In 2015, controversy over the Dietary Guidelines for Americans reached a new level when the government-appointed Dietary Guidance Advisory Committee (DGAC) recommended that those guidelines promote more sustainable diets, particularly those lower in animal-based foods. Although the committee found ample scientific evidence that such a shift would be a “win-win” for Americans' health as well as the environment, it met with fierce opposition on both counts, and not only from the livestock industry. This suggests how sustainable diet guidance poses a classic wicked problem, meaning one characterized by high levels of complexity, uncertainty, and epistemological conflict. While relationships between food, bodies, and environments are inevitably complex and uncertain, the controversy surrounding DGAC's recommendation offers an opportunity to explore how the scientific evidence on dietary sustainability is actually produced, and how it does or does not speak to other knowledge about eating for bodily and ecological health. To do this I look first at the research behind DGAC's endorsement of diets high in plant-based versus animal-based foods, and then at select responses in the public comments. The contrast not only highlights the incommensurability of modeled versus experiential evidence, but also suggests that efforts to promote more sustainable food consumption cannot credibly ignore questions (however unresolved) about what constitutes more sustainable production.