Some people eat and eat and never seem to gain weight -- until metabolic changes occur.
Many think it's because of age, but now new evidence suggests an association between weight gain and an overabundance of "gut bacteria" called Methanobrevibacter smithii.
Scientists performed breath analyses. People with more methane and hydrogen in their breath had more M. smithii.
"These hydrogen-producing organisms actually aid fermentation in the gut, which fermentation is another way to metabolize calories," said Dr. Ben Garrett, Providence St. Joseph Medical Center.
Researchers found people with more M. smithii had more body fat. Dr. Ben Garrett says these bacteria may help the body absorb calories more efficiently.
"If you do more fermenting you create more calories that are available to the person," said Garrett.
A medium apple has about 80 calories. According to this theory, somebody with high concentrations of the M. smithii bacteria would metabolize all 80 of the calories. Somebody who does not may metabolize just 70 calories. And over time someone who is ingesting all those extra calories would gain weight.
"If the difference is 100 calories a day, that's a pound of fat within a month or a month and a half," said Garrett.
Researchers hope their findings could someday lead to new ways to treat obesity.
But Dr. Garrett says while some people may be more efficient at absorbing calories, you can't put all the weight gain on bacteria.
"It's our own hands feeding our own mouths," said Garrett.