Environmental footprint of a low-fat vegan diet and Mediterranean diet: a secondary analysis of a randomised clinical trial.

Background: Plant-based foods, compared with animal-based foods, have a smaller environmental impact.

Objective: This secondary analysis used a randomised, crossover trial to calculate and compare the environmental impact of two plant-forward diets: low-fat vegan and Mediterranean.

Methods: 62 overweight adults were randomly assigned to a low-fat vegan or Mediterranean diet for 16-week periods, separated by a 4-week washout. 3-day dietary records were analysed, and greenhouse gas emissions (GHGEs) and cumulative energy demand (CED) were calculated for completers of the study. The calculated GHGEs and CED relationship to changes in body weight were analysed.

Results: Total calculated GHGEs and CED were reduced on the low-fat vegan diet by 57%, or by -1451.7 g CO₂-eq/person-day (95% CI -1776.5 to -1126.9; p<0.0001), and 55%, or by -9306.7 kJ/person-day (95% CI -11022 to -7591.5; p<0.0001), respectively. Total calculated GHGEs in the Mediterranean diet reduced by 20%, or by -451.3 g CO₂-eq/person-day (95% CI -859.7 to -42.8; p=0.03), and no change in calculated CED. In both treatment groups combined, changes in body weight correlated with changes in total CED (r=+0.41; p=0.004), even after adjustment for changes in energy intake (r=+0.34; p=0.02). The correlation between changes in body weight and changes in GHGEs did not reach statistical significance (r=+0.26; p=0.07).

Conclusion: Collectively, a low-fat vegan diet demonstrated statistically significant reductions in GHGEs, CED and weight loss compared with the non-plant-forward and Mediterranean diets, suggesting that a low-fat vegan diet can improve both human and planetary health.

Trial registration number: NCT03698955.