Oops, sorry I forgot to post a bit of an explanation.
There was a study done that shows that people who are obese have a larger number of a certain type of microbe in there stomach and if those microbes are found in mice as well. If those microbes are transferred to a mouse that is not overweight it will gain weight.
Apparently these microbes are responsible for increasing the use and absorption of food.
The clips explain it but are a bit dull.
I was going to link the article but I cannot sorry.
There is also some postuation that the gut flora balance affects hormones which regulate food absorption, processing of proteins, fats and sugars, etcetera.