Consumers generally underestimate the difference in GHG emissions between animal-based and plant-based items. Existing carbon labels primarily focus on enhancing metric knowledge (i.e., the general GHG emission levels of a food product) yet neglect to improve mapping knowledge (i.e., the relative differences between different food items). This research proposes the "item mapping" carbon labeling strategy, designed to link GHG emissions to food items (i.e., animal- vs. plant-based). Five pre-registered experiments document that the item mapping label (i) reduces the knowledge gap in GHG emissions between animal- versus plant-based food items and (ii) reduces animal-based food choices. The item mapping label is particularly effective at decreasing animal-based food choices for consumers with higher sustainable eating involvement, which is associated with higher knowledge overconfidence (i.e., consumers believe that they know more than they actually do). These findings offer insights into designing carbon labels that facilitate sustainable food choices.