Total plant food diversity and greenhouse gas emissions: a cross-sectional analysis of the National Diet and Nutrition Survey Year 9

While plant-based diets are generally considered more sustainable (1) , their greenhouse gas emissions (GHGE) may vary according to the diversity and type of plant foods consumed. There is limited evidence on the contribution of plant food diversity (PFD) to GHGE in population-based studies. This study examined differences in total GHGE across PFD tertiles, and differences in total plant and plant group diversity across GHGE tertiles. This cross-sectional analysis used dietary data from 648 UK adults in the National Diet and Nutrition Survey (NDNS) Year 9 (2016-2017). Participants completed four-day interviewer-checked food diaries. Plant-based ingredients from composite dishes were disaggregated and were categorized into eight plant food groups. PFD was assessed as the total count of unique plant food items consumed over four days. GHGE (gCO 2 -eq) represented four-day totals and were estimated by multiplying ingredient weights from disaggregated NDNS food codes to emission factors from life-cycle assessment sources (2) . ANOVA and ANCOVA tests were performed, with Bonferroni post hoc tests, across five models with sequential confounder adjustment, to compare differences in (a) total GHGE across PFD tertiles and (b) total and plant group diversity across GHGE tertiles. Participants in the high PFD group had higher total GHGE than those in the moderate and low groups in the unadjusted and demographic-adjusted model (unadjusted model means [SD] low: 3,101.5 [1,230.6]; moderate: 3,707.0 [1,166.9]; high: 3,987.5 [1,159.3] gCO 2 -eq ; p<0.01); post-hoc test confirmed significant differences across all pairwise comparisons in these two models (p<0.05). Differences were no longer significant after further covariate adjustment. There was a significant difference in total PFD across GHGE groups in the fully adjusted model (mean [SD]; low: 31.8 [11.8], moderate: 34.6 [9.7], high 34.9 [12.2] diversity counts; p=0.03). Pairwise comparisons showed a significantly higher total PFD in the moderate GHGE group, compared to the low group (p=0.03). Grains and fats/oils diversity were significantly different across GHGE groups in all models (all p≤0.01), with significantly higher plant group diversity in the moderate GHGE group, compared to low group (all p<0.05). Higher vegetable diversity was consistently shown in the high GHGE group, compared to both the low and moderate GHGE groups (p<0.05). Differences in total GHGE across PFD groups were not significant after adjusting for dietary and lifestyle factors, suggesting that these variables may better explain variations in total GHGE. After full adjustment, vegetable diversity was higher in the high GHGE group, whereas total PFD, grains and fats/oils diversity were only higher in moderate vs low GHGE groups. Overall, these findings highlight that not all plant food groups have the same environmental impact, and the importance of including confounding factors in GHGE assessment. Future research should explore the potential of substituting higher-emission plant foods with lower-emission alternatives.