Bone calcium balance following an Antarctic expedition

Expeditions involve high exercise volumes and severe energy deficits, which can decrease areal bone mineral density (aBMD), indicative of disturbed calcium homeostasis. This study investigated the effect of a prolonged Antarctic crossing on aBMD and bone calcium balance. Nine individuals (six men and three women) each hauled 85 kg sledges over 911 km in 47 days. Pre- and postexpedition whole-body aBMD was measured by DXA. Spot urine voids were collected for the measurement of calcium isotope ratio (δ^44/42Ca) upon waking pre- (-1 day), during (<i>day 15</i> and <i>day 30</i>), and post- (+2 days) expedition. Venous blood samples were obtained pre- (-14 days) and post- (+6 days) expedition. Body mass (-3.3%), body fat percentage (-21.4%), and aBMD of the arms, legs, rib, pelvis, and spine (-1.3% to -12.1%) decreased from pre- to postexpedition (<i>P</i> ≤ 0.032). Despite no overall change in group-level urine δ^44/42Ca between the start and end of the expedition (<i>P</i> = 0.960), large interindividual variability in the responses were observed with some participants increasing and some decreasing up to <i>day 30</i>. There was no difference in intact parathyroid hormone, total 25-hydroxy vitamin D, follicle stimulating hormone, luteinizing hormone, estradiol, or testosterone between pre- and postexpedition (<i>P</i> ≥ 0.067). Thyroid stimulating hormone and total T3 increased from pre- to postexpedition (both <i>P</i> = 0.02). The decline in aBMD was likely due to prolonged energy deficit. There was no associated change in bone calcium balance likely because δ^44/42Ca tracks short-term calcium fluxes from bone and can normalize rapidly, whereas aBMD reflects cumulative changes in mineralized bone mass.<b>NEW & NOTEWORTHY</b> This study provides new insight into bone calcium balance in extreme environments using a novel and noninvasive approach of naturally occurring calcium isotopes. Decreases in fat mass and body mass provide evidence that participants were in energy deficit, but most reproductive and thyroid hormones remained stable. Whole-body and regional aBMD decreased. Group-level urine δ^44/42Ca-a marker of bone calcium balance did not change across the expedition, but individuals displayed significant variability in responses.