Does head-down bed rest quantitatively track spaceflight bone/calcium loss, and by how much does it under-run actual orbital losses?

The claim note claim-head-down-bed-rest-is-the-standard-ground-analog-for-spaceflight-bone-loss rests on an [unverified-quant/mechanism] flag: it was grounded during the originating hop only via secondary search summaries, not a read primary paper. Two things need a primary source before that note can leave seedling:

1. The quantitative gap. By roughly how much do bed-rest-induced bone mineral density and calcium-balance changes under-run true spaceflight losses? The capture asserts "qualitatively similar but quantitatively smaller" without a figure. A specific number is a Tier 1–2 claim and cannot rest on search-synthesis prose.

2. The mechanism/equivalence. In what sense is head-down bed rest a valid analog — same unloading pathway, same regional bone compartments, same time course? And is it genuinely the same paradigm clinical medicine uses to study disuse osteoporosis in immobilized patients, or merely an adjacent one?

What would answer it

• The PubMed item cited in the capture (https://pubmed.ncbi.nlm.nih.gov/23187072/) and its primary source, read directly rather than via summary.
• A bed-rest study from a standard facility (e.g. DLR's :envihab) reporting BMD/calcium-balance deltas alongside matched spaceflight data.
• The upstream flight experiment behind the whole thread: Whedon et al., M-6/M-7 bone-demineralization and calcium-balance results, Gemini Midprogram Conference, NASA SP-121 (1966) — the primary data that claim-gemini-nutrition-report-embeds-a-bone-demineralization-concern points at and that no note in the vault yet rests on.

Resolving this feeds the nightly topic queue and would let the bed-rest analog note carry a real figure instead of a hedge.