Diet is a key lever for preserving human health and the environment. Without rapid and drastic changes in food systems, from production to consumption patterns, the environmental limits for the sustainability of ecosystems will be reached by 2050 and the incidence of chronic diseases will continue to rise. While the awareness of stakeholders seems to be strong and sustainable diets are well characterized, "food solutions" remain global (not individualized) and prospective work is based on scenarios. Indeed, no study has quantitatively assessed recent dietary changes and whether they are consistent with a transition to a more sustainable, healthier, more accessible and more environmentally friendly diet. There is an urgent need to assess, using a multi-criteria and transdisciplinary approach, the dietary transitions underway, the associated individual characteristics and to measure their impacts on health, the environment and in socio-economic terms. A better understanding of these aspects will enable us to propose actions with regard to the specific levers and barriers of population groups. The TRANSFood project aims to respond to these different issues by using a systemic approach "from field to plate" (agriculture, accessibility, consumption, etc.) based on two epidemiological studies (one cross-sectional, the other longitudinal), an experimental study and a consumer panel. More specifically, the specific objectives of the TRANSFood project include: 1) a study of dietary change trajectories over an 8-year period (covering the COVID-19 crisis and its aftermath) at the individual level (based on repeated data from the same sample) integrating food system specificities (agro-ecological and conventional farming practices). Recent changes will be assessed over an 8-year period in the NutriNet-Santé cohort in order to generate a typology of change on which subgroups of the French population (INCA3) will be located, 2) a detailed assessment of the sustainable values (economic, nutritional, health and environmental) of diets and their changes as well as the characterization of their determinants (socio-demographic, socio-economic, geographical). The environmental and health implications of these transitions will be assessed using life cycle analysis databases, a biodiversity indicator and a risk model in order to quantify the potential improvements that could be expected in the near future, 3) identification of barriers and levers and measurement of the actual individual willingness to change. A precise analysis of the levers and barriers to change will be carried out via the administration of questionnaires on the propensity to trade and via an experiment on willingness to pay. The socio-demographic (gender, age/generation, family situation), socio-economic (cost of diet, education, income etc.) and geographical (accessibility, supply) determinants of these trajectories will be highlighted, 4) the establishment of recommendations adapted to the population groups and trajectories identified by the implementation of multi-criteria models based on optimization under multiple constraints (environmental, nutritional, health, economic) which will make it possible to propose sustainable diets, according to the various characteristics, barriers and levers previously identified. The results of this project will be widely disseminated to the academic community but also to stakeholders and in particular to consumers.