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People who eat a healthy, sustainable diet may substantially lower their risk of premature death in addition to their environmental impact, according to a new study led by Harvard T.H. Chan School of Public Health. It is the first large study to directly evaluate the impacts of adherence to recommendations in the landmark 2019 EAT-Lancet report. The researchers have named the dietary pattern outlined in the report — which emphasizes a variety of minimally processed plant foods but allows for modest consumption of meat and dairy foods — the Planetary Health Diet (PHD).
The study will be published online June 10 in The American Journal of Clinical Nutrition.
“Climate change has our planet on track for ecological disaster, and our food system plays a major role,” said corresponding author Walter Willett, professor of epidemiology and nutrition. “Shifting how we eat can help slow the process of climate change. And what’s healthiest for the planet is also healthiest for humans.”
While other studies have found that diets emphasizing plant-based foods over animal-sourced foods could have benefits for human and planetary health, most have used one-time dietary assessments, which produce weaker results than looking at diets over a long period of time.
The researchers used health data from more than 200,000 women and men enrolled in the Nurses’ Health Study I and II and the Health Professionals Follow-Up Study. Participants were free of major chronic diseases at the start of the study and completed dietary questionnaires every four years for up to 34 years. Participants’ diets were scored based on intake of 15 food groups — including whole grains, vegetables, poultry, and nuts — to quantify adherence to the PHD.
The study found that the risk of premature death was 30% lower in the top 10% of participants most closely adhering to PHD compared to those in the lowest 10%. Every major cause of death, including cancer, heart disease, and lung disease, was lower with greater adherence to this dietary pattern.
In addition, the researchers found that those with the highest adherence to the PHD had a substantially lower environmental impact than those with the lowest adherence, including 29% lower greenhouse gas emissions, 21% lower fertilizer needs, and 51% lower cropland use.
The researchers noted that land use reduction is particularly important as a facilitator of re-forestation, which is seen as an effective way to further reduce levels of greenhouse gases that are driving climate change.
“Our study is noteworthy given that the U.S. Department of Agriculture has refused to consider the environmental impacts of dietary choices, and any reference to the environmental effects of diet will not be allowed in the upcoming revision of the U.S. Dietary Guidelines,” said Willett. “The findings show just how linked human and planetary health are. Eating healthfully boosts environmental sustainability — which in turn is essential for the health and wellbeing of every person on earth.”
Other Harvard Chan School authors included Linh Bui, Fenglei Wang, Qi Sun, Frank Hu, Kyu Ha Lee, and Marta Guasch-Ferre.
The study was funded by National Institutes of Health (NIH) research grants UM1 CA186107, P01 CA87969, R01 HL034594, U01 CA176726, U01 CA167552, R01 HL035464, R01 DK120870, and R01 DK126698.
“Planetary Health Diet Index and risk of total and cause specific mortality in three prospective cohorts,” Linh P. Bui, Tung T. Pham, Fenglei Wang, Boyang Chai, Qi Sun, Frank B. Hu, Kyu Ha Lee, Marta Guasch-Ferre, Walter C. Willett, The American Journal of Clinical Nutrition, June 10, 2024, doi: 10.1016/j.ajcnut.2024.03.019