Vitamin D Profile in
National Football League Players
Joseph C. Maroon,* MD, Christina M. Mathyssek,* PhD, Jeffrey W. Bost,*y
PA-C,
Austin Amos,* BA, Robert Winkelman,* BA, Anthony P. Yates,z
MD,
Mark A. Duca,z
MD, and John A. Norwig,§
Med, ATC
Investigation performed at the Department of Neurosurgery,
University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania, USA
Background: By maintaining phosphate and calcium homeostasis, vitamin D is critical for bone health and possibly physical performance.
Hence, vitamin D is important to athletes. Few studies have investigated vitamin D levels in relation to fractures and
performance in athletes, and no published study has included a multiracial sample of professional American football players.
Purpose: To assess vitamin D levels, including the prevalence of vitamin D deficiency/insufficiency, in professional American
football players and to evaluate the association of vitamin D levels with race, fracture history, and the ability to obtain a contract
position, which may be a marker for athletic performance.
Study Design: Cohort study; Level of evidence, 3.
Methods: Serum vitamin D levels of 80 professional football players from a single team in the National Football League were
obtained during the 2011 off-season (mean age, 26.5 6 3.7 years; black, n = 67 [84%]). These levels were used to compare injury
reports from the 2011-2012 and 2012-2013 seasons. Statistical analyses were performed to test if vitamin D levels were related to
race, fracture history, and the ability to obtain a contract position.
Results: Mean vitamin D level was 27.4 6 11.7 ng/mL, with significantly lower levels for black players (25.6 6 11.3 ng/mL) versus
white players (37.4 6 8.6 ng/mL; F1,78 = 13.00, P = .001). All athletes who were vitamin D deficient were black. When controlling
for number of professional years played, vitamin D levels were significantly lower in players with at least 1 bone fracture when
compared with no fractures. Players who were released during the preseason because of either injury or poor performance
had significantly lower vitamin D levels than did players who played in the regular season.
Conclusion: Black professional football players have a higher rate of vitamin D deficiency than do white players. Furthermore,
professional football players with higher vitamin D levels were more likely to obtain a contract position in the National Football
League. Professional football players deficient in vitamin D levels may be at greater risk of bone fractures.
Keywords: football; injury prevention; vitamin D; 25-hydroxyvitamin D; athletic training
Vitamin D deficiency is epidemic, with an estimated 1 billion
people worldwide affected.17
The general health consequences
of low vitamin D levels are well established in the
general population and have wide-ranging adverse health
effects that involve every organ system. Both deficient and
insufficient vitamin D levels have been associated with
a greater rate of cardiac morbidity and mortality.7,27,30
Vitamin
D levels\20 ng/mL are associated with a risk increase of
30% to 50% for developing colon, prostate, and breast cancer,
as well as with an increased mortality from these can-
cers.11,13
Increased rates of depression,2
suicide rates,29
and
many autoimmune diseases—including type I diabetes,20
multiple sclerosis,9,12,25
and rheumatoid arthritis24
—are
also associated with low vitamin D levels. In addition, the
consequences of vitamin D deficiency as it pertains to bone
health and muscle function are widely recognized.
Despite this knowledge, vitamin D levels have not been
widely assessed in team sport athletics such as football. Elite
athletes of high-impact sports, such as professional football
players, who put extreme demands on their musculoskeletal
system, require optimal musculoskeletal functioning. Yet,
the role of team athletic trainers and physicians for determining
vitamin D levels and the need for supplementation
y
Address correspondence to Jeffrey W. Bost, PA-C, Department of
Neurosurgery, University of Pittsburgh Medical Center, 200 Lothrop
Street, Suite 5C, Pittsburgh, PA 15213, USA (e-mail: bostj@upmc.edu).
*Department of Neurosurgery, University of Pittsburgh Medical Center,
Pittsburgh, Pennsylvania, USA.
z
University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania,
USA.
§
Pittsburgh Steelers Football Club, Pittsburgh, Pennsylvania, USA.
One or more of the authors has declared the following potential conflict
of interest or source of funding: Support was provided by Heindl Neuroscience
Foundation, Nelson Peltz Foundation, Lewis Topper
Foundation, Cameron Foundation, and Mylan Laboratory Foundation.
