Rethinking frailty as a disorder of mitochondrial adaptability, from energetic congestion to systemic vulnerability.

Frailty is a clinical syndrome of reduced physiological reserve in older adults for which no pharmacological treatment exists and whose cellular basis remains incompletely defined. As life expectancy rises without a comparable extension of healthspan, the absence of a mechanistic account able to guide targeted intervention is a growing clinical problem. The dominant model of primary mitochondrial bioenergetic insufficiency does not accommodate several features of the phenotype. Among the conditions most strongly associated with frailty in aging, obesity, particularly when coupled with sarcopenia, stands out for its rising prevalence and the depth of its systemic metabolic consequences. Drawing on a recent multi-omics characterisation of skeletal muscle in sarcopenic obesity and on the convergent literature in aging metabolism, organelle communication, and redox biology, we propose a complementary framework in which the proximate cellular abnormality of frailty is energetic congestion, a chronic mismatch between substrate input, energetic demand, and the capacity to dispatch the resulting flux through demand-driven oxidative metabolism. In this view the mitochondrion is not failing because fuel is scarce, but because energetic demand declines below the rate at which substrate continues to be delivered, so that substrate persists in relative rather than absolute excess, while mitochondrial adaptability is progressively impaired. The resulting cycle is self-amplifying, anchored in reverse electron transport, and generalises across skeletal muscle, adipose tissue, liver, heart and brain. Strategies that re-engage demand-driven metabolic flux through AMPK activation, substrate restriction, mild mitochondrial uncoupling, modulation of endoplasmic reticulum stress, and clearance of irreversibly congested cells are predicted to produce more durable benefits than energy supplementation, with structured exercise as the prototype of demand-driven recoupling. This perspective offers a path toward a precision pharmacology of frailty grounded in molecular stratification of patients.