Page 69 - Journal of Special Operations Medicine - Spring 2014

hormonal abnormalities, increased inflammation, and Intense exercise produces inflammatory cytokines that
reduced muscle strength, all of which will inhibit opti- contributes to postexercise muscle damage. Vitamin D
mal Operator functioning. modulates the postexercise inflammatory response, re-
ducing muscle weakness and enhancing muscle recovery
Bone Health between exercise sessions. In endurance runners, serum
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Essential to the regulation of calcium homeostasis and 25(OH)D levels were inversely correlated to inflamma-
bone metabolism, vitamin D facilitates absorption of tory cytokines, suggesting that maintaining optimal vi-
dietary calcium following activation by parathyroid tamin D status suppresses the inflammatory response
hormone (PTH). Poor calcium absorption from the associated with physical training. Further research
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gastrointestinal tract elevates PTH levels, forcing bone shows that inflammatory cytokines are elevated in active
to free enough calcium to raise extracellular levels and adults with poor vitamin D status, reinforcing a negative
promoting bone loss via excessive bone degradation. correlation between vitamin D levels and inflammatory
Elevated rates of bone remodeling weaken bones and cytokines. Thus, optimal vitamin D levels appear to
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increase fragility, raising the risk for stress fractures dur- regulate the postexercise inflammatory response, reduc-
ing military training. In a prospective study of men in ing the risk for muscle damage and potential injury.
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the Finnish Defense Forces, baseline measurements of
serum 25(OH)D were significantly lower in recruits who Increased risk for musculoskeletal injury observed in
developed stress fractures during basic training. Simi- vitamin D–deficient athletes may develop from muscle
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larly, female U.S. Navy recruits with serum 25(OH)D weakness or myopathy since chronic vitamin D defi-
levels lower than 39.9ng/mL had a higher risk of stress ciency has been associated with muscle weakness,
fractures during the first 6 months of active duty com- musculoskeletal pain, and poor balance. Vitamin D–
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pared with recruits with higher 25(OH)D levels. Elite supplemented athletes sustained fewer injuries and
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male combat recruits from the Israeli Defense Forces improved their muscle strength and power, while non-
who developed stress fractures had significant deficits supplemented peers had no change or slight regression
in dietary vitamin D intake compared with their peers in these measurements. In healthy adults, higher se-
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without stress fractures. rum 25(OH)D levels were correlated with upper and
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lower extremity muscle strength. Professional soccer
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While PTH levels did not differ between stress fracture players with vitamin D deficiency had lower leg muscle
groups in these Operators, other researchers have shown mass compared with players with higher vitamin D
that elevated PTH increases the risk for stress fractures in levels. Vitamin D deficiency has also been linked to
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Servicemembers. Prolonged physical activity increases atrophy of type II muscle fibers, which are responsible
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PTH in response to depressed serum calcium levels. As for bursts of speed and power. Therefore, correcting
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a result, combat Operators engaged in strenuous train- deficiency can increase type II muscle fiber diameter
ing may have increased requirements for vitamin D com- and translate to improved physical performance, as evi-
pared with the normal population. Failure to meet these denced in professional athletes. Eight weeks of vitamin
needs will lead to hormonal alterations and changes in D supplementation significantly improved vertical jump
bone turnover. In Lithuanian servicemembers, serum and 10-m sprint speed in deficient professional Euro-
25(OH)D showed a weak negative correlation with pean soccer players. Peak lower extremity function
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PTH levels that is likely to increase rates of bone de- has been observed with 25(OH)D levels greater than
gredation. Sex hormone binding globulin, a measure 40ng/mL, likely because type II muscle fibers are in-
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of estrogen and testosterone, was positively associated volved in balance and neuromuscular function. Along
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with both 25(OH)D and markers of bone turnover in these lines, vitamin D supplementation in deficient older
male Finnish recruits, suggesting that vitamin D is es- adults improved balance, reaction time, and functional
sential to hormonal regulation of bone homeostasis. 16 coordination.
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Physical Performance Androgen Synthesis
Recent evidence has established a role of vitamin D in One potential mechanism for improved physical per-
physical performance, showing an association of poor vi- formance with higher vitamin D status is its interaction
tamin D status with reduced muscle strength and increased with androgenic activity. Vitamin D may play a role
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injury risk. For example, football players with muscle in regulating testosterone production, as evidenced by
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injuries had a mean serum 25(OH)D level of 19.9ng/mL the identification of vitamin D receptors and metaboliz-
that was significantly lower than the mean level of ing enzymes in the testes. If testosterone production
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24.7ng/mL measured in uninjured team members. In decreases due to vitamin D deficiency, then physical per-
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another group of athletes, vitamin D supplementation formance could be impaired. The combination of strenu-
during the winter months was associated with fewer in- ous training and poor nutrition lower testosterone levels
juries compared with nonsupplemented athletes. and have been shown to result in loss of muscle strength
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Vitamin D Status in Soldiers and Physical and Cognitive Performance 61