Energy balance

Obesity is associated with a gut microbiota composition characterized by a higher Firmicutes-to-Bacteroidetes ratio and an increased capacity for energy harvest, which contributes to host adiposity.

Your gut bacteria may be more efficient at extracting energy from food than those of lean individuals, potentially contributing to weight gain. While you cannot directly 'eat' a new microbiome, the paper suggests that weight loss and dietary changes can alter this composition, potentially reducing this efficiency.