SIRT1–Redox Crosstalk in Aging: Adaptive Capacity and Cellular State Transitions

Aging gradually reduces the ability of cells to maintain and restore homeostasis under metabolic and oxidative stress. Sirtuin 1 (SIRT1) is an NAD+-dependent deacetylase that regulates metabolism, mitochondrial function, inflammatory signaling, and cellular stress responses. Its activity depends on NAD+ availability, redox state, post-translational regulation, subcellular localization, and metabolic context, arguing against a simple linear relationship between oxidative stress, SIRT1 loss, and cellular dysfunction. SIRT1 functional reserve is defined as the remaining capacity of SIRT1-dependent processes to respond to a defined cellular demand and is placed within a broader framework of adaptive capacity and adaptive threshold. An experimental protocol with measurable readouts is described to test this framework. SIRT1–redox–mitochondrial adaptive capacity is examined under transient, repeated, and persistent stress, with attention to incomplete recovery and transitions from adaptive to compensatory and persistent dysfunctional states. Neuronal vulnerability is also considered in relation to mitochondrial dependence and proteostasis. Rather than treating SIRT1 activation as universally beneficial, strategies that restore the conditions required for appropriate SIRT1 activity are discussed. The proposed framework remains a hypothesis and requires experimental and clinical validation.