Research shows that diets with more plant-based foods and fewer animal products generate fewer greenhouse gases (GHGs), consume less natural resources, and reduce the risk of chronic diseases such as type 2 diabetes, coronary heart disease, and stroke. However, the authors of this study questioned whether all plant-based diets have the same positive effects on health and the environment.

The goal of this study was to measure the health and environmental impacts of plant-based diets against several dietary indices to learn which specific foods and dietary patterns provide the most positive (and negative) outcomes. Researchers were specifically interested in understanding how diets impact diseases as well as GHG emissions and nitrogen-based fertilizer, irrigation water, and high-quality cropland use. They used data from the Nurses’ Health Study II, which tracked the diet and lifestyle of over 116,000 U.S. women in the nursing industry from 1989 to 2017. As part of this data, participants were asked to report their intake of 150 different foods every four years. 

Researchers scored participants’ diets using four rating systems: the Alternative Healthy Eating Index (AHEI), , the Plant-Based Diet Index (PDI), the healthy Plant-Based Diet Index (healthy PDI), , and the unhealthy Plant-Based Diet Index (unhealthy PDI). The AHEI gives a high score to diets rich in fruits, vegetables, legumes, whole grains, nuts, and a few sources of omega-3 fats, such as fishes. It gives a low score to diets containing large quantities of sugary beverages, red meat, processed meat, trans fat, and salt. The PDIs measure the following: 

• Overall PDI: Diets score higher for overall plant-based intake, regardless of food. Diets score lower for animal product intake.
• Healthy PDI: Diets only score higher for healthy plant-based foods including whole grains, fruits, vegetables, nuts, legumes, vegetable oils, tea and coffee. 
• Unhealthy PDI: Diets only score higher for unhealthy plant-based foods including fruit juices, sugary beverages, refined grains, potatoes, and sweets. 

Data from 65,625 participants revealed a significant correlation between a diet’s score on the AHEI and PDI indices and its environmental impact. Specifically, diets scoring in the top 20% of the AHEI and the healthy PDI produced lower GHG emissions and used less cropland, water, and fertilizer than the bottom 20%. Conversely, diets scoring in the top 20% of the unhealthy PDI required significantly more cropland and fertilizer than the bottom 20%. 

These findings indicate that unhealthy plant-based foods harm the environment more than healthy plant-based foods. Animal foods, however, had the largest environmental impact. In particular, red and processed meat generated more greenhouse gas emissions and consumed more cropland, water, and fertilizer than all other food groups.

Data from 90,884 participants revealed that diets with higher amounts of healthy plant-based foods decreased participants’ risk of developing cardiovascular disease. Participants with diets scoring in the top 20% of the healthy PDI were 29% less likely to develop cardiovascular disease than the bottom 20% of the healthy PDI. Similarly, participants scoring in the top 20% of the AHEI reduced their risk of cardiovascular disease by 23%. High scores on the AHEI were also associated with a lower risk of coronary heart disease, type 2 diabetes, stroke, and mortality. In contrast, participants who scored in the top 20% of the unhealthy PDI were 15% more likely to develop cardiovascular disease than those who scored lower on this index.

This study demonstrates that diets rich in healthy plant-based foods can lead to positive health outcomes and reduced environmental impact. However, a diet filled with unhealthy foods can lead to unwanted health and environmental outcomes, regardless of whether it’s plant-heavy. Because as many as 42% of people try a veg*n diet for health reasons and 18% for environmental reasons, it’s important for advocates to educate aspiring veg*ns about choosing the right plant-based foods to meet their goals. 

Advocates should also be mindful that these results are based on a niche set of participants who were mainly white women based in the U.S., and working as nurses. Outside this demographic, the benefits of healthy plant-based foods as well as the connection between environmental protection and health may vary. However, the findings are a helpful resource for veg*n advocates to use when supporting plant-based eaters along their journey.