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There is widespread interest in dietary strategies that lower environmental impacts. However, various forms of malnutrition are also widely prevalent. In a first study of its kind, we quantify the water-scarcity footprint and diet quality score of a large (>9000) population of self-selected adult daily diets. Here, we show that excessive consumption of discretionary foods—i.e., energy-dense and nutrient-poor foods high in saturated fat, added sugars and salt, and alcohol—contributes up to 36% of the water-scarcity impacts and is the primary factor differentiating healthier diets with lower water-scarcity footprint from poorer quality diets with higher water-scarcity footprint. For core food groups (fruits, vegetables, etc.), large differences in water-scarcity footprint existed between individual foods, making difficult the amendment of dietary guidelines for water-scarcity impact reduction. Very large reductions in dietary water-scarcity footprint are possible, but likely best achieved though technological change, product reformulation and procurement strategies in the agricultural and food industries.

Food systems vitally depend on croplands, which are a scarce natural resource. Croplands are also heterogeneous, differing in productive capability and in environmental context. Some are in regions of high biodiversity conservation importance, others in regions vulnerable to food insecurity. In this study, life cycle assessment was used to quantify cropland scarcity footprints, cropland biodiversity footprints and cropland malnutrition footprints for 9341 individual Australian adult daily diets. Dietary cropland scarcity footprints averaged 7.1 m2yr-e person−1 day−1, exceeding a target of 6.1 m2yr-e person−1 day−1, consistent with the proposed global cropland planetary boundary of 15% of the ice-free land area. Discretionary foods, which are energy-dense and nutrient-poor foods high in saturated fat, added sugars and salt, and alcohol and are not essential to a healthy diet, made the largest contribution, followed by fresh meats and alternatives, breads and cereals, fruit, dairy and alternatives and vegetables. Around 45% of the variation in cropland footprint between individuals was explained by differences in total dietary energy intake. Diets characterised by higher diet quality and lower cropland scarcity footprint required only 4.2 m2yr-e person−1 day−1 and recommended diets based on the food choices of this subgroup required 5.9 m2yr-e person−1 day−1. Eating within the global cropland planetary boundary appears realistic if Australians greatly reduce their intake of discretionary foods and moderate their food choices within the “meat and alternatives” food group.

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