A nutritious diet that includes lots of fruits and vegetables might not be the greenest in its environmental impact, according to a new study from France.
After analyzing 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 that widely embraced goals for the health of people and for the health of the planet are not necessarily perfectly compatible.
Growing fruit and vegetables doesn't produce as much greenhouse gas as raising cattle or livestock, the study confirms, but people who eat a primarily plant-based diet make up for that by eating more of those foods.
"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.
Greenhouse gases - which include carbon dioxide, methane and nitrous oxide - are produced by machines that burn fossil fuels. That gas is then released into the atmosphere, where it contributes to climate change.
Food production - including the use of farming equipment and transportation - is estimated to be responsible for 15 percent to 30 percent of greenhouse gas emissions in developed countries, the authors write in The American Journal of Clinical Nutrition.
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.
According to the U.S. Environmental Protection Agency, the average car emits about 423 grams of carbon dioxide every mile.
Overall, about 1,600 grams of carbon dioxide were emitted for every 100 grams of meat produced. That's more than 14 times the amount of greenhouse gas emitted during the production of fruit, vegetables and starches. It's also about 2.5 times as much greenhouse gas as that generated by 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 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."