In general, analyses of the greenhouse gas emissions of various diets use an idealized diet, rather than looking at data about what people actually eat. As a result, the way people eat in the real world may be very different from what the models assume. By using data from an ongoing nationally representative survey in the U.S., the authors of this study estimated the carbon footprint and nutritional quality of the diets of over 16,000 adults. 

 The study divided people’s diets into six categories:

1. Vegan, which didn’t include any animal products and represented 0.7% of respondents
2. Vegetarian, which included eggs and milk but no meat and represented 7.5% of respondents
3. Pescatarian, which included eggs, milk, and seafood, but no other meat, and represented 4.7% of respondents
4. Paleo, which didn’t include grains or dairy and represented 0.3% of respondents
5. Keto, which included less than fifty grams of non-fiber carbohydrates and represented 0.4% of respondents
6. Omnivore, which didn’t fit into any of the above and represented 86% of respondents

Because the survey only examined the last 24 hours of individuals’ diets, it overestimated the number of people who reliably eat a particular diet. Hispanic and Black adults were more likely to eat a keto diet compared to white respondents. Men were also less likely than women to consume a vegetarian or pescatarian diet. 

Vegan diets had the lowest carbon footprint, followed by vegetarian diets. Omnivore, paleo, and keto diets had higher carbon footprints. On average, an omnivorous diet led to one kilogram more of greenhouse gas emissions per thousand calories than a vegetarian diet did. As measured by both the Healthy Eating Index and the Alternate Healthy Eating Index, pescatarians had the healthiest diets, followed by vegetarians and vegans.

The authors also analyzed omnivorous diets to see how closely they followed the recommendations of the DASH and Mediterranean diets, which are commonly recommended eating patterns. The authors found that omnivores who ate more in accordance with the recommendations of the DASH and Mediterranean diets had healthier diets and lower carbon footprints. 

This study is the first of its kind to look at the carbon footprints of keto and paleo diets based on nationally representative samples from the United States. According to the results, veg*n diets appear to be healthier and have a lower carbon footprint than keto, paleo, or omnivorous diets. While the pescatarian diets were perceived to be healthier by the authors, they also had higher carbon footprints than veg*n diets.

The authors’ estimates show that, if a third of omnivores switched to consuming a vegetarian diet for one day, the savings would be equivalent to eliminating 340 million passenger vehicle miles. As such, advocates can use these results to raise awareness of how a vegetarian or vegan diet is a responsible choice for both your health and the environment.