While simple calculation of the numbers of calories consumed and expended often explains weight loss and gain, new research suggests that intestinal microbes may also play a role in the regulation of body weight (Ley RE et al. Nature. 2006;444:1022-1023;Turnbaugh PJ et al. Nature. 2006;444:1027-1031).
Two divisions of beneficial bacteria—Bacteroidetes and Firmicutes—reside in the gut, and the balance of the two seems to be important in determining an individual's propensity for obesity.
Hampton T. Studies Link Intestinal Microbes With Obesity. JAMA. 2007;297(4):352–353. doi:10.1001/jama.297.4.352