Low-fat vegan diet cut emissions and energy use in controlled trial, BMJ study reports

A randomized clinical trial has found that a low-fat vegan diet reduced food-related greenhouse gas emissions by 57% and cumulative energy demand by 55%, while also delivering improvements in weight and metabolic health among overweight adults.

• A randomized crossover trial of 62 adults compared low-fat vegan and Mediterranean diets over two 16-week periods with a washout phase in between.
• The vegan diet reduced greenhouse gas emissions by 57% and cumulative energy demand by 55%, compared with a 20% emissions reduction for the Mediterranean diet.
• Reductions in meat, dairy and egg consumption drove most of the environmental gains observed in the vegan diet group.

The findings, published in BMJ Nutrition, Prevention & Health, were based on a secondary analysis of a randomized crossover trial, offering real-world clinical evidence linking dietary choices to environmental outcomes.

“This is not just about nutrition anymore, it’s about systems biology and planetary health,” said Hana Kahleova, MD, PhD, Director of Clinical Research at the Physicians Committee for Responsible Medicine and lead author of the study. “We now have randomized clinical trial data showing that a single intervention, diet, can simultaneously reduce environmental impact and improve metabolic health.”

The trial included 62 overweight adults who were randomly assigned to follow either a low-fat vegan diet or a Mediterranean diet for 16 weeks, before switching to the alternate diet after a four-week washout period.

Participants recorded their food intake over multiple three-day periods, allowing researchers to calculate greenhouse gas emissions and cumulative energy demand using established environmental databases.

The analysis showed that the vegan diet reduced emissions by an average of 1,451.7 grams of CO2-equivalent per person per day, compared with a reduction of 451.3 grams for the Mediterranean diet. The difference between the two diets amounted to roughly 1,000 grams of CO2-equivalent per person per day.

Cumulative energy demand, a measure of the total energy required to produce food, also fell sharply under the vegan diet, decreasing by more than 9,300 kilojoules per person per day, while no significant change was observed under the Mediterranean diet.

Researchers found that the environmental gains were largely driven by the removal of animal-based foods. Reductions in meat consumption accounted for the largest share of emissions savings, followed by dairy and eggs. At the same time, participants increased their intake of legumes, fruits and vegetables, which are associated with lower environmental impacts.

“What’s striking is how consistent the signal is,” Kahleova said. “When you remove animal products, you’re shifting the entire metabolic and environmental burden of the diet.”

The study also examined the relationship between environmental impact and health outcomes. While changes in greenhouse gas emissions did not show a statistically significant correlation with weight loss, reductions in cumulative energy demand were positively associated with decreases in body weight.

Participants following the vegan diet showed greater improvements in weight, insulin sensitivity and cholesterol compared with those on the Mediterranean diet, reinforcing earlier findings from the same clinical trial.

The researchers noted that the study’s randomized crossover design strengthened the reliability of the findings, although they acknowledged limitations including self-reported dietary records and the relatively short 16-week intervention periods.

“A dietary shift is one of the most immediate and scalable tools we have,” Kahleova said. “It doesn’t require new technology, it requires applying what we already know from clinical science.”