Krebs cycle (TCA cycle)
At the heart of every cell, the same chemical sequence repeats without interruption: eight reactions looping with no beginning and no end, turning what you eat into usable fuel. This is the Krebs cycle. It is running right now, billions of times per second inside your body.
Described by Hans Krebs in 1937 (Nobel Prize 1953) and also called the citric acid cycle or TCA cycle, it is the metabolic crossroads where carbohydrates, fats and amino acids converge before their energy is captured as ATP.
Eight steps, two energy outputs
It starts when a molecule of acetyl-CoA binds with oxaloacetate to form citrate. The cycle then runs through eight reactions that release two molecules of CO₂ and regenerate oxaloacetate, ready to go again. Each turn: 3 NADH, 1 FADH₂, 1 GTP. These coenzymes carry electrons to the respiratory chain, where most ATP is actually made. The Krebs cycle is therefore not an ATP factory: it is an electron pump feeding the cell's real energy plant.
When the cycle catches
A cycle that slows down or seizes shows up as fatigue settling in with no clear explanation, recovery that drags, sometimes diffuse muscle pain. The critical cofactors are B vitamins (B1, B2, B3, B5), magnesium, iron and manganese. Some magnesium forms, such as malate, deliver a direct cycle intermediate, which explains their use in fibromyalgia and chronic fatigue states.
Related terms
Scientific sources
- Martínez-Reyes & Chandel, Mitochondrial TCA cycle metabolites control physiology and disease, Nat Commun 2020
- Arnold & Finley, Regulation and function of the mammalian tricarboxylic acid cycle, J Biol Chem 2023
- Inigo et al., Ins and Outs of the TCA Cycle: The Central Role of Anaplerosis, Annu Rev Nutr 2021