A weight-loss plan claiming that people should eat according to their ABO blood group, with type O as meat-eating hunters, type A as vegetarian agrarians, and so on. Proposed in a bestselling book, it wraps an evolutionary story around lists of supposedly beneficial and harmful foods for each type. Rigorous testing has found no evidence that blood type modifies anyone’s response to diet: in trials, adherents’ improvements track the diet’s general healthfulness, not their blood group, and cardiovascular risk markers improve equally across types on the same diet. Nutrition scientists classify it as a fad diet with a pseudoscientific mechanism. Its popularity demonstrates how a veneer of biological personalization can sell dietary rules that work, when they do, for entirely conventional reasons.