In head-down bed rest, bone-resorption markers and PTH suppression run at roughly half their spaceflight magnitude

A 30-day −6° head-down-tilt bed rest study directly quantified the gap between the ground analog and actual orbital flight for the biochemical readouts of bone loss. On the resorption side, the study reports that "During spaceflight, urinary markers of bone resorption increase to 100–150% of their preflight values," whereas "Bone resorption markers increase 50–75% in bed rest, whereas bone formation markers typically remain unchanged during bed rest." The same halving appears in endocrine control: bed-rest suppression of parathyroid hormone (PTH) "represent[s] about one-half the decrease that has been observed during spaceflight (∼60% decrease)," i.e. roughly a 30% PTH drop in bed rest against ~60% in flight.

Read together, these figures make the analog's direction reliable but its amplitude attenuated: bed rest moves the biochemical and endocrine markers the same way spaceflight does, at about half the intensity. This is the cleanest primary-source evidence that head-down bed rest under-runs true microgravity, and it is the turnover-marker leg of the broader claim-bed-rest-under-runs-spaceflight-bone-loss-depends-on-readout comparison. It supplies the specific figure that claim-head-down-bed-rest-is-the-standard-ground-analog-for-spaceflight-bone-loss previously asserted only as "quantitatively smaller," and it descends from the in-flight skeletal-loss concern first recorded in claim-gemini-nutrition-report-embeds-a-bone-demineralization-concern. The bone-mineral-density readout tells a less tidy story — see claim-bed-rest-bmd-loss-about-one-percent-per-month-hip-pelvis-heel and claim-spaceflight-bmd-loss-lower-limbs-about-0-8-percent-per-month.