This randomized cross-over study will identify physiological factors that underlie changes in bone metabolism that could affect skeletal injury risk, to include increases in parathyroid hormone (PTH) in response to militarily relevant exercise in females. The primary objective is to determine the hormone and calcium (Ca) response to multiple bouts of load carriage exercise in females. The investigators hypothesize that PTH will increase after multiple bouts of load carriage exercise and this increase will be due to disruption in Ca kinetics, specifically either a decrease in fractional intestinal Ca absorption (FCA) or changes in bone formation and/or resorption.
Initial military training (IMT) results in increased risk of stress fracture, particularly for females as up to 21% of females may sustain a stress fracture during IMT, an incidence that is approximately 4-fold higher than that for males. Young female adults will undergo two separate study periods in random order, one will include exercise and the other will not. Each study period will use dual stable Ca isotope methodology in order to determine Ca kinetic responses to a militarily relevant exercise, and dietary intake will be controlled. Kinetic analyses will include fractional intestinal Ca absorption (FCA), Ca flux into bone (Vo+) and out of bone (Vo-), as well as renal Ca handling. Serial blood samples collected during the study periods will also be used to determine hormonal responses to exercise. The treadmill exercise will consist of a 60 minute timed trial with load carriage (30% body weight) completed three times during the 6 day kinetic period. At the start of the study, blood will be collected for analysis of Ca and bone-related genetic markers, and habitual food intake and exercise will be assessed. The information gathered from this trial will be used to identify physiological responses that can be targeted by future interventions designed to improve bone responses to training and decrease injury risk in female Warriors.
Study Type
INTERVENTIONAL
Allocation
RANDOMIZED
Purpose
BASIC_SCIENCE
Masking
NONE
Enrollment
21
Treadmill exercise for 60 minutes with load carriage (weighted vest, 30% body weight) completed three times during the 6 day kinetic period.
United States Army Research Institute of Environmental Medicine
Natick, Massachusetts, United States
Hormone response and calcium kinetics
After dosing with dual stable calcium isotopes, changes in calcium kinetics (isotopic rations) will be measured in response to multiple bouts of a militarily relevant load carriage exercise or no exercise (rest) in females.
Time frame: Exercise or rest will occur on days 0, 2, and 4 and measures will be taken daily days 0-4. After a washout period, measures will be repeated 1-4 weeks later on days 0, 2, and 4 with whichever intervention was not performed during the first study period.
Fractional calcium absorption
Determine whether fractional Ca absorption (FCA) is associated with bone microarchitecture and strength in females undergoing multiple bouts of a militarily relevant load carriage exercise
Time frame: Bone measurement will be taken at baseline and FCA will be measured once during each intervention (exercise or rest) on day 0.
Single nucleotide polymorphisms
Evaluate whether single nucleotide polymorphisms (SNPs) previously identified to be associated with circulating 25OHD concentrations explain some of the variability in calcium kinetic responses to load carriage exercise.
Time frame: SNP measurement will be taken at baseline and calcium measurements will be taken daily on days 0-4 when undergoing exercise and daily on days 0-4 when there is no exercise (rest).
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