While the role of livestock in sustainable food systems is heavily debated, recent studies show that livestock can contribute to global nutrition security by converting leftover streams – products humans cannot or do not want to eat – into animal-source food (ASF). Where these studies clearly underpin livestock’s role in global food security, the current study aims to identify which combination of livestock systems, differing in production level, can optimally convert leftover streams into protein. To this end, we developed an optimization model containing a variety of livestock systems (pigs, dairy cattle, beef cattle, laying hens and broilers), differing in production level (low, mid and high), to enable better utilization of the various (low quality) leftovers. Included leftover streams consist of waste and processing co- products related to current food consumption in the EU and currently available grazing resources in the EU. The optimal use of these leftover streams requires mainly low productive dairy cattle, and provides 30 g animal protein/cap/day. Although this protein supply fulfils half of our daily protein requirement, it requires a shift in consumption patterns and farming practices. This study illustrates that using leftover streams optimally, improves the role of livestock in nutrition security.