506-P: Weight Loss Is Associated with Changes in Gut Microbiome: A Randomized, Crossover Trial Comparing a Mediterranean and a Low-Fat Vegan Diet in Overweight Adults

Background: Mediterranean and vegan diets improve body weight and gut microbiome composition. The aim of this study was to compare both diets head-to-head.

Methods: Randomized cross-over trial, conducted February-October 2019. Sixty-two overweight adults were assigned to each diet for 16-week periods in random order, separated by a 4-week washout. Body weight was the primary outcome. Secondary measures included changes in gut microbiome which was measured using 16S rRNA sequencing.

Results: Body weight decreased on the vegan compared with the Mediterranean diet (treatment effect -6.0 kg [95% CI -7.5 to -4.5]; p<0.001). The relative abundance of Bacteriodetes decreased (p<0.001 for both diets) and Eubacteria increased on both diets (p<0.001 for the Mediterranean and p=0.009 for the vegan diet). The relative abundance of Lachnospiraceae increased (p=0.03), the Firmicutes to Bacteroidetes ratio increased (p=0.04) and the butyrate-producing bacteria decreased (p=0.02) on the Mediterranean diet. The relative abundance of Proteobacteria decreased (p<0.001), and Enterobacteria and Ruminococcus increased on the vegan diet (p=0.04 and p<0.001, respectively). Changes in body weight correlated positively with changes in relative abundance of Firmicutes both on the Mediterranean (r=+0.36; p=0.01) and the vegan diet (r=+0.41; p<0.001) and with changes in relative abundance of Lachnospiraceae both on the Mediterranean (r=+0.40; p<0.001) and the vegan diet (r=+0.44; p<0.001). In addition, the changes in body weight correlated negatively with changes in relative abundance of Enterobacteria on the Mediterranean diet (r=-0.32; p=0.02) and Eubacteria on the vegan diet (r=-0.49; p<0.001).

Conclusions: A low-fat vegan diet lead to a greater weight loss compared with a Mediterranean diet. This may be partly explained by the difference in gut microbiome composition.