This paper calculates the carbon footprints and dietary quality score of six dietary patterns based on consumption data from the United States: vegan, vegetarian, pescatarian, keto, paleo and omnivorous diets. All of these diets were loosely defined (e.g. vegetarian diets are those with less than 14 grams of meat and seafood per day) to allow some deviation from the strict conventional definitions of these diets. Pescatarian diets scored as the most healthy, and vegan diets had the lowest carbon footprint.
The figure below illustrates the carbon footprint and dietary quality scores of the six dietary patterns. Note that for the dietary quality scores, two different scoring systems were used to test the robustness of the findings; both scoring systems are based largely on intake of certain types of food, rather than on detailed macro- or micronutrients composition of the diets, or on health outcomes.
Image: Average carbon footprint and two dietary quality scores for six dietary patterns. Both dietary quality scores can have a maximum of 100 points, with higher scores indicating better dietary quality. Diets are presented in order of highest to lowest carbon footprint (from left to right). Figure produced by TABLE based on data from Table 3 of O’Malley et al. (2023).
Keto diets, defined as having less than 50 grams of net carbohydrates per day, have the highest carbon footprint and either the lowest or third lowest dietary quality score, depending on the system used. Paleo diets, defined as having less than 14 grams of grains and legumes per day and less than 59 ml of dairy, have the second highest carbon footprint and the lowest or second lowest dietary quality scores, depending on the system used.
Carbon footprints of vegetarian, pescatarian, and other popular diets have been studied previously, but mostly as idealized versions modeled to meet dietary recommendations. Less is known about the footprints of popular diets as they are consumed by US adults, and thus the potential trade-offs with diet quality for free-living individuals.
This study estimated the carbon footprint and diet quality of popular diets as selected by a nationally representative sample of US consumers, including the recently trending keto- and paleo-style diets.
The 2005–2010 NHANES 24-h recall data were used to categorize individual adult diets (n = 16,412) into 6 types: vegan, vegetarian, pescatarian, paleo, keto, and all other diets, referred to here as omnivore diets. Average daily greenhouse gas emissions in kilograms of carbon dioxide equivalents per 1000 kcal (kg CO2-eq/1000 kcal) were calculated for each diet by matching our previously developed database to NHANES individual diet data. Diet quality was determined using the Healthy Eating Index (HEI) and the Alternate Healthy Eating Index. Survey-weighted ordinary least-squares regression was used to assess mean differences in diets.
The average carbon footprints of vegan (0.69 ± 0.05 kg CO2-eq/1000 kcal) and vegetarian (1.16 ± 0.02) diets were lower (P < 0.05) than those of the pescatarian (1.66 ± 0.04), omnivore (2.23 ± 0.01), paleo (2.62 ± 0.33), or keto (2.91 ± 0.27) diets. Mean HEI scores were highest for pescatarian diets (58.76 ± 0.79) and higher (P < 0.05) for vegetarian (51.89 ± 0.74) than for omnivore (48.92 ± 0.33) or keto (43.69 ± 1.61) diets.
Our results highlight the nuances when evaluating the nutritional quality of diets and their carbon footprints. On average, pescatarian diets may be the healthiest, but plant-based diets have lower carbon footprints than other popular diets, including keto- and paleo-style diets.
O’Malley, K., Willits-Smith, A. and Rose, D., 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. In Press, Corrected Proof.