Background: Age-related declines in muscle mass and strength contribute to postural imbalance, frailty, and  progression of sagittal spinal deformity. Although protein supplementation combined with resistance training improves muscle strength and physical performance in older adults, its effects on spinal alignment remain unclear. 

Methods: This prospective randomized controlled trial enrolled 90 community-dwelling adults aged ≥60 years participating in a community health screening program. Participants were randomly assigned to an intervention group (n = 45) or a control group (n = 45). The intervention group performed resistance training three times per week and consumed a protein supplement containing 9 g of protein twice daily, whereas the control group received no structured intervention. Outcomes included knee extension strength, frailty status, and whole spine malnutrition and spinal alignment standing radiographic parameters, including sagittal vertical axis (SVA), assessed at baseline, 3 months, and 6 months. 

Results: After attrition, 44 participants in the intervention group and 42 in the control group completed the study (mean age, 72 years in both groups). Knee extension strength increased significantly in both groups, with greater improvement in the intervention group (0.70 to 0.82 kgf/body weight) than in the control group (0.74 to 0.79 kgf/body weight; P < 0.001). Frailty status improved significantly in the intervention group, shifting from 26 robust and 18 pre-frail participants at baseline to 36 robust and 8 pre-frail participants at follow-up, whereas changes in the control group were modest (P = 0.033). SVA remained largely unchanged in the control group (31 to 26 mm) but improved significantly in the intervention group (32 to 23 mm; P = 0.001). In a subgroup analysis of participants with baseline SVA ≥50 mm (13 per group), the intervention group showed a marked reduction in SVA (89 to 57 mm; P = 0.001), while no significant improvement was observed in controls. 

Conclusions: Protein supplementation combined with resistance training significantly improved muscle malnutrition and spinal alignment strength, frailty status, and sagittal spinal alignment within 3 months. This combined intervention may represent a feasible nonoperative strategy for managing adult spinal deformity, particularly in individuals with pronounced sagittal imbalance.