Livestock production has an enormous impact on climate change emissions, resource use, habitat loss, and the availability of staples for consumers in developing countries. Despite this, macromarketers have paid little attention to environmentally sustainable diets. Although researchers in health studies have identified the need to mainstream plant-based diets, they downplay the socio-cultural meanings associated with meat and vegetable consumption. We propose the challenge of change in eating habits reflects a classic agency-structure tension and draw on Kurt Lewin’s force-field theory to examine five forces for/against the mainstreaming of sustainable diets (human health, environmental sustainability, morality, identity, and institutional factors). Policy solutions are identified with particular attention paid to expanding the size of the health vegetarian segment.