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 post-expedition whole-body aBMD was measured by DXA. Spot urine voids were collected for the measurement of calcium isotope ratio (δ44/42Ca) upon waking pre- (-1 d), during (day 15 and day 30), and post- (+2 d) expedition. Venous blood samples were obtained pre- (-14 d) and post- (+6 d) 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 post-expedition (p ≤ 0.032). Despite no overall change in group-level urine δ44/42Ca between the start and end of the expedition (p = 0.960), large inter-individual variability in the responses were observed with some participants increasing and some decreasing up to day 30. There was no difference in iPTH, total 25(OH)D, FSH, LH, oestradiol, or testosterone between pre- and post-expedition (p ≥ 0.067). Thyroid stimulating hormone and total T3 increased from pre- to post-expedition (both p = 0.02). The decline in aBMD was likely due to prolonged energy deficit. There was no associated change in bone calcium balance likely because δ44/42Ca tracks short-term calcium fluxes from bone and can normalise rapidly, whereas aBMD reflects cumulative changes in mineralised bone mass.
Keywords: Calcium isotopes; bone calcium balance; bone density; polar expedition.