Higher diet quality scores linked to lower emissions but come with trade-offs

Plant-based dietary patterns show the most favorable sustainability outcomes, reducing emissions, energy use, and land impact but increasing water use and costs, highlighting complex trade-offs in diet sustainability

A study published in The American Journal of Clinical Nutrition evaluates the associations between eight commonly used diet quality indexes (DQIs) and seven sustainability indicators in a nationally representative U.S. sample. This study addresses a critical gap in understanding how dietary patterns impact sustainability and inform potential policy actions.

Background

The association of diet and health is key to shaping medical practice guidelines and public health policy. The use of DQIs allows diet quality to be quantitatively expressed, with comparisons between diets. Newer DQIs include sustainability among their criteria, in addition to scoring for nutrient content and proportion, foods, and food groups.

Food production and consumption account for significant environmental impacts, contributing 35% of global greenhouse gas emissions (GHGE), 40% of land use, and 70% of freshwater consumption, with the US diet responsible for 20-25% of these metrics.

The US, with the highest food-related GHGE globally—70% above the global average—adopted the National Strategy on Hunger, Nutrition, and Health in 2022 to promote accessible, sustainable, and healthy diets. The strategy emphasizes research into the link between healthy eating and sustainability, recognizing that dietary patterns vary in environmental impact, necessitating a comprehensive evaluation of dietary quality indices through diverse sustainability indicators.

About the study

The present study used eight different indices. These included the Healthy Eating Index-2020 (HEI-2020), Alternate Healthy Eating Index-2010 (aHEI-2010), Mediterranean Diet Score (Med), Alternate Mediterranean Diet Score (aMed), Healthful Plant-Based Diet Index (hPDI), Planetary Health Diet Index (PHDI) for the United States, Dietary Approaches to Stop Hypertension Index (DASH), and Nutrient-Rich Foods Index 9.3 (NRF9.3).

These follow different guidelines, such as the Dietary Guidelines for Americans (DGA), the Mediterranean diet modified for Americans, a plant-based nutritious diet, adherence to sustainability recommendations, or the DASH diet that is meant to control blood pressure. The NRF9.3 is based on 12 nutrients for which daily values have been set.

Diet cost was also examined, using the Purchase-to-Plate Price Tool (PPPT) as well as derived prices for food eaten outside the home since this comprises over half of the food spending in the US.

Data on dietary habits was drawn from the National Health and Nutrition Examination Survey (2011–2018). This was analyzed against a context of sustainability indicators taken from public domain databases. The latter included greenhouse gas emissions (GHGE), water scarcity footprint (WSF), cumulative energy demand (CED), land, fertilizer nutrients, pesticides, and food prices.

Healthy diets vs. sustainability

The analysis demonstrates that, as reported earlier by multiple studies, higher diet quality was associated with lower daily GHGE per capita and lower land use. A healthier diet was also linked to lower CED in five of eight diets. The exceptions were HEI-2020, Med and aMed).

Fertilizer use was lower for healthy diets except for NRF9.3 and DASH. However, a healthy diet also required greater water utilization, as shown by a higher WSF, and was more costly.

Healthy eating tended to include more seafood, plant-based foods, fats, and oils but less red meat, refined grains, beverages, and sweeteners. While the former was linked to higher environmental impact, the opposite was true for the latter.

Dairy, egg, and poultry use did not show a uniform trend with either DQIs or sustainability. Pesticide use showed conflicting associations with the DQI.

Comparison of diets

The highest number of positive sustainability outcomes and fewest negative sustainability outcomes were associated with aHEI-2010 and hPDI, at five and two, respectively. The PHDI showed a 4:1 ratio, while Med and aMed both had a ratio of 4:2.

The least sustainable diets were HEI-2020 and DASH (3:2 each), and NRF9.3 (3:4). The commonly used DGA-based HEI-2020 uses more water and is more costly but has a lower GHGE, land use, and fertilizer use as expected from prior research.

Plant-based diets had the highest associations with sustainability parameters compared to nutrients or animal-based foods. This was reflected in the best quality-sustainability balance for aHEI-2020 and hPDI.

Nutrient-based diets and the DASH diet were less sustainable compared to those that were food-based or included foods and nutrients. The former was also linked to greater fertilizer and pesticide use.

The paper thus suggests that extending diets recommended or used in specific countries globally may have mixed environmental effects.

Conclusion

Healthier diets have long been thought of and presented as more sustainable, supported by dietary experts. The current study is the first to compare 56 DQI-sustainability parameter pairings directly.

The findings revealed a more complex pattern, suggesting trade-offs between diet quality and sustainability irrespective of specific dietary patterns. Some patterns do emerge, however, with GHGE, CED, land, and fertilizer use being universally or generally lower for healthier diets. Plant-based diets also appear to be more sustainable.