Effect of plant versus animal protein on body weight on a vegan and a Mediterranean diet: a secondary analysis of a randomised cross-over trial

Background A high animal protein intake has been associated with increased obesity risk, whereas adherence to plant-based dietary patterns is associated with body weight (BW) reduction.Methods We conducted a secondary analysis of a 36-week randomised cross-over trial to compare the effects of a low-fat vegan and Mediterranean diet on protein intake, amino acid profiles and BW in overweight adults (n=62). Three-day food record analysis provided protein and amino acid intake estimates, which were evaluated in relation to BW change.Results Total protein intake decreased on the vegan diet by 33.3 g (95% CI −39.4 to −27.2), while it did not change on the Mediterranean diet; effect size: −33.8 g (95% CI −45.0 to −22.5; p<0.001). Animal protein decreased on the vegan diet by 47.7 g (95% CI −53.6 to −41.8), with no change on the Mediterranean diet; effect size: −45.5 g (95% CI −57.1 to −33.9; p<0.001). Plant protein increased on the vegan diet by 14.4 g (95% CI 10.3 to 18.5), with no change on the Mediterranean diet; effect size: +11.7 g (95% CI +6.6 to +16.8; p<0.001). Changes in total and animal protein correlated positively with changes in BW (r=+0.34, p=0.008 and r=+0.40, p<0.001, respectively). Changes in plant protein correlated negatively with changes in BW (r=−0.37, p=0.004). Furthermore, changes in isoleucine, lysine, methionine and tyrosine intake correlated positively with changes in BW (r=+0.40, p=0.002; r=+0.41, p=0.001; r=+0.43, p<0.001 and r=+0.39, p=0.002, respectively). After applying a Bonferroni-corrected threshold for this family of three protein-level correlations (p<0.017), all three associations remained statistically significant.Conclusions Replacing a diet rich in animal protein with one rich in plant protein was associated with weight loss on a low-fat vegan diet in overweight adults. Because this change in protein quality occurred alongside dietary shifts, such as a reduction in fat and energy density and increased carbohydrate intake, these correlational findings are hypothesis-generating and do not establish that protein source itself drives BW change.Trial registration number NCT03698955.