Systematic reviews answer specific questions based on primary literature. However, systematic reviews on the same topic frequently disagree, yet there are no approaches for understanding why at a glance. Our goal is to provide a visual summary that could be useful to researchers, policy makers, and health care professionals in understanding why health controversies persist in the expert literature over time. We present a case study of a single controversy in public health, around the question: "Is reducing dietary salt beneficial at a population level?" We define and visualize three new constructs: the overall evidence base, which consists of the evidence summarized by systematic reviews (the inclusion network) and the unused evidence (isolated nodes). Our network visualization shows at a glance what evidence has been synthesized by each systematic review. Visualizing the temporal evolution of the network captures two key moments when new scientific opinions emerged, both associated with a turn to new sets of evidence that had little to no overlap with previously reviewed evidence. Limited overlap between the evidence reviewed was also found for systematic reviews published in the same year. Future work will focus on understanding the reasons for limited overlap and automating this methodology for medical literature databases.