A study that looked at whether all plant-based diets have the same positive effects on health and the environment has concluded that unhealthy plant-based diets, although they may be better than animal-based diets, are worse than healthy plant-based diets both for health and the environment.

The study, conducted by Muscius et al. end published in the Lancet Planetary Health in November 2022, involved over 90,000 participants (mostly white women based in the US, and working as nurses). It was titled “Health and environmental impacts of plant-rich dietary patterns: a US prospective cohort study.”

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. Specifically, four rating systems were used: 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, and it gives a low score to diets containing large quantities of sugary beverages, red meat, processed meat, trans fat, and salt. The PDIs score higher for overall plant-based intake, regardless of food, and an Unhealthy PDI would score higher for unhealthy plant-based foods including fruit juices, sugary beverages, refined grains, potatoes, and sweets.

Using data from 65,625 participants, the study found 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 fertiliser than the bottom 20%. 

Also, this study, using data from 90,884 participants, found 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.

Jordi Casamitjana
“Originally from Catalonia, but resident in the UK for several decades, Jordi is a vegan zoologist and author, who has been involved in different aspects of animal protection for many years. In addition to scientific research, he has worked mostly as an undercover investigator, animal welfare consultant, and animal protection campaigner. He has been an ethical vegan since 2002, and in 2020 he secured the legal protection of all ethical vegans in Great Britain from discrimination in a landmark employment tribunal case that was discussed all over the world. He is also the author of the book, ‘Ethical Vegan: a personal and political journey to change the world’.