According to the November 11, 2009 University of Washington press release “Bacteria in intestines play role key role in weight gain, study finds,” bacteria in the gut are generally divided into one of two classifications: Firmicutes or Bacteroidetes.
The gnotobiotic mice that received bacteria from the lean mice had more Bacteroidetes in their guts than Firmicutes.
However, when these mice were switched from a low-fat, plant-rich diet to a high-fat, high-sugar diet, the amount of bacteria reversed themselves within 24 hours, now having more Firmicutes than Bacteroidetes.
The Gordon study found that the gnotobiotic mice were more efficient at digesting food that is normally expelled from the body (waste foods, or foods not of a high nutritional value) when more Firmicules (specifically Erysipielotrichi and Bacilli) were present than Bacteroidetes.
That is, when the body has more Firmicultes in the intestines, a higher proportion of food is digested and absorbed into the body as calories. Thus, more Firmicultes means that it is easier to gain weight and more difficult to lose it.
It was stated in the paper, “Animals that have a higher proportion of Firmicutes convert a higher proportion of food into calories that can be absorbed by the body, making it easier to gain weight.”
The Washington University press release quotes Dr. Gordon.
He states, "Pinpointing triggers of obesity or malnutrition in humans is hard because there's a host of factors - genetic, cultural and environmental, such as diet - that are extremely difficult to control. Recreating the human gut ecosystem in mice gives us a way to control these variables. The information gained from these studies allows us to develop hypotheses that we can test in humans."
The press release also states, “Obesity affects an estimated 300 million people worldwide, according the World Health Organization. The condition increases the risk for a host of illnesses, including type 2 diabetes, heart disease, stroke and certain cancers.”