Are you fat? It may not be your fault, blame the bacteria in your intestines instead:
Ninety percent of the bacteria fall into two major divisions, or phyla: the Firmicutes and the Bacteroidetes. Previous research had shown that obese mice had higher levels of Firmicutes, and lean ones had more Bacteroidetes.
Analyzing the genomes of the bacteria, Gordon and graduate student Peter Turnbaugh concluded that the Firmicutes were more efficient at digesting food that the body can't.
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.
When the researchers transferred bacteria from obese mice into so-called gnotobiotic mice, which were raised in a sterile environment and had no bacteria in their guts, the mice gained more weight than did those receiving a similar amount of bacteria from lean mice, even though they were fed the same diet.