That gap narrowed, however, when the researchers looked at how many grams of carbon dioxide were emitted per 100 kilocalories (kcal) - a measure of energy in food.
The most greenhouse gas ― 857 grams ― was still emitted to produce 100 kcal of meat, but it was only about three times the emissions from a comparable amount of energy from fruit and vegetables.
Greens also ended up emitting more gas for the calories than starches, sweets, salty snacks, dairy and fats. It was also about as much gas as pork, poultry and eggs.
And when Darmon and her colleagues looked at what people actually ate to get a certain amount of energy from food every day, they found that the "highest-quality" diets in health terms ― those high in fruit, vegetables and fish ― were linked to about as much, if not more, greenhouse gas emissions as low-quality diets that were high in sweets and salts.
Overall, the documented diets were responsible for around 5,000 grams of greenhouse gas emissions per day per person.
Darmon said that's because people who eat a plant-based diet need to eat more produce to get the amount of energy they'd have in a piece of meat.
"I think to any reader it's surprising. One of the standard things we hear is that meat - particularly red meat - has the greatest greenhouse gas emissions," said Roni Neff, who studies how food contributes to climate change but was not involved with the new study.
But Neff, the director of research and policy at Johns Hopkins Bloomberg School of Public Health's Center for a Livable Future, cautioned against taking the findings too literally. "It's a lot more complex than that," she added.
For example, she pointed out that according to the study's calculations, people would need to eat about nine pounds of fruit and vegetables to make up for a smaller serving of meat, and that may be unrealistic.
But, Neff said, "I think they're raising a lot of important questions that need further investigation."