Supplementation of low-protein diets with plant protein and probiotics enhances muscle health by regulating gut microbiota and metabolomic profiles in SAMP8 mice†

Muscle health is crucial, especially for aging populations. This study investigates how plant protein and probiotic supplementation in a low-protein (LP) diet affects the gut microbiota, metabolome, and muscle health in aging SAMP8 mice, emphasizing the gut–muscle axis in older populations. Twenty-four 8-month-old male SAMP8 mice were divided into four groups: control (CON, standard diet), low-protein (LP), LP supplemented with plant protein (LP + P), and LP supplemented with plant protein and probiotics (LP + P + B). The experimental treatment lasted 8 weeks. Results showed that, by week 6, body weight increased significantly in all LP groups, with a trend toward higher body fat. Protein utilization and muscle strength improved significantly in the supplemented groups compared to the LP group (P < 0.05). The LP group showed elevated levels of inflammatory cytokines (IL-12p70, IL-6, TNF-α) (P < 0.05) in serum, which were reduced in the supplemented groups, especially in the LP + P + B group (P < 0.05). Gene expression related to muscle protein synthesis (mTOR, S6K1) and oxidative stress (CAT, Nrf2) was upregulated in the LP group but downregulated in the supplemented groups (P < 0.05). Immune-related genes followed similar patterns, with inflammation markers being significantly reduced after supplementation (P < 0.05). After supplementation, fecal beneficial bacteria (Bifidobacterium, Roseburia) and metabolites (butyric acid, indole-3-propionic acid, kyotorphin) increased, indicating enhanced gut health (P < 0.05). Specific bacterial species were correlated with metabolites and immune markers, highlighting their role in immune modulation (P < 0.05). These results show that supplementing plant protein and probiotics into an LP diet improved muscle strength, reduced inflammation, optimized gut microbiota, and boosted beneficial metabolism. The findings suggest that plant protein and probiotics can maintain muscle health and regulate immune responses in the elderly.