Shifts in dietary patterns are important for both health and the environment, and ruminant meat has been highlighted as a critical cornerstone in this respect. Here, nonlinear and multi-objective diet optimization based on the consumption of French adults (INCA3, n=1 125, 18-64 years old) have been used to model transitions towards nutritionally adequate and healthy dietary patterns under scenarios of either the direct removal or gradual reduction (in 10% steps) of ruminant meat, without or with preserving at best dietary habits, respectively. This has led to identify the nutritional issues when designing diets low in ruminant meat, and the dietary levers that make them nutritionally adequate and healthy. Overall, our main finding is that ruminant meat could readily be replaced by poultry. Such a gradual transition from ruminant meat to poultry should be accompanied by an in-depth restructuring to achieve a healthy diet, involving the early removal of processed meat and non-ruminant red meat and a gradual increase in healthy plant-based foods such as fruits and vegetables (reaching 800 g/d at the final step) and whole grain products (reaching 330 g/d). In terms of its expected environmental impacts, this transition would result in a gradual improvement in greenhouse gas emissions and land use (up to -29% and -36%, respectively), without changing cumulative energy demand but requiring an increase in water use (up to +47%). Finally, reducing ruminant meat forms part of a transitional feature that consists in replacing all red meat with poultry, which could result in healthy and nutritionally adequate dietary patterns aligned with reductions in greenhouse gas emissions.