From Vegan to Keto, Which of 6 Popular Diets Has

Researchers compared nutritional quality and environmental impact of 6 common eating styles. Here’s what they found.

Published March 3, 2023 08:01AM EST

Research keeps telling us that what we eat has a big impact on the planet. In fact, a 2021 United Nations-backed study found that 34% of greenhouse gas emissions come from the food system. Given the urgency with which we need to address this warming world of ours, food choices can play a starring role for anyone who wants to participate less in contributing to climate change.

Food choices are also tricky. With many factors to consider—from ethics and access to nutrition and the latest fads—many of us cycle from one diet style to the next. But if your priorities are carbon footprint and/or nutritional value, then recent research from Tulane University may be of interest.

The study compared six popular diets on both nutritional quality and environmental impact. The researchers compiled diet quality scores using data from more than 16,000 adult diets in the United States collected by the CDC’s National Health and Nutrition Examination Survey and looked at vegan, vegetarian, pescatarian, paleo, and keto-style diets, with everything else being categorized as omnivore.

Keto and Paleo Tank for Eco Impact and Nutrition

Many of us know that red meat has a high carbon footprint—remember how Democrats want to take away everyone’s hamburgers because of climate change? (They don’t. Kind of.) So it may not be too surprising that the meat-forward keto and paleo diets ranked as the least sustainable of the six diets examined. And also probably not surprising that those diets scored the lowest for nutritional value as well.

As Tulane’s Andrew J. Yawn explains in a press release about the study:

“The keto diet, which prioritizes high amounts of fat and low amounts of carbs, was estimated to generate almost 3 kg of carbon dioxide for every 1,000 calories consumed. The paleo diet, which eschews grains and beans in favor of meats, nuts, and vegetables, received the next lowest diet quality score and also had a high carbon footprint, at 2.6 kg of carbon dioxide per 1,000 calories.”

Diego Rose, the study’s senior author and professor and nutrition program director at Tulane University School of Public Health and Tropical Medicine, says that while scientists have looked at the nutritional impact of keto and paleo diets, this is the first time researchers have quantified the carbon footprints of each diet and compared them to other popular 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,” Rose said.

The vegan diet had the lowest carbon footprint, generating 0.7 kg of carbon dioxide per 1,000 calories consumed, less than a quarter of the impact of the keto diet. Vegetarian diets had the next lowest carbon footprint, and pescatarian diets followed.

As for health, the pescatarian diet yielded the highest score for nutritional quality, with vegetarian and vegan diets following behind.

The Omnivore Sweet Spot?

Included in the six diets was the catchall omnivore diet, which was represented by 86% of survey participants. Omnivore diets were smack dab in the middle for both quality and sustainability.

However, hidden within the omnivore category were the plant-forward Mediterranean and DASH diet (which steers eaters away from fatty meats)—for people who ate in these styles, both carbon footprints and nutritional quality scores improved.

Treehugger has long advocated for reducing meat consumption when and where you can—it doesn’t have to be vegan or bust. And the Tulane research backs this up. They found that on any given day, if a third of those on omnivore diets ate a vegetarian diet, on average, it would be equivalent to eliminating 340 million passenger vehicle miles.

Make no bones about it: Meat (especially red meat) just has a high carbon footprint. Beef is responsible for 8 to 10 times more emissions than chicken production and over 20 times more emissions than nut and legume production.

“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,” Rose said. “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.”

The study, “Popular diets as selected by adults in the United States show wide variation in carbon footprints and diet quality,” was published in The American Journal of Clinical Nutrition.