Circulating fasting total BA was not associated with obesity .
FGF19 was inversely and faecal BA excretion was positively associated with obesity .
Roux-en-Y gastric bypass ( RYGB ) and sleeve gastrectomy ( SG )  modulated BA metabolism , ie , increased BA and FGF19 .
Our results indicate that BA metabolism is altered in obesity .
Effects of Surgical and Dietary Weight Loss Therapy for Obesity on Gut Microbiota Composition and Nutrient Absorption
Evidence suggests a correlation between the gut microbiota composition and weight loss caused by caloric restriction . Laparoscopic sleeve gastrectomy ( LSG ) , a surgical intervention for obesity , is classified as predominantly restrictive procedure . In this study we investigated functional weight loss mechanisms with regard to gut microbial changes and energy harvest induced by LSG and a very low calorie diet in ten obese subjects ( n = 5 per group ) demonstrating identical weight loss during a follow-up period of six months . For gut microbiome analysis next generation sequencing was performed and faeces were analyzed for targeted metabolomics . The energy-reabsorbing potential of the gut microbiota decreased following LSG , indicated by the Bacteroidetes/Firmicutes ratio , but increased during diet . Changes in butyrate-producing bacterial species were responsible for the Firmicutes changes in both groups . No alteration of faecal butyrate was observed , but the microbial capacity for butyrate fermentation decreased following LSG and increased following dietetic intervention . LSG result ed in enhanced faecal excretion of nonesterified fatty acids and bile acids .  LSG , but not dietetic restriction , improved the obesity-associated gut microbiota composition towards a lean microbiome phenotype . Moreover , LSG increased malabsorption due to loss in energy-rich faecal substrates and impairment of bile acid circulation . This trial is registered with Clinical Trials.gov NCT01344525
Circulating fasting total BA was not associated with obesity .
FGF19 was inversely and faecal BA excretion was positively associated with obesity .
Roux-en-Y gastric bypass ( RYGB ) and sleeve gastrectomy ( SG )  modulated BA metabolism , ie , increased BA and FGF19 .
Our results indicate that BA metabolism is altered in obesity .