This study represents the first comprehensive untargeted evaluation of the association of metabolite profiles with physical function in patients on dialysis. Untargeted metabolomic approaches have been performed in multiple studies in those who are non-CKD older adults but have not been explored in those with CKD or new to dialysis18,19. In this study, we identified 20 metabolites for gait speed and 30 metabolites for grip strength with either a p < 0.05 or ES > 0.4. Only four metabolites were consistent between gait speed and grip strength (C7 carnitine, valine, phosphocholine isomer 2, n-acetylputrescine), but none of these metabolites achieved significance for both p value and ES. The nominal number of overlapping metabolites reinforces the notion that gait speed and grip strength are two different constructs. Composite scores were constructed by combining the multiple metabolites that achieved both p value and ES criteria (22 for grip strength and 12 for gait speed) to reflect physical function in patients who are incident to dialysis. The composite scores for the metabolites had diagnostic abilities per AUC scores of 0.91 and 0.88 respectively for grip strength and gait speed, better than clinical-demographic predictors alone and minimally different from combining clinical variables with the metabolites. Although the AUCs were promising, further work is required to validate the metabolite panel in a larger cohort. Collectively, these panels offer insight into the pathogenesis and potential target metabolites for improving physical function in those incident to dialysis, and if prospectively validated, diagnostic potential.
The intent of this pilot study was to determine if there were metabolites associated with physical function as captured by gait speed and grip strength. Gait speed is a multi-system measure that reflects muscle strength, bone-muscle interaction, neuromuscular control, and balance. The measure is considered the 5th vital sign in geriatrics, and in CKD is associated with morbidity and mortality1,5,20. In contrast, grip strength is an isolated measure of muscle function, also associated with all-cause mortality in patients undergoing hemodialysis21,22,23. We anticipated metabolites would be unique to each physical function construct because of the contrast in task requirements between gait speed and grip strength. Muscle contraction, required for both constructs, requires energy with three major sources: fatty acid oxidation, glycolysis, and protein metabolism and thus we focused on these pathways, although there were no metabolites identified in the glycolytic pathway.
Fatty acid oxidation metabolites were identified for both gait speed and grip strength. Gait speed identified four acylcarnitines (i.e., C7, C9, C5, C5:1), with ES ranging from 0.25 to 0.56 (positively associated with faster gait speeds), while grip strength identified two medium-long chain acylcarnitines (C16, C7), and nine medium-long chain lipid derivatives (i.e., C20:4 LPC, C22:6 LPC, C18:1 SM, C16:1 LPC plasmalogen, C20:4 LPE, C18:3 LPC, C16:0 LPC, C18:0 LPC, C22:0 SM) with ES ranging from 0.33 to 0.56 (positively associated with stronger gait strength). Fatty acid oxidation metabolites have been studied in the context of physical function in non-CKD populations, but there is a lack of consistent directions. In a study of 77 older men with a mean age of 79 years and average BMI of 28.4 kg/m2, higher acylcarnitine scores were associated with lower gait speeds19. In disease conditions such as heart failure, higher acylcarnitines were associated with a more pronounced disease state24,25. In a study with 43 community-dwelling older adults and age- and sex- matched controls higher plasma concentrations of medium- and long-chain acylcarnitines were associated with higher risk of lower extremity functional impairment by the short physical performance battery (SPPB) test that includes gait speed, but gait speed was not individually provided26. In this study, elevated acylcarnitines levels were positively associated with both gait speed and grip strength, which appears to be a unique feature in patients with CKD.
Grip strength was positively associated with taurine and valine, suggestive of altered protein metabolism which has been previously established in CKD27. Taurine is an amino acid with a moderate ES of 0.50, that defends against lipid induced oxidative stress. Taurine supplementation decreased lipid peroxidation marker malondialdehyde in rats with diabetes28,29. Treadmill running increased lipid peroxidation in rats but was mitigated by oral taurine supplementation30. The impact taurine may have on physical function draws from non-CKD preclinical and clinical studies, but is not yet noted in the CKD population and warrants further investigation. Additional metabolites that were associated with stronger grip strength were xanthurenate, kynurenic acid, and methylguanidine. Kynurenic acid is considered a uremic toxin, so the association with higher strength was surprising31. However, a study that isolated skeletal muscle mitochondria from healthy mice found that exposure to varying doses of L‐kynurenine, kynurenic acid, and methylguanidine decreased mitochondrial OXPHOS with no effect upon pyruvate dehydrogenase activity. Kynurenine metabolism utilizes a PGC-1α1-dependent mechanism to improve glucose oxidation and may have a beneficial effect upon skeletal muscle despite categorization as a uremic toxin32. Methylguanidine is exogenously provided through meat intake and endogenously by conversion from creatinine and arginine and has been suggested to be a uremic toxin33. Methylguanidine is the end-product of the reaction from guanidinoacetic acid to creatine to creatinine. Increased skeletal muscle utilization may place greater demand upon creatine and subsequent downstream accumulation of methylguanidine. Although this information supports the notion that metabolites along the kynurenine- tryptophan pathway could improve muscle strength, further studies are needed on the impact of uremic toxins on muscle metabolism and function.
Our study has several strengths and limitations. Strengths of this study are that both metabolite and physical function measures were concurrently collected from > 100 patients, distributed across sex and representative of an inner-city dialysis population. With the median number of days since the start of dialysis at 100 ± 46 days, we can exclude some of the variability in metabolites and muscle health with prolonged dialysis vintage. Another strength is use of a robust LC–MS platform for metabolite measures utilized extensively in kidney disease research34,35. The measurement of > 200 metabolites across all key cellular energy pathways provides a comprehensive overview in this discovery study. Limitations include being a cross-sectional analysis, use of plasma rather than muscle tissue, not accounting for inflammatory status, exclusion of drug metabolites, and a lack of healthy controls. Blood was collected pre-dialysis without controlling for diet or fasted/fed state at a single timepoint. We also acknowledge that selecting a set of metabolites out of 200 candidates based on unadjusted p < 0.05 and ES > 0.4 will identify up to 5% false positives in this sample. The scientific implementation and interpretation should be performed with careful consideration given the exploratory nature of the study and the potential influence of unaccounted confounding factors. A future validation study should identify additional confounding factors that may also contribute to differences in metabolic profiles and physical function. An additional limitation is the lack of a validation cohort which will be required prior to implementation of interventional studies to augment physical function.
In this study, we developed a composite score that represents physical function with metabolites unique for each measure of gait speed and grip strength in patients who are incident to dialysis. We identified 22 metabolites for grip strength and 12 metabolites for gait speed with composite score ES of 1.19 and 1.38, respectively. These were very large ES unique to each physical measurement indicating no overlap of specific metabolites, although alterations in fatty acid oxidation and protein synthesis were observed in both measures. Although some of the identified metabolites are known uremic toxins, many are novel and the role of these metabolites in muscle health is yet to be clearly elucidated. Further validation studies are warranted surrounding the selection of metabolites identified. Given the complexity of the systems involved in physical function, the use of panels of metabolites associated with physical function offers a fresh opportunity to offer insight into pathophysiology for those with CKD. A future direction of this work is to utilize these metabolic profiles to predict change over time (response/non-response) from an exercise or other interventional strategies (i.e., nutraceuticals) intended to improve physical functioning. Novel approaches such as this are vital to address both the physical and economic burden of impaired mobility in those with CKD.
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- Source: https://www.nature.com/articles/s41598-024-58522-9