Trade-offs between water use and greenhouse gas emissions related to food systems: an optimization study in French adults

Water use (WU) and greenhouse gas emissions (GHGe) are two main issues facing food systems. Still, they have rarely been studied together even though they are potentially conflicting because their levers for improvement are not necessarily the same. Data on food-related environmental pressures suggest that GHGe and WU can be improved jointly, but their potential conflicts and trade-offs have not been explored. This is what we studied here by a compromise programming approach, using multi-criteria non-linear optimization under a set of nutritional and epidemiological constraints. We used food consumption data of adults aged 18-64 years (n=1,456) from the French representative study INCA 3 (2014-2015) coupled with food environmental impact data from the Agribalyse (R) database. A full range of scenarios was identified by prioritizing the two objectives differently, giving weight from 0% to 100%, by 5-% steps, to GHGe improvement over WU improvement. Overall, we showed that it is possible to achieve significant joint reductions in WU and GHGe relative to their observed values: across the prioritization scenarios, WU reduction ranged from -36% to -14% as its prioritization decreased, while GHGe reduction varied less, from -44 to -52% as its prioritization increased. These joint important reductions in GHGe and WU required the removal of meat consumption (beef, pork, poultry and processed meat), while the consumptions of offal and dairy products remained moderate in order to meet nutrient reference values. However, the consumption of some foods varied according to the priority given to WU over GHGe reductions (namely, vegetables, fruit juice, dairy products, eggs, refined cereal, substitutes, offal and potatoes). Fish, whole grains, and fruit remained more constant due to the epidemiological constraints used. Whatever the scenario, the modeled diets were more plant-based than the observed diet from which they differed significantly (only 23-31% of common food consumptions), and were therefore healthier (63-76% reduction in distance to theoretical minimum risk of chronic disease). To conclude, while focusing solely on WU reduction induces a joint GHGe reduction that is near-maximal, the reverse is not true, showing that there is good alignment but also some divergence between these objectives. This suggests that food systems WU should be better considered in dietary guidelines for healthy and sustainable diet.