Citric acid cycle (Krebs cycle)

The citric acid cycle, also called the tricarboxylic acid cycle or Krebs cycle, is a central stage of energy metabolism inside mitochondria. Acetyl-CoA enters the cycle after being produced from fatty acids, glucose, ketogenic amino acids, and alcohol. The cycle does not produce most cellular energy directly; it generates NADH and FADH2, which feed the respiratory chain for ATP production.

The name comes from citrate, the first stable molecule in the cycle. Acetyl-CoA combines with oxaloacetate to form citrate, then the pathway proceeds through isocitrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and back to oxaloacetate. This loop does not simply “burn food.” It connects carbohydrate, fat, and amino acid metabolism into one integrated system.

The cycle depends on vitamins and minerals. Thiamine is required for pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, riboflavin is part of FAD, niacin is part of NAD, pantothenic acid is part of coenzyme A, and magnesium supports many enzymes. Fatigue and poor exercise tolerance are therefore not always only about calories. Deficiencies, mitochondrial function, anemia, thyroid status, and inflammation can all affect energy production.

The citric acid cycle is closely linked to oxygen even though oxygen is not directly used in its reactions. If the respiratory chain cannot oxidize NADH and FADH2, the cycle slows because oxidized cofactors become limited. In hypoxia, severe anemia, poor circulation, mitochondrial disease, and some toxic exposures, cellular energy can suffer even when food is available.

Before glucose-derived carbon enters the cycle, the pyruvate dehydrogenase complex must convert pyruvate into acetyl-CoA. This requires thiamine, lipoic acid, FAD, NAD, and coenzyme A. When this route is impaired, more pyruvate may be converted into lactate. High lactate or poor exercise tolerance can therefore reflect oxygen delivery, mitochondrial function, vitamin status, circulation, or medications, not only “lactic acid from exercise.”

Insulin resistance also changes substrate flow. Cells may use glucose less effectively, the liver may produce more glucose, and fatty acids may arrive in excess. The citric acid cycle is not broken in one simple spot, but mitochondria work in an overloaded environment. Improving insulin sensitivity, reducing visceral fat, and training muscle often matter more than trying to stimulate one enzyme with a supplement.

In keto and LCHF, the citric acid cycle remains essential. Fatty acids become acetyl-CoA, and the liver can divert part of acetyl-CoA into ketone body production. For acetyl-CoA to enter the cycle efficiently, oxaloacetate and mitochondrial function must be available. Low carbohydrate intake does not switch the cycle off. It changes substrate flow, glucose conservation, and the way different tissues share fuels.

The phrase “fat burns in the flame of carbohydrate” is an oversimplification. Oxaloacetate is important for acetyl-CoA entry into the cycle, but the body can maintain it through amino acids, lactate, glycerol, and other pathways. With a well-structured low-carbohydrate diet, fat oxidation works normally. Problems are more likely with low protein, micronutrient deficits, alcohol, liver disease, excessive training, or a sudden transition without adaptation.

The cycle also supplies intermediates for synthesis. Alpha-ketoglutarate is connected with glutamate and ammonia metabolism, succinyl-CoA is needed for heme synthesis, and citrate can leave the mitochondrion and support fatty acid synthesis in the cytosol. The cycle is therefore both catabolic and anabolic. When intermediates are removed for synthesis, anaplerotic reactions are needed to refill the cycle.

Supplements containing cycle intermediates, such as alpha-ketoglutarate, malate, or succinate, are not universal ways to “boost mitochondria.” They may be studied in sport, fatigue, or specific metabolic contexts, but the effect depends on why energy production is impaired. Poor sleep, iron deficiency, hypothyroidism, low B12, chronic inflammation, and under-eating will not be solved by one intermediate metabolite.

There is no single routine blood marker that tells an ordinary person the “speed of the citric acid cycle.” Clinicians may look indirectly at lactate, ammonia, organic acids, glucose, ketones, liver markers, ferritin, B12, thyroid function, and inflammation depending on the question. These results require context. Diagnosing a “cycle problem” from fatigue alone and self-prescribing a stack of metabolites is a weak strategy.

In practice, supporting the citric acid cycle means building the metabolic foundation rather than chasing a complicated supplement. Adequate protein, nutrient-dense fats, B vitamins, magnesium, iron when indicated, aerobic fitness, sleep, thyroid function, and avoiding excess alcohol all matter. On a low-carbohydrate diet, it is especially important not to reduce food to fat without micronutrients. Mitochondria need fuel, but they also need enzymes, cofactors, oxygen, and recovery.