The study investigates the human gut microbiota in obesity and after gastric bypass surgery. Using 454 pyrosequencing, researchers analyzed 184,094 16S rRNA gene sequences from 9 individuals, 3 in each of the normal weight, morbidly obese, and post-gastric bypass groups. Phylogenetic analysis revealed that the gut microbiota of obese individuals was dominated by Firmicutes and Bacteroidetes, with a significant increase in Gammaproteobacteria in post-gastric bypass individuals. Obese individuals also had higher numbers of H2-producing Prevotellaceae and H2-oxidizing methanogenic Archaea. The coexistence of these microbes suggests interspecies H2 transfer, which may enhance energy uptake in the gut. Post-gastric bypass individuals showed a shift in microbial communities, likely due to surgical changes and altered digestion. The study also found that methanogens were more abundant in obese individuals, and their presence may be linked to increased energy extraction from food. The results suggest that the gut microbiota plays a significant role in obesity and that gastric bypass surgery alters the microbial community in a unique way. The study highlights the importance of understanding the gut microbiota in the context of obesity and weight loss.The study investigates the human gut microbiota in obesity and after gastric bypass surgery. Using 454 pyrosequencing, researchers analyzed 184,094 16S rRNA gene sequences from 9 individuals, 3 in each of the normal weight, morbidly obese, and post-gastric bypass groups. Phylogenetic analysis revealed that the gut microbiota of obese individuals was dominated by Firmicutes and Bacteroidetes, with a significant increase in Gammaproteobacteria in post-gastric bypass individuals. Obese individuals also had higher numbers of H2-producing Prevotellaceae and H2-oxidizing methanogenic Archaea. The coexistence of these microbes suggests interspecies H2 transfer, which may enhance energy uptake in the gut. Post-gastric bypass individuals showed a shift in microbial communities, likely due to surgical changes and altered digestion. The study also found that methanogens were more abundant in obese individuals, and their presence may be linked to increased energy extraction from food. The results suggest that the gut microbiota plays a significant role in obesity and that gastric bypass surgery alters the microbial community in a unique way. The study highlights the importance of understanding the gut microbiota in the context of obesity and weight loss.