According to a study, keto and paleo diets, as eaten by Americans, are the “least sustainable” and have the lowest overall nutritional quality.

The study, carried out by researchers at Tulane University, compared popular diets on both nutritional quality and environmental impact.

The keto diet (a diet that prioritises high amounts of fat and low amounts of carbs) was estimated to generate almost 3kg of carbon dioxide for every 1,000 calories consumed. Meanwhile, the paleo diet, (a diet that promotes the inclusion of natural fats from pasture-fed livestock, fish and seafood as well as nuts and seeds) received the next lowest diet quality score and also had a high carbon footprint, at 2.6kg of carbon dioxide per 1,000 calories.

The researchers analysed diet quality scores using data from more than 16,000 adult diets collected by the Centers for Disease Control and Prevention’s (CDC) National Health and Nutrition Examination Survey.

Study authors assigned individual diets point values based on the federal Healthy Eating Index and average scores were calculated for those eating each type of diet.

Commenting on the findings, the study’s senior author, Diego Rose, said that while researchers have examined the nutritional impact of keto and paleo diets, this is the “first study” to measure the carbon footprints of each diet, as consumed by US adults, and compare them to other common diets.

“We suspected the negative climate impacts because they’re meat-centric, but no one had really compared all these diets – as they are chosen by individuals, instead of prescribed by experts – to each other using a common framework,” said Rose.

In contrast, the researchers found a vegan diet to be the least impactful on climate, generating 0.7kg of carbon dioxide per 1,000 calories consumed. This is less than a quarter of the impact of the keto diet. The vegan diet was followed by vegetarian and pescatarian diets in increasing impact, according to the researchers.

The pescatarian diet scored highest on nutritional quality of the diets analysed, with vegetarian and vegan diets following behind.

The omnivore diet, represented by 86 percent of survey participants sat in the middle of the pack of both quality and sustainability, with researchers stating that, based on the findings, “if a third of those on omnivore diets began eating a vegetarian diet, on average for any given day, it would be equivalent to eliminating 340 million passenger vehicle miles”.

“Climate change is arguably one of the most pressing problems of our time, and a lot of people are interested in moving to a plant- based diet,” explained Rose.

“based on our results, that would reduce your footprint and be generally healthy. Our research also shows there’s a way to improve your health and footprint without giving up meat entirely.”

Going forward, Rose said that they still have questions about how to encourage eating habits that are better for people and the planet.

“I think the next question is how would different policies affect outcomes and how could those move us toward healthier, more environmentally friendly diets?”