Balancing the needs for enhanced food security with low-carbon energy is among the most pressing challenges facing the world. Increased demand for food and energy place additional pressures on already stressed agricultural and natural resources. The food vs. energy dilemma is ultimately determined at the intersection of land productivity and land scarcity. This research examines how consumption and production of food for a growing human population can result in strikingly different land use patterns. Using a geographically distributed model across the US, we find that the type of food consumed, especially beef, is a major driver of agricultural land requirements, and that the type of meat consumed is as least as important as the total amount of meat consumption. If people continue to eat less beef and adapt other healthier food choices to reduce obesity or risk of chronic diseases, then significant amounts of land would no longer be required for food production. If land is spared following a shift in diet, then it could either revert to unmanaged habitat such as grasslands or forests or be used for biofuels. These different unamanaged and bioenergy landscapes would also have different C sequestration rates. Using geographically distributed data based on three global climate models and two biogeochemical models, we predict potential ecosystem C storage on spared lands, and compare that to the C benefits of bioenergy systems. We combine these results with changes in greenhouse gas emissions in the agricultural sector to determine the overall C implications of changing diets.