After analysing the eating habits of about 2,000 French adults, and the greenhouse gas emissions generated by producing the plants, fish, meat, fowl and other ingredients, researchers concluded in the American Journal of Clinical Nutrition that such a diet might not be the greenest in environmental impact.
"When you eat healthy, you have to eat a lot of food that has a low content of energy. You have to eat a lot of fruits and vegetables," said Nicole Darmon, the study's senior author from the National Research Institute of Agronomy in Marseille, France.
Growing fruits and vegetables doesn't produce as much greenhouse gas as raising cattle or livestock, but food production - including the use of farming equipment and transportation - is estimated to be responsible for 15% to 30% of greenhouse gas emissions in developing countries, the authors said.
Scientists have long advised people to switch to a plant-based diet to benefit the environment and their own health.
To more closely examine that premise, Darmon and her colleagues used food diaries from 1,918 French adults to compare the nutritional quality of people's real-world diets and how much greenhouse gas they produced.
From the diaries that were kept for seven days between 2006 and 2007, the researchers identified the 400 most commonly consumed foods. They then used a database to find out how much greenhouse gas was emitted to produce each one, measured as the grams of carbon dioxide equivalent per 100 grams of food.
All aspects of a food's lifecycle were taken into account, including how it was cooked, Darmon said.
"The only step that wasn't taken into account was the transport from the supermarket to the home," she added.
Living healthier but not always greener
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.
About 1,600 grams of carbon dioxide were emitted for every 100 grams of meat produced, according to the researchers.
That's more than 15 times the amount of greenhouse gas emitted during the production of fruits, vegetables and starches and about 2.5 times as much greenhouse gas as that from fish, pork, poultry and eggs.
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 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.
Overall, the documented diets were responsible for around 5,000 grams of greenhouse gas emissions per day per person.
Greens need to eat more
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.
Roni 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. For example, according to the study's calculations, people would need to eat about four kilograms of fruit and vegetables to make up for a smaller serving of meat.
"I think they're raising a lot of important questions that need further investigation," she said.