Journal Title (Medline/Pubmed accepted abbreviation): J Int Soc Sports Nutr
Page numbers: 38
Summary of Background and Research Design
Background: Vitamin D is necessary to maintain bone health, with suboptimal levels leading to poor calcium absorption and increases in parathyroid hormone (PTH) levels. Intense physical exercise such as combat training (CT) combined with weakened bone health could result in stress fractures. Women of combat age fall within the 14- to 30-year age bracket that consumes less vitamin D than other age groups. Because stress fractures may account for up to a 60% loss of female soldier recruits during CT programs, understanding the effects of intensive physical exercise on vitamin D levels is important.
Hypothesis/purpose of study: To evaluate the effects of military CT on vitamin D and PTH levels in female soldiers.
Subjects: 74 females entering basic CT randomized to the placebo group in a clinical study designed to evaluate the effect of iron supplementation on health and performance (age, 21 ± 4 yr; height, 162 ± 6 cm; weight, 62 ± 9 kg; non-Hispanic white, 39%; non-Hispanic black, 24%; Hispanic white, 11%)
Experimental design: Cohort analysis from a randomized, placebo-controlled study
Treatments and protocol: CT was conducted over 8 weeks from August to October in 3 phases (physical training, weapons training, and field exercises). Although most of the training occurred outside, participants wore a standard army combat uniform that limited sunlight exposure to the hands and face. Fasting blood samples were available from the original iron study and were analyzed for 25-hydroxyvitamin D (25[OH]D), the main biomarker of vitamin D status, and PTH.
Summary of research findings:
- Overall, mean vitamin D levels decreased from baseline during training (72.9 to 63.3 nmol/L; P < .05
- Ethnicity-by-time interaction effects were found (P < .05).
- In both non-Hispanic whites and Hispanic whites, vitamin D levels decreased from baseline.
- In non-Hispanic blacks, vitamin D levels did not change from baseline; however, this group had the lowest levels at both timepoints (P < .05).
- Overall, PTH levels increased from baseline during training (36.2 to 47.5 pg/mL; P < .05).
- These results were independent of ethnicity.
Interpretation of findings/Key practice applications:
The levels of 25(OH)D unexpectedly decreased during CT in the late summer, possibly from inadequate dietary intake and the clothing worn. Some authors have suggested that 75 nmol/L (30 ng/mL) of 25(OH)D be used as a cutoff for an optimal level. Based on this concentration, 57% of the women entered CT with suboptimal 25(OH)D levels, and 75% completed training below the cutoff. Ethnic differences in 25(OH)D levels paralleled the results of previous studies; however, PTH levels were not significantly different among the ethnic groups. There may be a more complex relationship between physical exercise, 25(OH)D levels, PTH levels, and bone health. However, the results of previous studies suggest that vitamin D supplementation may aid in preventing stress fractures.
This study was limited by a lack of information on sunscreen use and data collection from 1 round of CT during 1 season. Further information on functional consequences such as bone health and fracture incidence from decreased 25(OH)D levels is also necessary to determine any relationship between 25(OH)D status and stress fractures during CT.