Researchers identified gut microbiome signatures associated with frailty severity in older women, revealing microbial changes linked to poorer physical function, injurious falls, and mortality.
Study: Gut microbiota associates with frailty in older women. Image credit: In Green/Shutterstock.com
A recent paper published in the journal Nature Communications found that in older women, increasing frailty severity was associated with distinct gut microbiome signatures that differ from those previously seen in healthy aging.
Existing frailty tools miss important health dimensions
Frailty is a multidimensional syndrome related to aging, characterized by reduced physiological resilience and depleted reserves. Frail individuals are prone to falls, hospitalization, disability, and mortality. Multiple tools have been developed to measure frailty, including the Fried Frailty Phenotype, the Rockwood Frailty Index, and the Clinical Frailty Scale.
However, each captures different aspects of frailty but may not encompass its full functional, psychological, and physiological dimensions. The Charlson Comorbidity Index (CCI) is commonly used to stratify mortality risk, but again incorporates only disease-related risk.
Recognizing this, the authors developed and internally validated the Frailty Mortality Index (FMI). This incorporates three types of measures: functional, physiological, and psychological , associated with frailty and mortality risk. Variables include:
- Age and weight
- Physical function in terms of walking speed and chair stand
- Current smoking habit
- Mental quality of life
- Hospital stay duration
- CCI
Frailty shows distinct microbiome changes beyond healthy aging
Prior studies suggest that the gut microbiome may contribute to systemic inflammation, alterations in immune signaling and neuroendocrine communication, metabolic shifts, and reduced musculoskeletal function.
Aging is often accompanied by reduced gut microbial diversity, loss of beneficial commensals, and expansion of opportunistic or pro-inflammatory species. These shifts are associated with the loss of muscle mass and cognitive decline, and multiple chronic conditions.
Notably, although healthy aging is also accompanied by changes in the gut microbiome, frailty has been associated with a distinct microbial profile that differs from healthy aging. This motivated the current study, which aimed to identify FMI-associated changes in the gut microbiome linked to greater frailty severity and poorer frailty-related clinical outcomes.
The authors captured these shifts using clinical and metagenomic data from the Swedish SUPERB cohort. The FMI was developed in the full SUPERB cohort and subsequently applied to 2,081 women aged between 75 and 80 years with complete metagenomic data, who were followed up for a median of 8 years. Stool samples and clinical assessments were collected at baseline, while mortality, injurious falls, and hip fractures were assessed prospectively during follow-up.
Higher frailty scores predicted poorer clinical outcomes
The results showed that the FMI was more strongly associated with mortality risk compared to the established Charlson Comorbidity Index (CCI) in this cohort. Compared with women in the lowest FMI quartile ("no frailty"), those in the highest quartile ("severe frailty") had approximately a fivefold higher hazard of death, a 63% higher hazard of injurious falls, and 2.4 times the hazard of hip fracture.
Participants with high FMI scores were more likely to be older, to be current smokers, to have a higher CCI, and poorer physical function. They also had more markers of systemic inflammation (as indicated by the Systemic Inflammation Response Index (SIRI)), and were more commonly on medications including metformin and proton pump inhibitors.
Gut microbial diversity declined with increasing frailty
Higher FMI scores were associated with reduced microbial diversity, gene richness, and predicted functional capacity. Lower microbial diversity and gene richness were also associated with higher mortality and injurious fall risk, although neither showed a significant association with hip fracture. Gene richness reflects the gut’s microbial functional potential.
The adjusted analysis showed that variation in FMI explained 0.6% of the overall variation in gut microbiome composition, which is higher than the share explained by any of the other tested risk factors for frailty, such as BMI, smoking, or physical function.
The authors proposed that reduced diversity and gene richness of the gut microbiome may indicate that it is less able to adapt to disruptions, including colonization by pathogens. In keeping with this, the host may be more likely to suffer adverse outcomes linked to frailty.
Taxa-specific shifts in gut microbiome
Of the total taxa examined, 404 bacterial species were significantly associated with the FMI. After adjusting for gene richness, 63 species remained significantly associated with the FMI, including 34 species that were significant before and after adjustment. Conversely, 29 species were significantly associated with the FMI only after adjusting for gene richness, indicating an intrinsic linkage between them.
Further analysis suggested that although many microbial associations overlapped with comorbidity, the FMI-associated microbial signatures remained significant after adjustment for a broad range of baseline diseases, indicating that they were not explained solely by comorbidity and poor health status at baseline.
The major species with the greatest association with FMI included the opportunistic pathogens Enterocloster spp., Streptococcus mutans, an oral facultative anaerobe, and Clostridium Q symbiosum. Several of them were independently related to frailty-related measures including functional limitations and injurious falls, as well as mortality risk. Conversely, several potential butyrate-producing species, including Faecalibacterium prausnitzii, were negatively associated with FMI and linked to better physical and mental function.
In an independent Chinese cohort, 52.3% of FMI-associated species showed directionally concordant associations with at least one physical function measure or mortality. Among the top 5% of FMI-associated species, 11 (55%) showed statistically significant associations in the same direction, supporting the consistency of the findings across geographically distinct populations.
Metabolic signatures reflected shifts toward frailty
The researchers examined gut metabolic modules (GMMs), representing selected metabolic pathways in the gut microbiota. This showed that the GMMs with strong positive associations with FMI were involved in amino acid breakdown and anaerobic respiration. Additional positively associated pathways included lactate metabolism, propionate production, and trimethylamine/trimethylamine-N-oxide metabolism. In contrast, several metabolic pathways negatively associated with FMI were linked to lower mortality and injurious fall risk.
Exploratory models identified the taxa most strongly associated with FMI. Several taxa contributed as much as gut microbiome gene richness or clinical markers in the prediction models. The authors note, however, that these machine-learning analyses were exploratory and intended to qualitatively prioritize features rather than establish independent biological effects.
Further studies are needed before clinical translation
The study had several limitations. Because the microbiome analyses were cross-sectional, the researchers could not determine whether changes in the gut microbiome preceded frailty or developed as a consequence of it, preventing any conclusions about causality. The SUPERB dataset also lacked some of the information needed to directly compare the Frailty Mortality Index (FMI) with other established frailty indices.
The microbiome analyses focused on bacterial species and predicted metabolic pathways rather than directly measuring metabolic byproducts, while the potential roles of archaea, bacteriophages, and strain-level variation remain unclear. In addition, dietary intake, an important factor influencing the gut microbiome, was not assessed.
The findings may also have limited generalisability because the discovery cohort consisted exclusively of community-dwelling Swedish women aged 75–80 years. Although the independent Chinese replication cohort included both men and women, it lacked the information needed to calculate the FMI directly, meaning only species-level associations with physical function and mortality could be evaluated.
New frailty index revealed microbiome patterns linked to health
The FMI integrates multiple measures (functional, physiological, and psychological) of frailty to estimate mortality risk. Using the FMI, the study identified species-level variations in the gut microbiome that are linked to increasing FMI scores, and to adverse outcomes associated with frailty and mortality risk.
While several findings were supported by concordant associations in an independent Chinese cohort, the results remain observational and do not establish that microbiome changes cause frailty. Future studies using more diverse populations, longitudinal sampling, external validation, and strain-specific analyses are required to validate these findings before they can be translated into preventive or therapeutic interventions for frailty.
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