The global rise in average age is increasing the prevalence of age-related conditions like muscle loss (sarcopenia) and cognitive decline. Dementia is a growing concern, but understanding cognitive changes during healthy aging is crucial. Exercise is known to mitigate these declines, but adherence can be challenging for older adults. Therefore, interventions providing broad physical and mental benefits are needed. Older adults often experience reduced dietary protein intake due to factors such as social isolation and decreased digestive efficiency. Additionally, they exhibit anabolic resistance, a blunted response of muscle protein synthesis to protein intake. Experts recommend higher daily protein intake (1–1.3 g/kg/day) for older adults compared to the general adult recommendation (0.8 g/kg/day). Resistance exercise is a potent anabolic stimulus for skeletal muscle, synergistically enhancing the effects of protein supplementation. Emerging research suggests a significant role for the gut microbiome in both cognitive and physical function during aging. The gut microbiome's resilience diminishes with age, increasing vulnerability to disease and lifestyle changes. Animal studies have demonstrated the ability of gut microbiota to influence body composition, highlighting its role in metabolic phenotypes. The gut microbiome's influence on anabolic resistance is being investigated through mechanisms such as protein digestion, gut barrier function, and inflammation. Prebiotics, selectively utilized by gut microbiota, have shown promise in improving physical function (hand grip strength, exhaustion) and frailty in older adults. Preliminary evidence suggests a beneficial effect on cognition. This study aimed to evaluate whether gut microbiome modulation, using a prebiotic, improves muscle function and cognition in older adults already undergoing BCAA supplementation and resistance exercise. Utilizing twin pairs helps control for genetic and environmental variance, strengthening the study design.

The literature review supports the hypothesis that the gut microbiome plays a crucial role in age-related muscle loss and cognitive decline. Studies have shown a decrease in gut microbiome resilience with age, linking this to altered species richness and increased inter-individual variability. Landmark animal studies using fecal transplants have demonstrated the profound impact of gut microbiota on body composition. Several mechanisms have been proposed for the gut microbiome's influence on anabolic resistance, including roles in protein digestion and absorption, gut barrier function, and inflammation. Previous research has demonstrated the positive effects of prebiotics on improving aspects of frailty in older adults, including handgrip strength and exhaustion levels. However, there's a scarcity of research specifically focusing on the combined effects of protein and prebiotic supplementation on both muscle function and cognition in older populations, making this study significant.

The PROMOTE trial was a double-blind, placebo-controlled, randomized controlled trial (RCT) conducted remotely. 36 twin pairs (72 individuals aged ≥60) were recruited from the TwinsUK cohort. Eligibility criteria included being aged 60 or older and having previously reported low dietary protein intake (<1 g/kg body weight/day). Exclusion criteria aimed to avoid confounding factors such as recent antibiotic use and significant gastrointestinal issues. Participants were randomized, one twin to each arm (prebiotic or placebo), in a paired design. All participants received daily branched-chain amino acid (BCAA) protein powder and were encouraged to perform resistance exercises twice weekly. The prebiotic arm additionally received a daily prebiotic supplement (inulin and fructo-oligosaccharides) while the placebo arm received maltodextrin. The supplements were provided in identical sachets to maintain blinding. The study was delivered remotely using postal packs, video teleconferencing, online questionnaires, online cognitive testing (CANTAB), and online food diary software (MyFood24). Data collected included baseline and end-of-study measurements of chair rise time (primary outcome), grip strength, Short Physical Performance Battery (SPPB) score, International Physical Activity Questionnaire (IPAQ) data, Simplified Nutritional Assessment Questionnaire (SNAQ) appetite score, and cognitive test results (CANTAB). Stool samples were collected at baseline and the study's end for gut microbiome analysis. Shotgun metagenomic sequencing was employed to characterize gut microbiota composition and function. Statistical analyses included linear mixed-effects models to compare intervention groups, adjusting for baseline measures and twin clustering. Correlation analyses investigated the relationship between changes in microbiota features and changes in muscle function and cognition.

A total of 72 participants (36 twin pairs) completed the study. The prebiotic supplement was well-tolerated, with mostly mild adverse events (abdominal bloating) reported more frequently in the prebiotic group. There was no significant difference between the prebiotic and placebo groups in the primary outcome of chair rise time (β = 0.579; 95% CI -1.080-2.239 p = 0.494). However, a significant improvement in cognition was observed in the prebiotic group compared to the placebo group (β = -0.482; 95% CI, -0.813, -0.141; p = 0.014). Specifically, the paired associates learning test showed significantly fewer errors in the prebiotic group (β = 7.55; 95% CI: 4.65-10.46; p=0.001). Gut microbiome analysis revealed significant changes in microbiota composition in response to the prebiotic intervention. Specifically, there was a significant increase in relative *Bifidobacterium* abundance in the prebiotic group. Further analyses revealed correlations between changes in several microbiota features and changes in chair rise time and cognitive scores, though these correlations were less strong than the prebiotic's effect on paired associate learning. Despite the paired design, and evidence that twin pairs had more similar microbiota than non-twin pairs, intra-twin dissimilarity did not change significantly over the study period.

This RCT provides evidence that a readily available and inexpensive prebiotic intervention can improve cognitive function in older adults. The lack of a significant effect on muscle strength may be attributed to several factors. The 12-week intervention period might have been insufficient for observable muscle remodeling, highlighting the need for longer-term studies. The sample size calculation was based on previous trials not incorporating microbiome interventions, potentially leading to underpowering. Future studies should consider using both composite measures of muscle strength and function, and also more direct and specific assessments of muscle strength. The specific prebiotic used may not be optimal for influencing muscle health, underscoring the need for further research to identify the most effective prebiotic for this purpose. The significant improvement in cognition supports the growing body of evidence for a gut-brain axis. The increase in *Bifidobacterium* abundance, specifically within the *Actinobacteria* phylum, aligns with other studies linking this phylum to improved cognition, though the association remains complex. This research highlights the potential of gut microbiome-targeted interventions for enhancing cognitive health in older adults.

This study demonstrates that a prebiotic supplement, when combined with protein supplementation and resistance exercise, significantly improves cognitive function in older adults. Although it didn't show significant benefits on muscle strength within the 12-week timeframe, the feasibility of remote delivery and positive effects on the gut microbiome are noteworthy. Future research should focus on longer intervention durations, larger sample sizes, and the exploration of different prebiotics and their combined impact on both muscle and cognitive function in older populations. Further research is needed to clarify the complex relationship between gut microbiota, especially *Bifidobacterium* abundance, and cognition.

This study has several limitations. The 12-week intervention period may have been insufficient to observe significant changes in muscle strength. The sample size calculation, based on previous trials without microbiome interventions, may have been underpowered. The remote study design, while advantageous for inclusivity, limited the ability to measure muscle mass and might have introduced some measurement error. Missing data was higher for CANTAB cognitive tests, possibly due to the complexity of home administration. The TwinsUK cohort's predominantly female and healthy volunteer bias might limit generalizability. Finally, the use of a maltodextrin placebo, not perfectly energy-matched to the prebiotic, is a potential limitation.