How well do Americans eat for the planet? New index reveals gaps and opportunities in sustainable diets

In a recent study published in The American Journal of Clinical Nutrition, a group of researchers utilized the newly developed Planetary Health Diet Index for the United States (PHDI-US) to evaluate adherence to the Planetary Health Diet and discern crucial areas for improvement in U.S. adults’ diets to enhance dietary sustainability and quality.

Study: Cross-sectional measurement of adherence to a proposed sustainable and healthy dietary pattern among U.S. adults using the newly developed Planetary Health Diet Index for the United States. Image Credit: j.chizhe Shutterstock


The global food system, a significant contributor to climate change, calls for innovative, sustainable dietary solutions, particularly given the substantial environmental impacts of animal-based foods, predominantly occurring at the farm stage. The EAT-Lancet Commission has proposed the Planetary Health Diet to mitigate such impacts, emphasizing the consumption of plant-based foods. PHDI measures adherence to this diet but requires validation and adaptation for the U.S. population. Developing a tailored PHDI-US is pivotal to assessing and enhancing adherence, addressing disparities in dietary sustainability and health quality, and informing subsequent interventions, endorsing global environmental conservation. Further research is crucial for adapting such tools for diverse populations, facilitating sustainable and healthy dietary practices, promoting food security, and contributing to global environmental preservation.

About the study

The study analyzed data from the National Health and Nutrition Examination Survey (NHANES) 2017-2018 participants, prioritizing individuals aged 20 or older and dismissing unreliable dietary recalls. Utilizing a sophisticated, multi-stage probability sampling, dietary information was acquired via computer-assisted, in-person interviews focusing on 24-hour dietary recall. 

The Food Patterns Equivalent Database 2017-2018 quantified intake, which informed PHDI-US components, encompassing sixteen components with a maximum score of 150. The modifications ensured the compatibility of PHDI-US with various dietary intakes, making it a more inclusive tool. Adjustments included accommodation for vegan and vegetarian diets, substituting certain components with plant proteins and fortified soymilk, and including palm oil in the Saturated Fats component.

Statistical analyses conducted via SAS software and adhering to NHANES procedures concluded that p-values <0.05 were significant. The study meticulously evaluated the U.S. population’s adherence to the Planetary Health Diet, applying descriptive statistics, independent t-tests, and Bonferroni corrections. Construct and concurrent-criterion validity was assessed through various scientific methods, ensuring the reliability of PHDI-US in measuring dietary sustainability and quality. This comprehensive approach reaffirms the adaptability and alignment of PHDI-US with the Planetary Health Diet, contributing significantly to environmental conservation and global health initiatives.

Study results

In an in-depth analysis of the total PHDI-US scores, substantial disparities in variations were revealed across different race or ethnicity groups; Non-Hispanic Asian individuals achieved the highest mean total score at 56.0±1.24, whereas Non-Hispanic Black individuals scored significantly lower, with a mean score of 41.9±0.78. This lower score was prevalent across all racial or ethnic groups except the “Other” race group, with a mean score of 46.1±1.31 (p = 0.0101). 

An intriguing pattern was observed when analyzing PHDI-US scores in conjunction with the Participant Income Ratio (PIR). Scores were found to generally escalate with increasing income levels. Notably, individuals in the highest PIR group obtained a PHDI-US score of 48.5±0.72, significantly surpassing the 1.31-2.50 PIR group, which scored 44.8±0.52 (p = 0.0004). Linear regression analyses revealed that nutrients predominantly found in animal-based foods were inversely related to PHDI-US scores, while those abundant in plant-based foods showed a positive correlation. Surprisingly, polyunsaturated fat did not significantly correlate with total PHDI-US scores.

The comprehensive linear regression analysis identified low yet meaningful correlations between nutrient intake and total PHDI-US scores. Nutrients inherent in animal-based foods, such as cholesterol, vitamin B12, and saturated fat, shared a negative association with total PHDI-US scores, suggesting a lower consumption of such nutrients could yield higher adherence to a planetary health diet. Conversely, nutrients abundant in plant-based foods, namely, vitamin C and fiber, were positively associated with PHDI-US scores, indicating a higher consumption correlates with adherence to the planetary health diet. 

The Principal Component Analysis and linear regression displayed more than one linear combination of PHDI-US components significantly contributed to data covariation. Furthermore, the total healthy eating index (HEI)-2015 and PHDI-US scores demonstrated a significantly positive association (ß = 0.67, SE = 0.03, p <0.0001; R2 = 0.39). Notably, individuals who abstained from smoking achieved significantly higher mean total PHDI-US scores compared to regular smokers (p <0.0001). Female participants and older participants, particularly those aged 60 and above, also exhibited significantly higher mean total PHDI-US scores.

To evaluate internal consistency among the sixteen PHDI­-US components, Cronbach’s alpha was applied, yielding a value of 0.54. Correlations between PHDI-US components were mostly significant but low, ranging between 0.000 and 0.387. The item-total correlations between the components and total PHDI-US scores were significant and generally moderate, ranging from 0.165 to 0.452. These analyses have provided nuanced insights into the relationships between diet, demographic factors, and adherence to the principles of the Planetary Health Diet, contributing to an understanding of sustainable eating patterns and nutritional intake.

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