore, intestinal calcium transport increases steeply between 20 and 32 ng/mL.^12,22-24 Risk factors for vitamin D deficiency and insufficiency include limited dietary intake of vitamin D and diminished sun exposure and absorption of UVB radiation (wavelengths 290-315 nm), which can be due to living in northern latitudes (above 35°), having darker skin pigmentation, consistently covering the body when outside, or consistently using UVB ray blocking sunscreen.^25-27

Research in athletes

Vitamin D deficiencies have been well documented in children, the elderly, the obese, and those with dark skin or reduced sunlight exposure. More recently, studies have indicated that young adult athletes also have an increased incidence of vitamin D deficiency.^13,28,29 In a cross-sectional survey of 18 Australian female elite gymnasts (aged 10-17 years), Lovell et al²⁹ found that 83% of the gymnasts who primarily trained indoors had vitamin D levels below adequate, and that these gymnasts had a higher incidence of bony stress injuries in the year prior to the testing than those with adequate vitamin D levels. Ward et al³⁰ studied vitamin D levels in 99 girls aged between 12 and 14 years to investigate the relationship between serum vitamin D levels and muscle power and force, measured using jumping mechanography. They found a positive relationship between vitamin D levels and muscle power and force. Close et al³¹ conducted a randomized double-blind placebo-controlled study in 10 professional male athletes (70% had insufficient or deficient vitamin D levels at baseline). They found that vitamin D supplementation of 5000 IU D₃ over eight weeks significantly increased total serum 25(OH)D (mean ± standard deviation increase from 11.6 ± 10 ng/mL to 41.2 ± 10 ng/mL) and significantly improved athletes’ 10-m sprint and vertical jump performance compared with their own baseline measures. The placebo group did not show significant serum vitamin D or performance changes.

However, not all studies found an association between vitamin D levels and muscle performance. For example, in a different but similar study among 30 young athletes, Close et al³² failed to find improvements in muscle performance after 12 weeks of supplementation. The authors hypothesized that higher serum vitamin D levels may have been necessary to see improvement.

A study conducted in 2011 by Hamilton et al³³ investigated a multiracial sample (n = 342) of professional male soccer players in Qatar and found a high rate of vitamin D deficiency/insufficiency (84% had vitamin D levels <30 ng/mL and 12% had levels <10 ng/mL, indicating severe deficiency); yet, they found no unique association between vitamin D levels and isokinetic lower limb peak torque. Interestingly, in a previous study by Hamilton et al³⁴ from 2010, the researchers found a very high rate of vitamin D deficiency in athletes in Qatar, which prompted them to instigate a nationwide education and supplementation program for team staff and players. The players who still had severely deficient vitamin D levels in 2011 may have been noncompliant with this program, raising questions as to whether they are representative of the general population of athletes. Furthermore, the group that was severely deficient in vitamin D differed from the insufficient and adequate groups in total and lean body mass, but not in height or body fat percentage. This could indicate a different body type that, in turn, may be associated with a difference in racial composition between vitamin D groups.

The researchers found significant differences in vitamin D levels between the racial groups, but did not provide information about the racial composition of each vitamin D level group. This information could aid in interpretation of their findings, as recent research indicates that different races may have a different bioavailability of vitamin D (ie, African Americans typically show low serum vitamin D levels when measured with the standard tools, yet, paradoxically, have higher bone density than whites).³⁵

Vitamin D levels in football players

In professional US football players, who regularly endure high-impact hits and put great demand on their musculoskeletal system, vitamin D levels and associated health and performance outcomes have only recently received research attention. Two cross-sectional studies, one unpublished and one published, have looked at the association between vitamin D levels and musculoskeletal health and performance.

Shindle et al³⁶ measured serum vitamin D levels of all 89 players on a single National Football League team and looked for associations between vitamin D level and occurrence of muscle injury during the previous season. They found that only 19.1% of the players had adequate vitamin D levels, 50.6% had insufficient levels of vitamin D, and 30.3% had deficient levels. The mean vitamin D level in non-African American players was 30.3 ng/mL, compared with 20.4 ng/mL in African American players (p < .001). In addition, they found that players who had at least one muscle injury (defined as a strain, tear, or pull that led to at least one missed practice or game) during the prior season had significantly lower vitamin D levels than players with no muscle injury during the same time frame (19.9 ng/mL vs 24.7 ng/mL, p < .05).

We recently published our research findings on vitamin D levels in a different team of 80 NFL players (mean age 26.5 ± 3.7 years, 84% African American) in the American Journal of Sports Medicine.³⁷ In this study, we measured serum vitamin D levels during the 2011 offseason and evaluated associations between vitamin D levels and bone fracture history during the 2011-2012 and 2012-2013 seasons, as well as ability to obtain contracted employment, which may be a proxy for performance.

