How many calories do gut bacteria burn? It has become well-accepted that microbiota of the gut (a.k.a. gut bacteria) consume calories that are ingested and can have significant effects on energy metabolism in humans. For example, **if you take the feces from mice that have had a gastric bypass and transplant those into the colon/GI tract of obese mice, then the obese mice lose weight** (See here for article about this study). **Are there any known estimates on how many calories the typical gut microbiota in an adult human consumes?** That is, about how many calories your gut microbiota use per day? For more information there is a freely available source on Pubmed, "The Human Microbiome, Diet and Health Workshop Summary."

A recent study by Bahr et al. found that "healthy" gut microbes increased non-aerobic resting metabolism by 16%, compared to gut microbes being treated by risperidone. Risperidone is an antipsychotic drug that causes weight gain and Bahr et al. were trying to determine why. In addition to to the article, their university issued a non-technical press release on the article.

This article doesn't exactly answer your question but does provide a good staring place.