Stavnichuk's −0.8%/month lower-limb figure is the slope of a linear meta-regression — the paper's own Discussion flags a hint that bone loss may decelerate over mission duration

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The −0.8%/month lower-limb figure most readers quote as "the" spaceflight bone-loss rate is not a simple average; it is a fitted slope from a constant-rate model. Stavnichuk et al.'s Methods specify: "Between-study meta-regression was performed assuming a random effects model: yi = β0 + β1xi + εj + ηj, where β0 was fixed at 0 (0% from pre-flight on day 0 of spaceflight), β1 describes the relationship between xi (mission duration) and outcome yi." With the intercept fixed at zero, the slope β1 is the only free parameter in the model's mean structure (the inter-study variance τ² is estimated separately, by the DerSimonian and Laird estimator) — bone loss is assumed proportional to time by construction, and "−0.8%/month" is that proportion. Figure 3d confirms the figure's status directly, plotting "slope coefficients β ± 95% CI for meta regressions"; the −0.8% value is the estimate for missions longer than 30 days, which the authors call their "most conservative and precise."

The paper's own Discussion complicates the assumption the model is fit under: "Bone loss in the lower limbs was progressive; however, long-duration missions reported less bone loss than intermediate duration missions, suggesting that microgravity-induced bone loss may diminish with time." The authors immediately qualify this as an "optimistic interpretation" that "should be cautioned by" four named considerations: that individual-level heel bone loss was proportional to flight duration; that the highest individual bone loss came from an intermediate-duration flight, "likely overestimating bone loss in this subgroup"; that the increase in bone formation markers "was highly variable and meta-analytic estimates differed from individual studies"; and that longer-duration missions included ISS flights that "benefited from advanced nutrition and exercise." The paper does not leave the idea there, however — its closing Summary lists the deceleration among the study's contributions, stating that the authors "provide evidence that bone loss may diminish during longer duration flights." So the caveat and the claim sit side by side in the same paper: its own authors note that the constant-rate kinetics underlying their headline slope may not describe the data it is fit to.