An index of oral-type gut genera is associated with lower muscle mass and with 3- year muscle loss in community-dwelling older adults: a cohort study

Abstract Background Evidence on the gut-muscle axis is mostly cross-sectional and framed around microbial diversity. We examined whether a prespecified index of oral-type gut genera — organisms normally resident in the mouth whose faecal appearance marks a failing gastric acid barrier — was associated with muscle mass, muscle function and subsequent 3-year muscle loss. Methods In the Kyotango Longevity Cohort Study, 782 community-dwelling adults aged 65 or older had faecal 16S rRNA profiling and a muscle assessment; 493 were reassessed a mean of 3.35 years later. Exposures were alpha diversity, community composition, 130 genera, and an index prespecified as the mean standardised centred log-ratio abundance of Streptococcus, Veillonella, Fusobacterium, Granulicatella, Actinomyces, Rothia and Haemophilus. Appendicular skeletal muscle mass (ASM) was the primary outcome; ASM/height², ASM/BMI, grip strength, knee extension strength and gait speed were secondary, with the false discovery rate controlled across them and across change in each at follow-up. Covariates were entered in five prespecified blocks, ending with gastrointestinal medication, constipation and disease. Longitudinal models were repeated with censoring weights. Results Per 1 SD of the index, ASM was lower by 0.075 SD (95% CI 0.034 to 0.116) adjusted for age, sex and survey year, and by 0.067 SD (0.029 to 0.105) fully adjusted. Every secondary outcome survived correction except grip strength (q = 0.102): ASM/BMI − 0.059, gait speed − 0.088, ASM/height² -0.046 and knee extension − 0.067 (all q ≤ 0.026). Annual change in ASM was − 0.125%/year (-0.235 to -0.015) fully adjusted and − 0.180%/year (-0.292 to -0.067) weighted, but change in grip strength, knee extension strength and gait speed was unrelated to the index (q = 0.36 to 0.88), as was change in every diversity measure. Proton pump inhibitor use raised the index by 0.49 SD (0.18 to 0.81); no inflammatory marker was associated with it. Conclusions An index of oral-type gut genera was associated with lower muscle mass, weaker knee extension and slower gait, and with subsequent loss of muscle mass but not of strength or speed; diversity was associated with none of these. The associations are small and observational. The gut-muscle axis may be better studied at taxonomic resolution and against continuous measures.