The American Journal of Sports Medicine, Vol. 43, No. 5
DOI: 10.1177/0363546514567297
Ó 2015 The Author(s)
1241
to obviate potential detrimental effects of low vitamin D levels
on performance and increased risk of injury are not
clearly established. The primary aim of this study was to
identify the vitamin D levels in professional football players
and to identify the prevalence of deficient, insufficient, and
adequate levels. In addition, we associated vitamin D levels
with race, age, number of National Football League (NFL)
seasons played, history of bone fractures, and performance.
Vitamin D has a key role in maintaining a healthy musculoskeletal
system. After the conversion of vitamin D to
25-hydroxyvitamin D—or 25(OH)D—in the liver, it will
further convert to its active form 1,25(OH)2D in either
the kidneys or other tissues, such as 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 renal
production of 1,25(OH)2D is used for calcium and phosphorus
metabolism. Both calcium and phosphorus are important
to bone and muscle health.17
The mechanism underlying the effect of vitamin D on
bone health is relatively well understood. Adequate calcium
and phosphorus levels promote bone mineralization.
The interaction of 1,25(OH)2D with vitamin D receptors
markedly increases intestinal absorption of calcium and
phosphorus. Furthermore, lower vitamin D levels are associated
with increased levels of parathyroid hormone. Parathyroid
hormone employs several mechanisms to regulate
and achieve optimal blood calcium levels, among which is
the activation of osteoblasts, which stimulates transformation
of preosteoclasts to osteoclasts. Osteoclasts dissolve
the mineralized collagen matrix in the bone, which weakens
the bone structure and increases risk for fractures.
The mechanism of how vitamin D affects skeletal muscle
performance is not well understood,5
but several pathways
seem to play a role. Multiple studies showed that low vitamin
D negatively affects handling, binding, and storage of
calcium in the muscle sarcoplasmic reticulum. Hence, one
proposed role of vitamin D is to increase calcium accumulation
in the sarcoplasmic reticulum. Furthermore, phosphate
imbalance, as induced by low vitamin D, was shown to cause
muscle weakness, which can be reversed with vitamin D
supplementation. Last, there is a direct effect of vitamin D
on the muscle cells via the vitamin D receptor that induces
new protein synthesis.5
There is currently no consensus on the ideal level of
vitamin D in humans as measured by serum level of
25(OH)D,3
neither for general health nor for sport-specific
benefits. A commonly used categorization defines vitamin
D deficiency as 25(OH)D levels \20 ng/mL; insufficiency,
between 20 and 32 ng/mL; and adequacy, .32 ng/mL.3,8
This categorization is supported by studies showing that
25(OH)D is inversely associated with levels of parathyroid
hormone, 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.10,16,17,19
Unless otherwise specified, comparison
studies in this article use the above categorization.
Risk factors for vitamin D deficiency and insufficiency
include limited dietary intake of vitamin D and diminished
sun exposure and absorption of ultraviolet B (UVB) radiation
(wavelengths, 290-315 nm). UVB skin absorption can be limited
by training or working indoors, living in northern latitudes
(.35°), having darker skin pigmentation, consistently
covering the body, or using UVB ray–blocking sunscreen.4,14,18
Vitamin D deficiencies have been well documented in
certain risk groups, such as children, elderly, women wearing
burqas in Muslim countries, the obese, and those with
dark skin. More recently, studies have indicated that athletes,
including elite athletes, have an increased incidence
of vitamin D deficiency.1,22,23
In a recently published abstract, Shindle et al28
assessed
vitamin D levels and skeletal muscle injuries in 89 professional
American football players playing for a single NFL
team. They found that only 19.1% of the players had adequate
vitamin D levels; 50.6% had insufficient levels; and
30.3% had deficient levels. The mean vitamin D level was
30.3 in nonblack players and 20.4 ng/mL in black players
(P \ .001). In addition, they found that players who had at
least 1 muscle injury throughout the prior season had significantly
lower vitamin D levels than did players with no muscle
injury during the same time frame.
The purpose of the current study was to confirm the
observations of Shindle et al by studying vitamin D levels
in an additional group of professional football players
using similar vitamin D deficiency criteria. We also compared
bone fracture history and ability to obtain contracted
employment by vitamin D level.
