The role of livestock in sustainable food systems is heavily debated. Recent studies, however, 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 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 animal protein. To this end, we developed an optimisation 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 utilisation of the various (low quality) leftovers. Leftover streams included food waste and food processing co-products related to current EU food consumption, and currently available grazing resources in the EU. Optimal conversion of leftovers results in a protein supply of 39 g/cap/day – 60% of daily protein requirements – from ASF (mainly milk). This protein supply is higher than found in previous studies (7-27 g/cap/day) indicating the importance of optimising the role of livestock production. Our results showed that dairy production is most efficient in converting a combination of leftovers and grass resources into animal protein. This however, requires a shift in consumption patterns towards more dairy and less meat. Maintaining current ASF ratios reduces the amount of animal protein available to 26 g/cap/day. This study, therefore, illustrates that using leftover streams optimally improves the role of livestock in nutrition security.