In missions longer than 30 days, spaceflight bone-mineral-density loss runs about 0.8% per month at the lower limbs

A systematic review and meta-analysis of spaceflight-only bone-mineral-density (BMD) data, pooling multiple missions and skeletal sites, reports that on missions longer than 30 days the lower limbs lose "−0.8% [−1.1, −0.5] per month," while the upper limbs and thorax barely change at "−0.1% [−0.2, 0.0] per month." Expressed as total post-flight loss, the lumbar spine and pelvis fall by "−6.2% [−6.7, −5.6]." The strongly weight-bearing lower skeleton demineralizes; the non-weight-bearing upper skeleton essentially does not — a regional gradient consistent with mechanical unloading as the driver.

These are the spaceflight-side reference figures against which the ground analog is judged. The meta-analysis is spaceflight-only and, as the capture verified directly, does not itself run a statistical bed-rest-versus-flight comparison in its main text; it supplies the orbital numbers that the bed-rest figures in claim-bed-rest-bmd-loss-about-one-percent-per-month-hip-pelvis-heel and claim-bed-rest-bone-turnover-markers-run-half-spaceflight-magnitude are paired against. That pairing is the substance of claim-bed-rest-under-runs-spaceflight-bone-loss-depends-on-readout. The whole comparison traces back to the in-flight concern recorded in claim-gemini-nutrition-report-embeds-a-bone-demineralization-concern and the ground program of claim-pauline-beery-mack-became-nasas-bone-density-pi-at-seventy.