MATERIALS AND METHODS
Study Design
Previously collected data on 83 active NFL players from
a single team were reviewed following University of Pittsburgh
Institutional Review Board approval. The data
included the most recent 25(OH)D serum levels obtained
as part of the team’s routine health screening during the
2011 off-season to and including the 2012 preseason training
camp, along with injury reports of the athletes for the
2011-2012 and 2012-2013 seasons. Three athletes were
excluded owing to unavailability of blood work results,
leaving a sample size of 80 athletes for further analyses.
De-identified medical records were reviewed that included
age, race, number of years in the NFL, and history of bone
fractures.
Measures
Categorical Vitamin D Levels. Athletes were categorized
according to their vitamin D levels as deficient (\20 ng/mL),
insufficient (20-32 ng/mL), or adequate (.32 ng/mL).
Race. Race was categorized as African American/black,
white, and Polynesian. Because of the low number of Polynesians
(n = 2), they were combined with the white athletes.
Bone Fractures. We collected information on selfreported
injuries both pre– and mid–NFL career that
included bone fractures obtained during athletic and nonathletic
activities.
1242 Maroon et al The American Journal of Sports Medicine
Statistical Analysis
Statistical analyses were performed with SPSS version 20.0.
Results are presented as mean 6 standard deviation, unless
indicated otherwise. Groups were compared with analysis of
variance or chi-square statistics. Post hoc analyses of significant
F value were performed with the Tukey honestly significant
difference (HSD) test. To determine if vitamin D
levels were associated with bone fractures, we used analysis
of covariance. In a separate analysis of the 66 players that
remained with the team for the 2012-2013 season and
started preseason training camp, we identified those who
were selected to play in the regular season and those who
were released during preseason by August 31, 2012.
RESULTS
Sample characteristics (N = 80) are shown in Table 1. The
serum vitamin D levels ranged from 8 to 59 ng/mL, with
68.8% of the team (n = 55) having vitamin D levels that
were less than adequate. The ages of athletes ranged
from 22 to 37 years.
The majority of the athletes (84%; n = 67) were black.
The black athletes had significantly lower vitamin D levels
than did whites (25.6 6 11.3 vs 37.4 6 8.6 ng/mL; F1,78 =
13.00, P = .001) and had a much higher rate of deficient
or insufficient vitamin D levels (77.6% vs 23.1%). Notably,
all athletes with vitamin D levels categorized as deficient
were black, as were 91% of the athletes with insufficient
vitamin D levels (Table 1).
The number of NFL seasons played ranged from 0 (no regular
season played) to 15 seasons completed. The numbers of
seasons in the NFL and vitamin D levels were significantly
correlated (r = 0.388, P  .01), with more seasons played
being associated with higher vitamin D levels. This significant
association is also reflected through categorical vitamin
D levels (Table 1). Post hoc group comparisons with the
Tukey HSD test revealed that athletes with adequate vitamin
D levels had played significantly more NFL seasons
than athletes with deficient levels (P = .005). Athletes with
deficient and insufficient vitamin D levels did not differ significantly
in the number of NFL seasons played.
Twenty-one athletes (37.5%) had experienced a bone
fracture, of which 9 experienced more than 1. The vitamin
D levels did not differ between athletes who had a bone fracture
and those who did not (26.7 6 13.2 vs 27.8 6 10.8 ng/
mL; F1,78 = .19, ns). However, for obvious reasons, bone fractures
are strongly related to the number of NFL seasons
played. When correcting for number of NFL seasons played,
vitamin D levels were significantly lower in the athletes
who experienced a bone fracture (F2,77 = 7.75, P = .001).
At the end of the 2011-2012 season, 14 players left the
team—hence, no information for the subsequent season
was available for them—and 66 started preseason training
camp at the same facility for the 2012-2013 season. Of those
66 players, 45 were placed on the regular season roster and
played in the regular season of 2012-2013, while 21 were
released before the regular season started. The 21 players
who were released during 2012-2013 preseason had significantly
lower vitamin D levels when compared with the players
who made the team (F1,64 = 27.60, P \ .001) and were
more likely to be vitamin D deficient (Table 1).
