What is a “Sustainable” Diet?

New research on which foods have the highest and lowest impact on the environment.

Originally published in Food, Nutrition & Science.

Foods with a heavy environmental impact tend to have lower nutritional quality and cost more, according to a recent study from Aix-Marseille Université in France and published in the June issue of the Journal of the Academy of Nutrition and Dietetics. The study set out of identify foods with compatible sustainability dimensions as defined by the Food and Agriculture Organization – in other words, foods that are at once kind to the environment, high in nutrition and affordable. 

To do this, researchers looked at 363 of the most commonly consumed foods in the Second French Individual and National Study on Food Consumption, their environmental impact indicators (like greenhouse gas emissions), and prices. Nutritional quality was also examined. Researchers gave the highest scores to foods with the best values for all three variables (environmental impact, nutritional quality and price).

Not surprisingly, meat, fish, and eggs and dairy products had the strongest influence on the environment, and starchy foods, legumes, and fruits and vegetables had the least impact. Fruits and vegetables had the highest nutritional quality, and foods high in fat, salt and sugar had the lowest. The nutritional value of fish and fish products was higher than meat and poultry. All meat, fish and eggs were expensive; starchy foods were cheap, by comparison.

While these findings are in line with those of previous studies – many have found that, for instance, animal production emits more greenhouse gases than the production of plant-based foods – they have not factored in issues of cultural acceptance, nutritional impact, and cost issues. In countries where there is a high risk of nutritional deficiency, for example, meat may need to be an important part of the diet. Likewise, the higher cost of many fruits and vegetables, even though they have a higher nutritional quality, could make the cost of a plant-based diet prohibitive in certain socio-economic populations. Therefore, foods that are truly sustainable per the definition used in the study would need to meet all three criteria. 

“The sustainable diet concept is not limited to the ‘environmental friendly’ message; it also includes affordability and nutritional adequacy. The cultural acceptability is another important dimension that was indirectly taken into account in this study by the fact that only food highly consumed within the French population were considered,” say study co-authors Dr. Nicole Darmon and Dr. Gabriel Masset.

Researchers found that 94 (26%) of the examined foods meet all three criteria, including most plant-based foods, 100% fruit juice, vegetable oils and starchy foods. Plant-based foods that did not meet the maximum score had higher emissions (dried fruits), lower nutritional quality (bread with high salt content) or cost more per kilogram (figs, mango, asparagus). Animal products like milks, soups containing meat or fish, and yogurts with no added sugar obtained the highest score too.

Interestingly, when expressed per kilogram, the three dimensions of sustainability (environment, nutrition and cost) were compatible with each other. This was not the case, however, when expressed per 100 kcal. When recalculated based on price per 100 kcal, only 42 foods (12%) meet the criteria. Most fruits and vegetables were too expensive to make the list. Milks, yogurts, juice, vegetable oils and starchy foods all kept the maximum score. Most animal products did not meet the criteria on either list. 

Therefore, choosing the best option to identify sustainable foods must depend on the intended application, say Darmon and Masset. Restricting foods based on kcal may not be realistic in certain cultural environments, whereas this messaging may work in others, in particular for nutrition professionals that are used to working on an energy basis. 

“The option needs also to depend on the local context. For example, portion sizes could be used in the U.S. since nutrition facts are usually given for such ‘unit’. The important message is the results need to be analyzed from different angles, to make sure no misinterpretation or wrong conclusions are drawn from the analyses,” say Darmon and Masset.

Healthier foods tended to have lower values of greenhouse gas emissions and lower prices/100 g, which was good news. Yet, when considering price per kcal, the virtuous cycle was absent. This was linked to the fact that most “healthy foods”, in particular fruit and vegetables, contain little energy and are therefore expensive sources of calories. On the other hand, sweet foods are very cheap sources of calories and have relatively low greenhouse gas emissions, though their poor nutritional quality prevented these foods from being identified as “sustainable”. Overall, starchy foods appeared to be the “most sustainable” foods according to the three dimensions analyzed, and whatever the unit used for price.

“Again, because of those possible divergences between nutrition and the environment, it is crucial to promote all sustainability dimensions at the same time,” say Darmon and Masset.

Darmon and Masset additionally suggest that their results be integrated at the diet level to help identify culturally acceptable food combinations that are at once nutritious, environmentally sound and affordable. Why? Reasoning solely at the food level ignores that foods with a low environmental value per 100g but consumed in important amounts (for instance fruit and vegetables) can be in practice as impacting as foods with a high environmental value per 100g but are consumed in small quantities (like cheese). In addition, nutritional recommendations make sense at the diet level, and reaching these requires the mixing of different types of food.

“We need to have more studies on the sustainability of self-selected diets, rather than on theoretical – and often unrealistic – diets. One option is to identify the more sustainable individual dietary patterns within a target population, since such patterns may be more acceptable by individuals from the target population. This kind of analysis, also called ‘positive deviance’, was recently applied in the French context. We found that observed shifts between the Average and the ‘More Sustainable’ diets were in agreement with most food-based dietary guidelines, which indicated that following the existing public health–oriented recommendations could be compatible with reducing the greenhouse gas emissions of the diets. The results showed that a reduction in diet-related greenhouse gas emissions by 20% while maintaining high nutritional quality seems realistic. This goal could be achieved at no extra cost by reducing energy intake and by increasing the share of plant-based products, without eliminating the consumption of animal products,” add Darmon and Masset.