Diet-related illness in the U.S. has risen to crisis levels, with today’s parents projected—for the first time—to enjoy longer lives than their children. Policymakers have responded with a range of novel proposals, many revolving around the assumption that increasing the relative price of unhealthful foods via taxes on soda, fat, sugar or caloric density will encourage more healthful eating. Yet there is little evidence that diet-targeted price interventions have had a meaningful impact on health outcomes. The reason for this, I argue, is both expected and largely overlooked in the academic and policy literatures: consumers have been motivated by price interventions to eat differently, but differently isn’t necessarily better. Price interventions created and crafted solely on the basis of the nutritional value of the targeted dietary item(s) are vulnerable to health-neutral substitution effects, wherein a price intervention motivates consumers to substitute items of similar dietary value—candy or beer for soda, for example—leading to little net change in overall dietary quality. This Article proposes a novel food tax strategy, one that focuses on substitution effects. Instead of targeting foods with the least or most nutritional value, legislators and other policymakers should direct taxes to foods with the least or most relatively healthful substitutes. As proof of concept, this Article introduces a specific substitution-based food tax proposal, supported with empirical results from an original randomized controlled trial: food taxes should seek to motivate substitution between grocery food and the relatively less healthful option of restaurant food. The health impact of such a shift will likely exceed that of a direct tax on soda, fat, sugar or calories given the high cross-elasticity of demand for restaurant food and its significant health inferiority. And because this shift may be accomplished by subsidizing groceries, by lowering taxes on groceries or by increasing taxes on restaurants, it presents a more politically flexible intervention than traditional consumer-side food taxes.