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.

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

Eligibility based on female anatomical sex characteristics