We estimate the impact on greenhouse gas emissions (GHGE) of shifting from the current average United States diet to four alternative diets that meet the 2010 Dietary Guidelines for Americans (DGA). In contrast to prior studies, which rely on process-based life-cycle-analysis GHGE estimates from the literature for particular food items, we combine a diet model, an environmentally extended input-output model of energy use in the U.S. food system, and a biophysical model of land use for crops and livestock to estimate food system GHGE from the combustion of fossil fuels and from biogenic sources, including enteric fermentation, manure management, and soil management. We find that an omnivore diet that meets the DGA while constraining cost leaves food system GHGE essentially unchanged relative to the current baseline diet (985 000 000 tons of CO2 eq or 3191 kilograms of CO2 eq per capita per year), while a DGA-compliant vegetarian and a DGA-compliant omnivore diet that minimizes energy consumption in the food system reduce GHGE by 32% and 22%, respectively. These emission reductions were achieved mainly through quantity and composition changes in the meat, poultry, fish; dairy; and caloric sweeteners categories. Shifting from current to healthy diets as defined by the DGA does not necessarily reduce GHGE in the U.S. food system, although there are diets, including two presented here and by inference many others, which can achieve a reduction in GHGE.