Partitioning the contributions of body composition to energy requirements in chronic spinal cord injury: A mediation analysis.

CONTEXT/

OBJECTIVE

Resting metabolic rate (RMR) is reduced after spinal cord injury (SCI), but the contributions of fat-free mass (FFM) loss and injury-related physiology remain unclear. We quantified the FFM-mediated and residual components of the SCI-RMR gap and tested whether RMR-FFM scaling differs between SCI and controls.

DESIGN

Cross-sectional.

SETTING

University-based research center.

PARTICIPANTS

Adults with chronic SCI (n = 55) were age- and sex-matched to controls (n = 55).

INTERVENTIONS: N/A.

OUTCOME MEASURES: Primary outcomes were the proportion of the SCI-RMR association mediated by FFM, the residual direct effect of SCI, and group differences in RMR-FFM allometric scaling. The RMR-FM association was also examined.

RESULTS

RMR, FFM, and bone mineral content were lower in SCI than in controls, whereas fat mass (FM) was higher (p ≤ 0.004). In the final FFM-only model, reduced FFM mediated 73.4% of the SCI-RMR association (indirect β = -0.34, p < 0.001), with a residual direct effect accounting for 26.6% (β = -0.12, p = 0.057). RMR scaled sublinearly with FFM in controls (α = 0.77, p = 0.022 vs. α = 1.0) and SCI (α = 0.74, p < 0.001 vs. α = 1.0), with no group × FFM interaction (p = 0.79). The RMR-FM association differed by group (group × FM, p = 0.025), with FM positively associated with RMR in SCI (α = 0.13) but not in controls (α = -0.07).

CONCLUSION

These findings quantify the FFM-mediated and residual components of the SCI-related reduction in RMR, supporting FFM-based energy estimation. The residual component (∼27%; ∼90 kcal/day) may be clinically meaningful for avoiding overestimation of energy needs, while group differences in the FM-RMR association may further refine prediction models.