Calcium Kinetic Responses to Exercise

Study Overview
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Brief Summary
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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.

Detailed Description
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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.

Official Title
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Skeletal Hormone and Calcium Kinetic Responses to Load Carriage Exercise in Females

Conditions
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Nutritional Status, Bones

Intervention / Treatment
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* Behavioral: Exercise


Participation Criteria
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Eligibility Criteria
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Inclusion Criteria: Women aged 18-42 years No current or prior military service Exercise 2-5 x/wk Stable body weight for 2 months (±5 lbs) Body mass index (BMI) between 19-26 kg/m2 VO2max between 35-50 ml·kg-1·min-1 Willing to discontinue use of dietary supplements and abstain from alcohol for the duration of the study Have not donated blood within the last 8 weeks Exclusion Criteria: History of endocrine disorders (e.g., diabetes, uncontrolled thyroid dysfunction, hypoparathyroidism, or hyperparathyroidism) History of bone-modifying disorders (e.g., osteogenesis imperfecta, osteopetrosis, or rickets) History of cardiovascular or renal disease Pregnancy or lactation in the last 6 months Routine use of medications known to affect bone or calcium metabolism (e.g., thiazide diuretics, bisphosphonates, oral steroids) A very restrictive diet or severe food allergies

Ages Eligible for Study
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Minimum Age: 18 Years
Maximum Age: 42 Years

Sexes Eligible for Study
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Female

Accepts Healthy Volunteers
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Accepts Healthy Volunteers

Study Plan
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How is the study designed?
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Design Details

Primary Purpose: Basic Science
Allocation: Randomized
Intervention Model: Crossover Assignment
Masking: None (Open Label)


Arms and Interventions

| Participant Group/Arm | Intervention/Treatment |
| --- | --- |
| No Intervention: Rest<br> | |
| Active Comparator: Exercise<br>Militarily relevant exercise | Behavioral: Exercise<br>* Treadmill exercise for 60 minutes with load carriage (weighted vest, 30% body weight) completed three times during the 6 day kinetic period.<br>|

What is the study measuring?
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Primary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| 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. | 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. | 

Secondary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| 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 | 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. | 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). |