DISCUSSION
In this study, we identified vitamin D levels of professional
football players and focused on the prevalence patterns of
TABLE 1
Descriptive Statistics and Group Differences Among Athletes
With Deficient, Insufficient, and Adequate Levels of Serum Vitamin Da
Vitamin D Category Group Differences
Total Deficient Insufficient Adequate F(2, 77) P Value
Sample 80 21 (26.3) 34 (42.5) 25 (31.3)
Vitamin D level, ng/mL 27.4 6 11.7 13.6 6 3.6 25.9 6 4.3 41.0 6 7.3
Age, y 26.5 6 3.7 25.5 6 3.5 26.0 6 3.2 27.9 6 4.1 3.08 .052
No. of NFL seasons 4.6 6 3.9 3.0 6 3.8 4.3 6 3.5 6.5 6 3.9 5.38 .007
x2
(2) P Value
Race 15.814 \.001
Black 67 21 (31.3) 31 (46.3) 15 (22.4)
White/Polynesianb
13 0 (0) 3 (23.1) 10 (76.9)
Bone fracture .352 ns
No 50 12 22 16
Yes 30 9 12 9
Released in preseason 2012
No 45 5 22 18 19.327 \.001
Yes 21 13 6 2
a
Deficient, serum vitamin D \20 ng/mL; insufficient, 20 to 32 ng/mL; adequate, .32 ng/mL. Values are presented as n (%) or mean 6 SD.
b
Because of the low number of Polynesians (n = 2), white and Polynesian players were combined.
Vol. 43, No. 5, 2015 Vitamin D in NFL Players 1243
vitamin D deficiency and its correlates. We found vitamin
D deficiency and insufficiency to be significantly greater
among black football players as compared with white players
in a single professional football team. Furthermore, low
vitamin D was associated with an increased number of
bone fractures and being released during preseason play.
Our study results corroborate the finding by Shindle et al
that low vitamin D levels are common among professional
football players. The overall prevalence of vitamin D deficiency
in our study (\20 ng/mL; 26.3%) was comparable
with the 30.3% prevalence found by Shindle et al. We also
found a comparable percentage of players with abnormally
low (deficient or insufficient) vitamin D levels (32 ng/mL):
68.8% in our study and 80.9% in that of Shindle et al.
Prevalence of vitamin D deficiency in this cohort of NFL
football players (26.3%) is far less than the estimated prevalence
of 41.6% reported in the general population by Forrest
and Stuhldreher,10
and the players’ vitamin D level
(27.4 6 11.7 ng/mL) was higher than that of the general
population of men (20.1 6 7.9 ng/mL). Importantly,
though, Forrest and Stuhldreher’s study population consisted
of only 11.1% blacks, while our sample consisted of
84% blacks. Since dark skin is a risk factor for low vitamin
D levels, comparisons between samples with unequal race
distribution are likely biased. Yet, a ‘‘within race’’ comparison
between our sample of football players and the general
population (ie, whites with whites and blacks with blacks)
also shows that vitamin D deficiency in our sample of professional
football players is less prevalent for both blacks
(31.3% in our sample vs 82.1% in US population) and
whites (0% in our sample vs 30.9% in US population).
The comparatively lower prevalence of vitamin D deficiency
of professional football players’ is encouraging and
may represent increased sun exposure during practice
and game play.21
Additionally, professional football players
generally have a higher socioeconomic status, owing
to advanced financial means that allow access to healthy
food, which others have reported is associated with higher
vitamin D levels.26
Despite the healthier vitamin D profile
in our sample when compared with that of the general population,
a substantial number of professional football players
were still vitamin D deficient, and the majority was
vitamin D insufficient.
Consistent with previous literature,10,15
black football
players had a much higher prevalence of inadequate vitamin
D levels when compared with white players (77.6% vs 23.1%).
