Crabtree effect

The Crabtree effect is the inhibition of cellular respiration by an excess of glucose, a phenomenon originally observed in yeast but now recognized as a central feature of cancer and rapidly proliferating normal cells. 2 Ray Peat described it as the suppression of respiration by glycolysis, often framed as the simple opposite of the Pasteur effect, in which oxygen limits glycolysis to the rate that allows its product to be consumed oxidatively. 2 However, Peat argued that unlike the logical, normal control system of the Pasteur effect, the Crabtree effect tends to lower cellular energy and adaptability, representing a maladaptive state. 4 He noted that embryonic tissues sometimes behave in this manner, leading to the suggestion that glycolysis is closely related to growth. 1

While the Crabtree effect is usually thought of as something that happens in tumors and some highly glycolytic tissues when given large amounts of glucose, Peat expanded its relevance by considering the role of lactate. 2 He argued that lactate, produced by normal tissues when deprived of oxygen or disturbed by a stress reaction, makes the Crabtree effect a very general phenomenon. 1 The "respiratory defect" seen on the organismic level during hyperventilation is very similar to a "systemic Crabtree effect" that happens during stress, in which respiration is shut down while glycolysis is activated. 2 Since oxidative metabolism is many times more efficient for producing energy than glycolysis, Peat considered it maladaptive to shut it down during stress. 2 He went so far as to propose that lactate is not merely a sign of the respiratory defect in cancer, as Otto Warburg thought, but a cause of it, just as it is the immediate cause of the respiratory derangement of hyperventilation. 4

Mechanistically, lactate formation from glucose increases when anything interferes with respiratory energy production, but lactate itself can suppress cellular respiration through a variety of mechanisms, creating a vicious cycle. 5 Peat identified nitric oxide and calcium excess as the main endogenous antirespiratory factors in stress, though free unsaturated fatty acids are also clearly involved. 1 The presence of lactate also mobilizes free fatty acids, which then suppress the oxidation of glucose, a phenomenon known as the Randle effect. 2 Peat noted that estrogen promotes this effect, and Danny Roddy has specified that estrogen shifts cellular metabolism over to glycolysis, such that even if oxygen is present, the cell cannot use it, an effect he identified as the Crabtree effect. 2 When the mitochondrion's ability to consume pyruvate and NADH is limited, pyruvate itself accepts hydrogen from NADH, forming lactic acid. 5 If too much lactate accumulates, it slows glycolytic ATP production, and a cell with defective respiration will die unless an alternative electron sink, such as the synthesis of fatty acids via fatty acid synthase (FAS), is available. 5

Peat contrasted the toxic effects of lactic acid with the protective effects of carbon dioxide, noting they have opposing effects. 2 He traced a wide range of pathologies to an excess of lactic acid and the related features of hyperventilated physiology, including the panic reaction, shock lung, vascular leakiness, brain swelling, and ultimately multiple organ failure. 4 He was critical of the medical use of lactated Ringer's solution, arguing it is toxic and that its use as a buffer is a matter of convenience rather than physiology, as it damages mitochondria. 2 The Crabtree effect, in Peat's framework, is thus not an isolated quirk of tumor metabolism but a fundamental, systemic derangement of energy production driven by glycolytic stress, with lactate acting as both a product and a central toxin that perpetuates a respiratory defect. 2