Two new studies show that obese people have different intestinal bacteria than slim people. What's more, the microbes in an overweight body are much more efficient at extracting calories from food.
One study looked at mice, the other looked at humans. In both, a family of bacteria known as firmicutes were more plentiful in the obese (20 percent more). Bacteria called bacteroidetes were also much more abundant in those of normal weight (the obese had almost 90 percent fewer bacteroidetes).
Most likely because of the firmicutes, the obese mice were more efficient at taking calories out of complex sugars and depositing those calories in fat. When these microbes were transplanted into the normal-weight mice, those mice started to gain twice as much fat.
As obese people lost weight, their bacteroidetes increased, while the numbers of firmicutes decreased.