Dark skin is a known risk factor for low vitamin D levels
owing to inhibited UVB absorption and the increased amount
of melanin skin pigmentation.4,14,18
Diet also plays a role
influencing race-related vitamin D levels. Studies in the general
population suggest that from puberty and onward,
median vitamin D intakes of blacks in America are below recommended
intakes in every age group of the general popula-
tion.15
Our results, showing a generally higher-than-normal
vitamin D profile across both races, may be due to access to
better nutrition. Future research into vitamin D levels in
athletes will need to assess both vitamin D intake and the
amount and quality of sun exposure to provide a more conclusive
insight regarding vitamin D deficiency in athletes. Furthermore,
some research suggests that between-race vitamin
D–level comparisons can be influenced by the fact that blacks
and whites differ in their physiology regarding their ability to
bind and metabolize vitamin D.15
Our finding that lower vitamin D levels are associated
with more fractures is consistent with previous research
conducted in athletes. A limited number of studies have
investigated vitamin D levels in athletes in relation to
bone fractures. In an Australian cross-sectional survey
among 18 female elite gymnasts (age, 10-17 years), Lovell
found that 83% who primarily trained indoors had vitamin
D levels below adequate and had a higher incidence of bony
stress injuries in the year before the testing than did those
gymnasts with adequate vitamin D levels.23
In this study, we found that players who were released
before the official season started and hence did not make
the roster had significantly lower vitamin D levels than
did players who made the team. This finding suggests an
association between vitamin D levels and suboptimal performance.
Previous studies have investigated performance
in athletes of other sports. For example, using jumping
mechanography, Ward et al31
studied vitamin D levels in
99 girls who were 12 to 14 years old to investigate the relationship
between serum vitamin D levels and muscle
power and force. They found a positive relationship
between vitamin D levels and muscle power and force. In
a recent randomized placebo controlled study with 61 professional
male athletes and 30 age-matched nonathletes,
Close et al6
found 62% of the athletes deficient in vitamin
D at baseline. With vitamin D supplementation, there was
an increase of serum total 25(OH)D from baseline (11.6 6
10 to 41.2 6 10 ng/mL; P \ .01), and performance levels,
including sprints and vertical jump, in the treatment
group significantly improved when compared with baseline
measures and the placebo control group. Note, however,
that we did not control for potential confounding variables
or moderators of this association, such as age and a player’s
general health and nutrition status. It is crucial for future
studies to assess and account for these potentially confounding
variables.
Despite the established evidence that adequate vitamin
D levels are critical to the musculoskeletal system, few
studies have examined vitamin D levels in (professional)
athletes, and no study has been published on athletes in
high-impact sports, such as American football. Our study
is one of the first to describe vitamin D levels in professional
football players and to assess vitamin D levels in
relation to the ability to obtain a contract position, which
may be a marker for athletic performance.
This study has several limitations. First, the sample
size is relatively small for the effect size that we expect.
A larger sample size would increase power and reduce
the likelihood of type II errors; hence, our study may
underestimate associations. Second, the observational
cross-sectional nature of our study does not allow us to
draw causal inferences. Longitudinal cohort studies and
experimental studies are necessary for a better understanding
of the role of vitamin D in fracture risk and performance.
Also, we measured vitamin D levels at a single
time point. While vitamin D levels are considered to stay
relatively stable across years, repeated measurement
1244 Maroon et al The American Journal of Sports Medicine
may provide a more reliable estimate of the vitamin D profile
of an athlete. Third, we did not have information on
what athletes may have supplemented with vitamin D.
Last, several other variables for which we were not able
to control may influence vitamin D levels, fractures risk,
and/or ability to obtain a contract position. Variables of
interest include general health status, body mass index,
the type of fracture (acute vs stress), player’s position,
and actual playing time. Future research should aim to
include these relevant variables as well as a more direct
and measurable assessment of muscle strength and performance
to confirm or reverse our finding.
CONCLUSION
The vitamin D profile of professional NFL football players in
our sample was more favorable than that of the general population;
yet, vitamin D deficiency and insufficiency levels
were found to be significant in the subset of black players
evaluated. We found that vitamin D levels were inversely
related to bone fracture prevalence. Furthermore, low vitamin
D levels were associated with a higher risk of getting
released during preseason, possibly indicating poorer performance.
Based on these findings, routine monitoring and
optimization of vitamin D levels should be considered as
part of the routine care of NFL players, with special attention
to black athletes.
ACKNOWLEDGMENT
The authors thank James Bradley, MD, and Ryan Grove,
ATC, for their valuable assistance.
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Vol. 43, No. 5, 2015 Vitamin D in NFL Players 1245