Are healthier diets more sustainable? A cross-sectional assessment of 8 diet quality indexes and 7 sustainability metrics

BACKGROUND: A comparison of commonly used diet quality indexes (DQIs) and their association with multiple indicators of sustainability has not been performed, which limits policy action. OBJECTIVES: To evaluate the associations between 8 DQIs and 7 sustainability indicators in a nationally representative sample in the United States. METHODS: In this cross-sectional analysis, dietary data from the National Health and Nutrition Examination Survey (2011-2018, n = 18,522 ≥20 y) were merged with data on greenhouse gas emissions (GHGE), cumulative energy demand (CED), water scarcity footprint (WSF), land, fertilizer nutrients, pesticides, and food prices from multiple publicly available databases. Diet quality was measured using 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). The relationship between each DQI and sustainability indicator was assessed using linear regression. RESULTS: Greater scores on most or all DQIs were associated with lower daily per capita GHGE (β = -0.908 to -0.250 carbon dioxide equivalents per 1 unit increment in diet quality z-score), CED (β = -2.699 to 0.202 MJ), land (β = -0.002 to <-0.001 ha), and fertilizer nutrients (β= -0.026 to 0.007 kg). By contrast, greater scores on most or all DQIs were associated with greater WSF (β = 343-649 L equivalents) and diet cost (β = 0.037-1.125 US$), and had mixed associations with pesticide use (β = -0.001 to <-0.001 kg). Greater scores on aHEI-2010 and hPDI were associated with the greatest ratio of favorable-to-unfavorable sustainability outcomes (5:2 each), followed by PHDI (4:1), Med and aMed (4:2 each), HEI-2020 and DASH (3:2 each), and NRF9.3 (3:4). CONCLUSIONS: All DQIs had sustainability trade-offs, but those that emphasized plant-based foods, rather than nutrients or animal-based foods, were associated with more favorable sustainability outcomes.