Weight loss fads like keto and paleo diets have become extremely popular over the last few decades, but a new study has news that advocates may find unpalatable. Researchers compared several popular diets for their overall nutritional quality and carbon footprint, finding that both keto and paleo diets provided the lowest nutritional quality and had the highest environmental impact.  

The study assessed the diet quality scores obtained from over 16,000 US adults through the Centers for Disease Control and Prevention’s (CDC) National Health and Nutrition Examination Survey. The team at Tulane University, Louisiana, classified individuals as having one of six diet types: omnivore, vegetarian, pescatarian, vegan, keto, and paleo. These diets were given point values based on the federal Healthy Eating index and average scores were calculated for people eating each diet.

This study represents the first of its kind to examine both the carbon footprint of keto and paleo diets and to compare them with other common diets among US adults.

Keto (short for ketogenic) diets seek to push the body into ketosis – a metabolic state that occurs when your body burns fat for fuel, rather than glucose – by relying on a strict low-carbohydrate diet. Normally, our bodies use glucose from food for energy, but when it runs out of carbohydrates the liver starts to break down fat stores instead. The energy it gets from this process comes from molecules called ketones. 

Keto diets were originally developed to treat children with epilepsy, but they soon entered the mainstream media where they were transformed into a popular option for weight loss. Keto proponents boast a range of health benefits including weight loss, feeling less hungry, and increased energy levels – but researchers have also warned about potential dangers for some time. The issue is that, to date, only short-term results have been studied. It is still not clear whether the diet is effective in the long run and whether it is safe. 

Now, new research from Tulane University has poured more cold water on this form of diet. The team estimated that the keto diet generates almost 2.91 kilograms (6.4 pounds) of carbon dioxide for every 1,000 calories consumed. The paleo(short for paleolithic) diet, a modified version of the keto diet that allows for some carbs from fruits and vegetables, was found to have the next highest carbon footprint, generating 2.6 kg (5.73 lb) of carbon dioxide per 1,000 calories. 

In a statement, Diego Rose, professor and nutrition program director at Tulane University School of Public Health and Tropical Medicine, explained that “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.”

Conversely, vegan diets were found to have the least impact on the climate as they only generated 0.7 kg (1.5 lbs) of carbon dioxide per 1,000 calories – less than a quarter of a keto diet's impact. Vegetarian and pescatarian (vegetarian diets that allow for the consumption of fish) came next. 

Interestingly, pescatarian diets had the highest overall nutritional quality and were ahead of both vegetarian and vegan ones.

Omnivorous diets, consumed by about 86 percent of the surveyed population, came in the middle for both nutritional quality and carbon emissions. The study found that if a third of the omnivores switched to a vegetarian diet, assuming a shift in domestic production, the savings would be the equivalent of eliminating 340 million passenger vehicle miles. If this change could be sustained for a year, then it would help reduce US carbon emissions amount by 4.9 percent of the reduction needed to match the current targets in the Paris accords. 

If omnivores switched to emphasize more of a plant-based diet, such as a Mediterranean diet, then both the overall carbon footprint and nutritional quality scores were improved. 

“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 stated. “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 was published in the American Journal of Clinical Nutrition.