Human colonic microbiota associated with diet, obesity and weight loss

Background: It has been proposed that the development of obesity in humans is influenced by the relative proportions of the two major phyla of bacteria (Bacteroidetes and Firmicutes) present in the large intestine.
Objective: To examine the relationships between body mass index, weight loss and the major bacterial groups detected in fecal samples.
Design: Major groups of fecal bacteria were monitored using fluorescent in situ hybridization (FISH) in obese and non-obese subjects under conditions of weight maintenance, and in obese male volunteers undergoing weight loss on two different reduced carbohydrate weight-loss diets given successively for 4 weeks each.
Results: We detected no difference between obese and non-obese individuals in the proportion of Bacteroidetes measured in fecal samples, and no significant change in the percentage of Bacteroidetes in feces from obese subjects on weight loss diets. Significant diet-dependent reductions in a group of butyrate-producing Firmicutes were, however, detected in fecal samples from obese subjects on weight loss diets.
Conclusions: Diets designed to achieve weight loss in obese subjects can significantly alter the species composition of the gut microbiota, but we find no evidence that the proportions of Bacteroidetes and Firmicutes among fecal bacteria have a function in human obesity.