Head-down bed rest of 42–90 days produces bone-mineral-density losses averaging about 1% per month at the hip, pelvis, and heel

A primary NASA-affiliated bed rest study of 13 subjects held in continuous head-down tilt for 42 to 90 days, with DXA scans before and after, found regional bone-mineral-density (BMD) loss "averaging 1% per month in the hip, pelvis, and heel." The authors report that these "BMD changes were consistent with earlier bed rest and spaceflight studies," and that "Recovery data were also consistent with data obtained after spaceflight" — placing bed rest and flight in the same family of skeletal response and recovery.

This is the bone-density leg of the ground-analog comparison, and it sits in tension with the turnover-marker leg. Where claim-bed-rest-bone-turnover-markers-run-half-spaceflight-magnitude shows a clean ~2x under-run, this ~1%/month BMD figure sits close to — not clearly below — the meta-analytic spaceflight lower-limb rate of ~0.8%/month in claim-spaceflight-bmd-loss-lower-limbs-about-0-8-percent-per-month. The two numbers, however, come from different populations and durations and are not a matched head-to-head measurement. The unresolved instrument gap is carried by claim-bed-rest-under-runs-spaceflight-bone-loss-depends-on-readout and the open verification in question-matched-head-to-head-bed-rest-vs-spaceflight-bone-comparison. This figure gives claim-head-down-bed-rest-is-the-standard-ground-analog-for-spaceflight-bone-loss its first primary-sourced DXA number.