Climate change is in the agenda of food policy. Livestock is recognized to be by far the biggest contributor to food-related greenhouse gas emissions (GHGE). This article examines the potential impact of an environmental tax at the consumption level on GHGE mitigation and its nutritional consequences on diet quality and the balance between plant based and animal proteins. We consider a carbon taxation policy proportional to emissions, with a focus on methodological aspects by introducing the use of different functional units for environmental taxation: the usual tax rate per weight of product, and a tax rate according to the nutritional unit of per animal protein. We use scanner data on food purchases of French households from 1998 to 2010. Using price elasticities allows to study substitutions and complementarities between 21 food groups clearly identified according to their environmental and nutritional content. Our results indicate that taxation according to the emissions potential of the animal protein reaches a higher reduction of GHGE than on a per weight basis. Moreover, nutritional adequacy to guidelines is overall maintained. However animal protein replacement by vegetal proteins is not achieved by such taxation policies. This debate can be crucial for the efficiency of food taxation to improve sustainability.