Head-down bed rest under-runs spaceflight bone loss by about twofold on turnover markers but nearly matches it on monthly BMD-loss rate

Head-down bed rest tracks spaceflight bone loss in direction but under-runs it in magnitude — yet the size of that under-run depends on which instrument is read. On the biochemical and endocrine readout the gap is clean and roughly twofold: bone-resorption markers rise 50–75% in bed rest versus 100–150% in flight, and PTH suppression in bed rest is "about one-half the decrease that has been observed during spaceflight" (see claim-bed-rest-bone-turnover-markers-run-half-spaceflight-magnitude). On the DXA-measured bone-mineral-density rate the gap nearly closes: bed rest loses ~1%/month at the hip and pelvis (claim-bed-rest-bmd-loss-about-one-percent-per-month-hip-pelvis-heel) while pooled spaceflight loses ~0.8%/month at the lower limbs (claim-spaceflight-bmd-loss-lower-limbs-about-0-8-percent-per-month).

The honest answer to "by how much does bed rest under-run flight" is therefore "it depends on the instrument," not a single multiplier. Crucially, the BMD comparison is not a matched head-to-head: the ~1%/month and ~0.8%/month figures come from different populations, durations, and skeletal sites, so their near- equality may be an artifact of unmatched study design rather than a real convergence. This note qualifies the flat "quantitatively smaller" summary in claim-head-down-bed-rest-is-the-standard-ground-analog-for-spaceflight-bone-loss and answers the quantitative half of question-bed-rest-analog-quantitatively-tracks-spaceflight-bone-loss. The matched study that would settle the BMD leg is routed to question-matched-head-to-head-bed-rest-vs-spaceflight-bone-comparison.