Two methods for jointly evaluating environmental impact and nutritional content of diets tested in new study on Swedish diets

In the first method, the climate impact and the land occupation caused by the production of the different diets are shown separate from the nutritional intake (se figure below). However, climate impact and land occupation are normalised to sustainable levels so that 1 on the vertical axis in the diagram corresponds to sustainable levels of these impacts.  The sustainable level of greenhouse gas emissions were defined as 50% of the yearly total per capita emission space of approximately 1–2 tonne CO₂e based on remaining carbon budgets as suggested by the IPCC, while the sustainable level of land occupation was defined as Swedish agricultural land availability in 2013 divided upon the Swedish population. Regarding nutrients, the 1 on the vertical axis corresponds to the recommended intake, or upper levels for average daily intake in case of nutrients that should be limited, e.g. salt. The authors argue that this simultaneous presentation of normalised nutrient intake and environmental impacts is an illustrative and transparent way of showing how different diets provide nutrients at different ‘environmental costs’.

In the second method, the authors used nutrient density scores to first compute the nutritional quality of the diets and then the environmental impact of each diet was expressed per nutrient density score (see figure below). Six different versions of nutrient density scores were used. Using nutrient density score provides a joint assessment of different nutrients, but this method is less transparent and results are highly dependent on how the nutritional density score is designed. The authors highlight the need for quantitative methods for assessing the environmental and nutritional impacts of diets to be transparent and easy to interpret in order to give valuable decision support. They argue that this method illustrates in a clear and straightforward way how changes in the diet designed to make it more sustainable (e.g.  a reduction in meat), would affect both environmental parameters and the nutritional content of the diet.

Abstract

This study examined two methods for jointly considering the environmental impact and nutritional quality of diets, which is necessary when designing policy instruments promoting sustainable food systems. Both methods included energy content and 18 macro- and micronutrients in the diet, the climate impact, land use and biodiversity damage potential. In Method 1, the content of different nutrients in the diet was normalised based on recommended intake or upper levels for average daily intake and presented together with the environmental impacts, which were normalised according to estimated sustainable levels. In Method 2, the nutritional quality of different diets was considered by calculating their nutrient density score, and the environmental impact was then expressed per nutrient density score. Three diets were assessed; a diet corresponding to Nordic recommendations, the current average Swedish diet and a lifestyle Low Carbohydrate-High Fat (LCHF) diet. Method 1 clearly showed that the climate impact was far beyond the sustainable level for all diets, while land use was within the sustainability limit for the recommended diet, but not the other two. Comparisons based on nutrient density scores depended on the score used, but the current and LCHF diets had more impact than the recommended diet (less livestock products) for all but one score. Over- and under-consumption of nutrients were clearly shown by Method 1 but not possible to distinguish with Method 2, as normalisation was not possible, making it difficult to evaluate the absolute scale of the impacts when nutrient density scores were used. For quantitative information on the environmental and nutritional impacts of diets as support in decision-making processes, it is important that data presentation is transparent. There is limited value in reducing results to a low number of indicators that are easy to read, but difficult to interpret, e.g. nutrient density score. Method 1 allows combined assessment of diets regarding environmental impact and nutritional intake and could be useful in dietary planning and in development of dietary recommendations and other policy instruments to achieve more sustainable food systems.

Citation

Röös, E., Karlsson, K., Witthöft, C., Sundberg, C., 2015, Evaluating the sustainability of diets–combining environmental and nutritional aspects. Environmental Science & Policy, 47:157-166. DOI: 10.1016/j.envsci.2014.12.001

Read the full paper here. You can also find other resources relating to sustainability assessments of diets on our website here, here and here. And there are also some resources more generally related to sustainability of different diets here and here.