We found that only 31.3% of the players had adequate vitamin D levels, 42.5% had insufficient levels of vitamin D, and 26.3% had deficient levels. Mean vitamin D level was 27.4 ± 11.7 ng/mL, with significantly lower levels in African Americans (25.6 ± 11.3 ng/mL) than in whites (37.4 ± 8.6 ng/mL; F(1,78) = 13, p = .001). Inter­estingly, all athletes who were vitamin D deficient were African American.

Furthermore, when controlling for number of professional years played, vitamin D levels were significantly lower in those with at least one bone fracture during their football career compared with no bone fracture (F(2,77) = 7.75, p = .001). Players who were released during the 2012 preseason had significantly lower vitamin D levels than players who played in the regular 2012-2013 season (F(1,64) = 27.60, p < .001).

Implications and future directions

Our findings, along with those of Shindle et al, raise questions about the role of prevention and proactive treatment with vitamin D supplementation to obtain and maintain recommended serum vitamin D levels. An important first step, however, is to establish a causal relationship between vitamin D levels and bone fractures. By design, our cross-sectional study on US football players cannot be used to draw any causal inferences regarding bone fractures and serum vitamin D levels in athletes. However, there are only a limited number of other studies that have used randomized controlled designs reporting evidence of fracture prevention using vitamin D supplements in a similar age group. For example, the previously described study in military personnel of similar age and physical conditioning found that vitamin D supplementation benefited bone health by reducing stress fracture rates.¹⁰

When considering the potential benefits of vitamin D supplementation in athletes, it is important to consider the vitamin D source and the specific physiologic requirements of this group, as well as fracture prevention. A recent review study of vitamin D with regard to bone health and athletic performance reported that perhaps the most important risk factor for deficient and insufficient vitamin D levels was time spent outside, rather than nutrition or skin pigmentation.³⁸ Indoor athletes such as gymnasts and possibly NFL players during the winter months at higher latitudes (which applies to both of the teams that have been studied thus far) may be particularly at higher risk for low vitamin D levels due to limited sun exposure. Research is needed to clarify whether increased outdoor training time could be used to sufficiently increase vitamin D levels to adequate levels.

According to a 2010 report from the Institute of Medicine,³⁹ the typical young adult vitamin D daily requirement is 400 IU and the recommended dietary allowance is 600 IU (maximum of 4000 IU), while the Endocrine Society recommends double to triple that amount of supplementation.⁴⁰ It has been suggested that certain subgroups, such as older adults, require increased intake of vitamin D due to increased physiological demands. The literature thus far does not indicate that athletes require a higher vitamin D intake than the general population to obtain adequate serum vitamin D levels.

The bone health benefits of raising vitamin D levels appear limited to increasing deficient and insufficient levels to adequate levels (usually defined as levels greater than 30 to 32 ng/mL). Vitamin D levels that are higher than adequate have not shown additional bone health benefits in the general population. In fact, vitamin D supplementation as a health benefit demonstrates a U-shaped relationship, with elevated risks associated with both hypo- and hyper­vitaminosis.³⁸ Some preliminary findings on athletic performance indicate that athletes may benefit from higher than adequate serum vitamin D levels specifically to optimize muscle function and performance, while others state there is no evidence that athletes would benefit from elevated serum vitamin D levels.³⁸ Little research has been conducted on this matter, however.

Lastly, while several trials in athletes have shown beneficial effects of vitamin D supplementation on bone density, they did not provide convincing evidence for fracture prevention, since total bone strength is only partly defined by bone density.³⁸ Future research needs to address other contributing factors, such as protein and calcium intake and their interactions, for fracture prevention.

Mark A. Duca, MD, is a clinical assistant professor of medicine at the University of Pittsburgh School of Medicine in Pennsylvania, and serves as a team physician for the Pittsburgh Steelers Football Club. Christina M. Mathyssek, PhD, received her PhD in psychology in 2014 from the Erasmus University Rotterdam in the Netherlands and currently works as senior research principal at the University of Pittsburgh Medical Center. Jeffrey W. Bost, PA-C, is a research director and clinical instructor at the University of Pittsburgh Medical Center. Joseph C. Maroon, MD, is professor and vice chair of the Department of Neurological Surgery and Heindl Scholar in Neuroscience at the University of Pittsburgh Medical Center and team neurosurgeon for the Pittsburgh Steelers.

Acknowledgements: The authors are grateful to the Heindl Neuroscience Foundation, Nelson Peltz Foundation, Lewis Topper Foundation, Cameron Foundation, Mylan Laboratory Foundation, and John and Cathie Garcia Foundation for their financial support for this research.

Disclosure: Joseph Maroon, MD, is an unpaid consultant and the neurosurgeon for the Pittsburgh Steelers National Football League (NFL) team, and is medical director of World Wrestling Entertainment (WWE). Mark A. Duca, MD, is a team physician for the Pittsburgh Steelers NFL team.

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