I read an interesting study recently regarding the effect of dieting on the microbiota in rats. What they found is that after the diet was over, the microbiota took nearly five months to recover to its previous state. During that period, the metabolic rate of the rats was basically reduced, forcing the rats to fatten up again if they ate the same caloric intake as they did previously.
This phenomenon, yo-yo dieting, is familiar to anyone who has tried to lose weight before. Despite your best efforts, it becomes increasing difficult to maintain your body at the reduced weight since the body physiologically wants to return to its previous weight.
More interesting is that these researchers managed to determine the cause, at least in the rats. A particular group of bacteria were diminished in the rats after dieting, and these microbiota had to do with steroid biosynthesis and flavonoid production. When these were missing, the rats metabolic rates (how much they burned in a day while at rest) dropped, leading to future weight gain.
So what does it mean? Well, it certainly seems like dieting can indeed lead to rebound weight gain. So vigilance is required for anyone who is actively trying to lose weight. It may take active effort for months after dieting just to maintain your weight while your microbiota sorts itself out. This is certainly an interesting line of research though, and it provides some promise regarding future treatments for obesity.
You can read more about the study here.