Changes seen in gut bacteria content and distribution with obesity: causation or association?

OBJECTIVE: In the microbiota of both obese animals and humans there is an increased ratio of the gram positive Firmicutes to the gram negative Bacteroidetes (the obesity pattern). To assess if altering this ratio in animals and humans would prevent obesity or reduce body weight in the obese subject this review was preformed.
METHODS: A survey of all the available English language literature utilizing Medline on this topic was obtained and critically reviewed. The key words that were utilized were gut microbiota, diet and obesity.
RESULTS: In both humans and animals changes in diet, particularly the utilization of the high fat, high calorie Western diet, utilization of artificial sweeteners and disruption of the diurnal rhythm will quickly change the microbiota from a thin to an obese pattern. In animals, the transfer of an obese microbiota to germ free animals and thin animals results in obesity and the introduction of a lean microbiota will result in weight loss in obese animals. However, in humans similar changes in the gut microbiota induced with probiotics and prebiotics have not been shown to result in weight loss. In both animals and humans the most dramatic changes in the gut microbiota occur following weight loss resulting from a gastric bypass where there is a restoration to a normal Firmicutes to Bacteroidetes ratio. These changes could either be due to the dramatic change in the composition of the diet which occurs following this surgery or due to down-regulation of the Farnesoid X Receptor which causes a decrease in bile acid production and an elevation of the gut pH which in turn allows the regrowth of bacteria associated with weight loss which were previously unable to grow in the acidic intestinal environment caused by excess production of bile acids.
CONCLUSION: In both humans and animals there are characteristic changes in the gut microbiota associated with obesity. In animals but not in humans altering the microbiota can result in weight loss and weight gain which does not occur in humans. This suggests that in humans the changes in gut microbiota are an association with rather than the cause of obesity.