This article examines the role of evidence in influencing food and nutrition-related public health policy, and starts to chart a way through the political economy of knowledge and evidence within this nexus. We propose an analytical framework for untangling the influence of food industry interests and public health concerns in the policy process, presenting a guiding structure for how an issue might move between contested and uncontested policy spaces, finding that the inherent uncertainty in public health research on complex food systems presents opportunities for contestation by different interest groups. We then use our framework to understand the political economy of the recent sugar-sweetened beverage tax in Mexico, in which public health policies have been adopted despite going against an apparent interest of elements in the food industry. This kind of evidence, given the right framing, has the potential to break some current deadlocks in creating healthier food systems.