The effect of nutritional quality on comparing environmental impacts of human diets

Several studies support the general conclusion that plant-based diets have a lower environmental impact than animal-based diets. These studies, however, do not account for the nutritional quality of diets. The main objective of our study, therefore, was to explore if accounting for nutritional quality affects the comparison of the environmental impacts of human diets varying in their percentage of animal-source food products (ASFP). We also explored whether meals or daily diets are equally suitable to compare environmental impacts of diets. Fifty peer-reviewed studies were found that examined the environmental impact of diets, generally using life cycle assessment (LCA). Only 12 of these studies were reviewed, based on five criteria: study contains more than one scenario; diet scenarios vary in their percentage of ASFP; the weight of each food product was provided; the study assessed global warming potential and/or land use; diet scenarios are not designed for specific (health) groups. For each diet described in the reviewed studies, we quantified the daily intake of nine qualifying and three disqualifying nutrients. Global warming potential and land use, as provided by the reviewed studies, were expressed in four ways: per day, per daily protein intake capped to the recommended intake level of 57 g; per daily protein intake uncapped; and per NRD9.3 (i.e. a composite nutrient score of a diet). We concluded that the nutrient intake resulting from a meal cannot be used to assess the nutritional quality of a daily diet and, hence, the environmental impact of meals cannot be compared to that of daily diets. Studies on meals were therefore excluded from further analysis. Our results further show that daily diets that had higher percentages of ASFP were associated with higher (excess) intakes of total protein and lower values of NRD9.3. Diets that had higher percentages of ASFP were associated with higher GWPs and LU's per gram protein capped and per unit NRD9.3. Without capping protein to the recommended intake level, GWP and LU per gram of protein were generally lower for diets that had higher percentages of ASFP. Without capping, diets with higher percentages of ASFP are credited for overconsumption of protein. Since overconsumption of protein does not benefit health, we recommend capping to the recommended intake level. The effect of using NRD9.3 rather than day as functional unit was small for GWP. For LU we found no effect. When using NRD9.3 as functional