The cartoon character Popeye the Sailor was capable of superhuman feats of strength after eating a can of spinach. Popeye ate spinach because the association of spinach with strength was a product of the first national nutrition crisis in the United States: the 1920s fight against child malnutrition. Spanning the first three decades of the twentieth century, the malnutrition crisis arose from the confluence of many different events including the invention of nutrition science and new standards for height and weight; international food crises created by world war; the rise of consumerism, advertising, and new forms of mass media; and Progressive reformers' conviction that education was a key component of any solution. The history of the malnutrition crisis presented in this essay synthesizes disparate histories concerning advertising, public health, education,consumerism, philanthropy, and Progressive Era reform with original analysis of a major nutrition education program sponsored by the Commonwealth Fund in the 1920s. Because the character of Popeye came to embody one of the nutritional norms advocated in the 1920s, I refer to the influence of culturally constructed social norms on children's beliefs about health and nutrition as the Popeye Principle. The history of the malnutrition crisis demonstrates the importance of understanding the cultural and economic conditions surrounding childhood nutrition, the use and influence of numerical norms, and the mutually reinforcing influences on children's nutritional norms from their parents, peers, teachers, and culture.