Mixed farming systems are of interest in the search for sustainability because of their species diversity and the potential for synergy from integrating crops and livestock, through nutrient cycling. However, their ability to maximize food production while maintaining their agroecological benefits has been little addressed in the literature and deserves to be explored further. The issue of nutrient cycling raises questions about resource allocation between food crops, feed crops and animal products. This study, based on a whole farm system experiment conducted for about fifteen years in north-eastern France, assesses the biotechnical processes and food production performance of two integrated mixed system configurations. These configurations differed both in their types of production (diversity in both livestock and crops), and in their overall strategies (dairy cattle striving for self-sufficiency vs diversified system maximizing food-crop output). Taking a Nitrogen metabolic approach, the study evaluates biotechnical processes (by ecological network analysis and nutrient balances) and food production efficiency. Our results show that: (1) the two systems metabolism is exclusively based on crop-livestock integration and renewable inputs (biological nitrogen fixation, atmospheric depositions); and (2) the configuration geared to maximizing food production is not the more productive but is the more efficient. In both cases, food efficiency at farm system scale is higher than the food efficiency of each production. This confirms the importance of combining systemic and analytical approaches to better understand and act on the development of performance at different levels. We also show the importance, for a self-sufficient system, of having stocks in reserve to cope with unfavourable years. Finally, our study confirms the value of integrated mixed farming systems in terms of agroecology, but highlights the need for a closer consideration of the role of livestock in those systems (ruminant vs monogastric) to produce food and the trade-offs between food production and nutrient cycling.