There is increasing interest in growing and eating foods with a lower carbon footprint as climate change effects become more widely known. Currently, the food system is a substantial contributor to environmental problems. Changing these food systems in the future could positively impact both population health and environmental sustainability, with dietary choices playing an important role. However, there is little information on the carbon footprint of actual American diets and how they relate to diet quality.

A new research paper looks at several popular diets and what it costs the environment to follow any of them, contrasting with their real-life benefits regarding diet quality.

Study: Popular diets as selected by adults in the United States show wide variation in carbon footprints and diet quality. Image Credit: alphaspirit.it / Shutterstock

Introduction

Modern agriculture significantly contributes to the unstable health and sustainability scenario brought about by climate change. Consumer demand could drive future trends in food production. An earlier study showed that about one in seven people in the USA would change their dietary patterns to promote environmental sustainability.

This begs the question, which diet pattern is the most sustainable as well as healthy? The current study, published in the American Journal of Clinical Nutrition, attempts to classify six diets – vegan, vegetarian, pescatarian, paleo, keto, and others – by these parameters.

The data came from the NHANES (2005-2010) study that analyzed individual 24-h dietary recall data in a nationally representative survey measuring the health and nutritional status of the US population. First, the scientists calculated the carbon footprint – the greenhouse gas emissions (GHGEs) – as kg of carbon dioxide equivalents (kg CO2-eq), per 1000 kcal. To assess diet quality, they also looked at two scores, the Healthy Eating Index (HEI) and the Alternate Healthy Eating Index (AHEI).

Earlier research has shown that plant-based diets are the greenest. Similarly, replacing some meat with plant foods may reduce the GHGEs. In this study, the United States Department of Agriculture Food Patterns Equivalents Database was used to convert the actual foods eaten into food equivalents in various food groups for the purposes of standardization and comparison.

For obvious reasons, people who would have fitted more than one diet type were not included in this analysis. The GHGEs were calculated using the databases on food impacts and food recall impacts on the environment in terms of nutritional studies. These were earlier developed by the same group and named dataFIELD and dataFRIENDS, respectively.

What did the study show?

The study showed that vegan and vegetarian diets had the least impact on the environment, at 0.7 and 1.2 kg of kg CO2-eq, respectively. Pescatarian diets showed a footprint of 1.66, which was eclipsed by the omnivore's 2.23, keto's 2.6, and paleo's 2.92 scores. Thus keto and paleo diets, both of which became very popular over a short period, have low diet quality and a larger carbon footprint than the other diets.

This could be because they rely on animal-based foods and cause significant changes in blood lipids, mainly a rise in low-density lipoproteins (LDL).  

When omnivore diets were classified by Mediterranean or DASH (Dietary Initiative Against Hypertension) parameters, those that were closest to either of the above were found to be linked to lower GHGEs, but positively with diet quality scores. The first includes plenty of seafood, legumes, fruits, vegetables, and little red meat, with practically no processed meat. The DASH diet also stresses low saturated and total fat, with more fiber.

Omnivore diets made up the most significant number of responses.

In addition, the HEI score was higher with fish-based diets and better with vegetarianism than omnivore or keto diets, at ~59%, 52%, and 49%, respectively.    

Vegetarian or fish-based diets were more common among females and non-Black participants, but Black and Hispanic individuals were more likely to eat keto than omnivore diets. Young adults were more likely to be vegan, but older people preferred fish over omnivore diets.

Middle-aged adults were likelier to be omnivores, especially those with less than a college education. Those without a high school degree were also less likely to eat mainly fish.                                                                                   

What are the implications?

This study, based on common diets, found that fish-based diets were the healthiest. However, the overall palm goes to plant-based diets for their low demands on the planet.

"For any given day, if a third of the omnivores consumed a 2000 kcal vegetarian diet, and assuming accompanying shifts in domestic production, the savings would be equivalent to eliminating 340 million passenger vehicle miles. If such a change were implemented year-round, this would amount to 4.9% of the reductions needed to meet the original US targets in the Paris accords."

Considering individual preferences, the best way out for the majority of people, who follow an omnivore diet, might be something closely aligned to either the Mediterranean or DASH diet. Further research will help identify what dietary factors should be addressed most urgently to mitigate practices that foster climate change in the areas of food production and consumption.

Journal reference:

• O'Malley, K. et al. (2023). Popular diets as selected by adults in the United States show wide variation in carbon footprints and diet quality. The American Journal of Clinical Nutrition. doi: https://doi.org/10.1016/j.ajcnut.2023.01.009. https://www.sciencedirect.com/science/article/pii/S0002916523005117