The Effects of Soy Protein–Rich Meals on Muscle Health of Older Adults Are Linked to Gut Microbiome Modifications

ABSTRACT Background Sarcopenia is characterized by accelerated muscle mass and function loss in older adults. The role of nutritional interventions in sarcopenia is uncertain. This study investigates whether a soy protein–rich diet can enhance muscle health in older adults via gut microbiota changes. Methods A 12‐week randomized controlled trial was conducted with 84 older adults from a long‐term care facility. Participants in the intervention group consumed three daily meals containing 10 g of soy protein (totalling 30 g/day), while the control group maintained their usual diets. Faecal samples from 53 participants were collected at Weeks 0, 6 and 12. We assessed changes in muscle function, gut microbiota composition and faecal short‐chain fatty acids (SCFA). Results The intervention group showed preserved calf circumference, while the control group experienced a decrease (W12‐W0: Intervention, 0.56 ± 0.22 cm; Control, −0.91 ± 0.26 cm, p (interaction) < 0.001). Metagenomic analysis revealed significant alterations in gut microbiota among intervention participants who showed improvement in muscle performance parameters. The intervention increased SCFA‐producing bacteria ( Roseburia faecis , Intervention: 0.42 ± 0.21%, Control: −0.06 ± 0.16, p (interaction) < 0.05; Agathobaculum butyriciproducens , Intervention: 0.02 ± 0.007%, p (time) < 0.01, Control: −0.04 ± 0.01) and decreased species associated with poorer muscle outcomes ( Alistipes putredinis , Intervention: −0.88 ± 0.40%, Control: 0.62 ± 0.63, p (interaction) < 0.05; Eubacterium_sp_CAG_38 , Intervention: −0.64 ± 0.28%, Control: 0.10 ± 0.22, p (interaction) < 0.05). Functional pathway analysis showed enrichment of anaerobic amino acid degradation pathways and vitamin biosynthesis, with depletion of inflammatory pathways, particularly lipopolysaccharide biosynthesis. Microbiome phenotype prediction revealed a decrease in aerobic bacteria abundance in the intervention group (W12‐W0, Intervention: −0.004 ± 0.002; Control: 0.001 ± 0.001, p (interaction) < 0.05). Interaction (group × time) for SCFA was not statistically significant; within‐group increases at Week 6 were observed in only the intervention group (butyric acid, Intervention: 0.74 ± 0.34 mg/g, p (time) < 0.05, Control: 0.12 ± 0.43 mg/g; isobutyric acid, Intervention: 0.14 ± 0.08 mg/g, p (time) < 0.05, Control: 0.08 ± 0.10 mg/g; isovaleric acid, Intervention: 0.27 ± 0.14 mg/g, p (time) < 0.05; Control: 0.16 ± 0.20 mg/g), with partial reversal by Week 12. These changes, positively correlated with improved muscle function parameters, suggest intervention benefits on gut health and muscle function. Conclusion A soy protein–rich intervention improved muscle health in older adults through beneficial gut microbiota. These findings support the gut–muscle axis hypothesis and suggest dietary soy protein may alleviate sarcopenia by promoting a healthier gut microbiome.