Krebs cycleCitric acid cycle · TCA cycle

The Krebs cycle runs inside the mitochondria, taking acetyl-CoA derived from carbohydrate, fat or protein and stripping it through a series of reactions that yield carbon dioxide and the electron carriers NADH and FADH2. Those carriers then feed the electron transport chain, where the great majority of ATP is actually made.

It matters because it is the common junction of all fuels. Carbohydrate and fat metabolism converge here, which is the biochemical reason the body can switch between them — and why metabolic flexibility is a coherent concept rather than a slogan.

In practice its capacity scales with mitochondrial density, so the way to improve it is the way to build mitochondria: consistent aerobic training, with a large share of it at easy intensity.