The Effect of Routine High-Soy-Protein Soymilk Intake on the Skeletal Muscle Index (SMI) in Japanese Pre-Frail Elderly Individuals with an Ordinary Life: A Post Hoc Analysis of a Randomized Controlled Trial

BACKGROUND/OBJECTIVES: The primary outcome of a 12-week randomized controlled trial studying the effects of continuous high-soy-protein (HSP) soymilk consumption in Japanese frail and pre-frail elderly individuals has previously been reported. The authors of this post hoc study aimed to conduct a statistical analysis focusing on changes in the skeletal muscle index (SMI) and evaluate the effect of HSP soymilk intake on appendicular skeletal muscle mass (ASM). METHODS: A full analysis of all subjects (n = 68, HSP group: n = 34, control group: n = 34) and a subgroup analysis of only pre-frail elderly individuals (n = 58, HSP group: n = 29, control group: n = 29) were performed. The following nine subgroup analyses were conducted: frailty phenotype, basal metabolic rate, walking speed, step counts, total energy expenditure (TEE), total energy intake (TEI), estimated protein intake, age, and sex. RESULTS: In the overall analysis (primary combined cohort), the SMI showed no significant change between the HSP and control soymilk groups (p = 0.18); however, a significant difference in the change in the SMI between the HSP and control groups was revealed for pre-frail elderly subjects (mean difference in SMI change: 0.08 kg/m² [95% CI 0.004, 0.15], p = 0.044). Furthermore, in four distinct subgroups restricted to the pre-frail elderly individuals-step counts (≥5000 steps/day), TEE (≥30 kcal/kg BW/day), TEI (≥30 kcal/kg BW/day), and male sex-the HSP group also showed significant differences in SMI change. CONCLUSIONS: In this study, no significant improvement in the SMI was observed across all subjects (frail and pre-frail elderly individuals); however, an exploratory subgroup analysis suggested that continued daily HSP soymilk intake was associated with potential benefits in pre-frail elderly individuals with high physical activity and energy intake levels. These findings are preliminary and require further research.