Eating less livestock products can reduce agricultural resource use, but it has also been argued that a portion of livestock products does sustainably contribute to food security. Given that diet change impacts arise through modification of food production systems, evaluating such changes requires explicit modelling of system dynamics. This paper uses a novel global integrated optimisation model, “Aalto OptoFood”, to investigate changes in water use when replacing livestock protein with plant-based protein sources in human diets and reallocating associated cropland and pasture. Our findings reveal that the first cuts are the deepest – moderate reductions in livestock protein decrease water consumption more than suggested by comparing global average crop and livestock water productivities. Further reductions show diminishing returns, with no clear optimum. In terms of water use, it is beneficial to eat less livestock products, even if there is no clear case for or against universal adoption of a vegan diet.