The Tricarboxylic Acid Cycle as a Central Regulator of the Rate of Aging: Implications for Metabolic Interventions

Certain metabolic interventions such as caloric restriction, fasting, exercise, and a ketogenic diet extend lifespan and/or health span. However, their benefits are limited and their connections to the underlying mechanisms of aging are not fully clear. Here, these connections are explored in terms of the tricarboxylic acid (TCA) cycle (Krebs cycle, citric acid cycle) to suggest reasons for the loss of effectiveness and ways of overcoming it. Specifically, the metabolic interventions deplete acetate and likely reduce the conversion of oxaloacetate to aspartate, thereby inhibiting the mammalian target of rapamycin (mTOR) and upregulating autophagy. Synthesis of glutathione may provide a high-capacity sink for amine groups, facilitating autophagy, and prevent buildup of alpha-ketoglutarate, supporting stem cell maintenance. Metabolic interventions also prevent the accumulation of succinate, thereby slowing DNA hypermethylation, facilitating the repair of DNA double-strand breaks, reducing inflammatory and hypoxic signaling, and lowering reliance on glycolysis. In part through these mechanisms, metabolic interventions may decelerate aging, extending lifespan. Conversely, with overnutrition or oxidative stress, these processes function in reverse, accelerating aging and impairing longevity. Progressive damage to aconitase, inhibition of succinate dehydrogenase, and downregulation of hypoxia-inducible factor-1α, and phosphoenolpyruvate carboxykinase (PEPCK) emerge as potentially modifiable reasons for the loss of effectiveness of metabolic interventions.