Of the six popular diets tested, the keto and paleo diets were found to be the least sustainable — and had the lowest diet quality scores — according to ScienceDaily.

For those on a keto or paleo diet, this can be difficult to swallow.

A new study from Tulane University that compared popular diets on both nutritional quality and environmental impact found that the keto and paleo diets consumed by American adults scored lowest in overall nutritional quality and highest in carbon emissions.

The keto diet, which prioritizes high amounts of fat and low amounts of carbohydrates, was estimated to produce about 3 kg of carbon dioxide for every 1,000 calories consumed. The Paleo diet, which eschews grains and beans in favor of meat, nuts and vegetables, received the next lowest food quality score and also had a high carbon footprint, at 2.6 kg of carbon dioxide per 1,000 calories.

Research, published American Journal of Clinical Nutrition, The Diet Quality Score was compiled using data from more than 16,000 adult meals collected by the CDC’s National Health and Nutrition Examination Survey. Individual diets were assigned point values ​​based on the Federal Healthy Eating Index, and average scores were calculated for those consuming each type of diet.

Study senior author Diego Rose, professor and director of the nutrition program at Tulane University School of Public Health and Tropical Medicine, said that while researchers have examined the nutritional effects of keto and paleo diets, this is the first study to measure the carbon footprint of each. Diets, as consumed by US adults, and compare them to other common diets.

Rose said

At the other end of the spectrum, a vegan diet was found to have the least impact on the climate, producing 0.7 kg of carbon dioxide per 1,000 calories, less than a quarter of the impact of a keto diet. Vegetarian and pescatarian diets were followed with increasing influence.

Pescatarian diets scored highest on the nutritional quality of the foods analyzed, followed by vegetarian and vegan diets.

Omnivore food – the most common food, represented by 86% of survey participants – sits in the middle of the pack for both quality and sustainability. Based on the results, if one-third of those on an omnivorous diet were to start eating a vegetarian diet, for any given day, it would be equivalent to eliminating 340 million passenger vehicle miles.

Notably, however, when omnivores chose the plant-forward Mediterranean or fat-restricted meat-restricted DASH diet version, both carbon footprint and nutrient quality scores improved.

“Climate change is arguably one of the most pressing issues of our time, and many people are interested in moving to a plant-based diet,” Rose said. “Based on our results, this will reduce your footprint and generally stay healthy. Our research shows that there is a way to improve your health and footprint without giving up meat entirely.”

A 2021 UN-backed study found that 34% of greenhouse gas emissions come from the food system. The bulk of these emissions come from food production, with beef responsible for 8-10 times more emissions than chicken production and 20 times more emissions than nut and legume production.

While the environmental effects of specific foods have been widely studied, Rose said the study is important because “it considers how individuals choose popular diets composed of different types of foods.”

Going forward, Rose still has questions about how best to encourage eating habits that are good for people and the planet

“I think the next question is how will different policies affect outcomes and how can they lead us to a healthier, more environmentally friendly diet?” Rose said.

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