Crewmembers participated in one of three bone loss countermeasure regimens while in space. The ARED (advanced resistive exercise device) (n=16) and iRED (interim resistive exercise device)(N=7) groups were treated only with ARED and iRED. The bisphosphate group (n=7) was treated with both ARED and the bisphosphate Alendronate, a commonly used anti-resorptive drug. Alendronate treatment started several months before spaceflight. In- flight exercise started after an initial adjustment period of about 30 days. Each analytical run required analysis of additional quality control solutions, including a bracketing standard, two international calibration standards, a natural sample (IAPSO seawater), a calibration curve of standards of varying spike-sample ratio, and frequent blank correction. In addition, it required preparation of all standards and samples out of a single batch of tightly controlled trace metal grade molarity acid

The effect of three treatment groups – bisphosphonate, Interim Resistive Exercise Device (iRED) and Advanced Resistive Exercise Device (ARED) was compared for the relative amount of bone loss. Refrigeration was not required for any of these samples.

RESULTS:
The results confirmed that Ca isotopes act as an accurate, near-real time monitor in space flight and allowed to address the five primary questions posed at the beginning of the study. Ca isotope data indicated that amount of bone loss correlated to intervention type. On average, exercise-only crewmembers lost bone, particularly in the first 30 days of spaceflight. Individuals who did heavier resistance exercise (using the advanced resistive exercise device, ARED) maintained BMB better than those that did lower resistance exercise (using the interim resistive exercise device, iRED). This difference became more pronounced as the length of the mission increased, highlighting the need for vigorous exercise facilities in long-term space missions for bone health.

The results of this study were consistent with data from the bed rest populations. This confirmed that variation of dietary d44/42Ca is a minor concern. Inflight d44/42Ca was significantly lower than preflight baseline in both the iRED and ARED groups, and slightly but significantly higher in the Alendronate group (Table 1) (P= 0.01). Inflight d44/42Ca measurements in the ARED and iRED groups were not significantly different overall (P=0.09). However, the two groups show distinct trends: d44/42Ca of both groups fell rapidly during the 30-day adjustment period before inflight exercise started. After flight day 30, d44/42Ca rose significantly in the ARED group (+0.17‰, P=0.02) but not in the iRED group (+0.03, P=0.81). The ARED group maintained higher d44/42Ca values than the iRED group for the remainder of spaceflight. No significant differences in d44/42Ca based on age, sex, initial BMI, or time in space were detected.

Analyses of urine samples from 30 ISS crewmembers undergoing various bone loss countermeasures showed a similar relationship between the isotopic composition of naturally occurring Ca and BMB that was observed in bed rest studies. Each individual crewmember had a baseline average Ca isotope ratio before going into space. For crewmembers whose only bone loss preventative intervention was exercise, the Ca isotope ratio typically decreased – indicating an increase in bone loss--shortly after spaceflight began, remained low during spaceflight, and returned to pre-flight values upon return to Earth.