|Credit: Image created by Ivana Semova, PhD|
Certain gut microbes, known as Firmicutes, whose abundance varies from individual to individual, seem to be critical in fat absorption in the stomach and may be a major cause behind the obesity pandemic.
So far this has only been tested in zebrafish (a model organism not so different from us) but there are previous studies in humans that do suggest that the abundance of these microbes may be critical for us too. If so, it would explain how the long-term diet and not just the short-term one affects weight because the abundance of these bacteria would mostly be affected by long term trends, beginning in childhood.
Ivana Semova, Microbiota Regulate Intestinal Absorption and Metabolism of Fatty Acids in the Zebrafish. Host & Microbe 2012. Freely available at the time of this citation ··> LINK [doi:10.1016/j.chom.2012.08.003]
Main source: Science Daily.