Many people worry about putting on a few pounds during the holiday season. But when you reach for a Christmas cookie, keep in mind that you're not the only one who's going to enjoy that tasty treat: It will also get eaten by the bacteria living in your gut.
And it turns out that the kind of bacteria living there may affect how much weight you gain. Until a couple of years ago, scientists didn't have the tools to figure out exactly what lives in a person's digestive tract. But with new genetic probes, they can do a kind of census.
The researchers found that "obese bacteria" might lead to more weight gain. The bacteria have genes that make them better at harvesting calories from food.
Jeffrey Gordon studies bacteria at Washington University in St. Louis. He says that the trillions of bacteria help us break down food that, otherwise, we couldn't digest.
"They're able to persist in the gut despite this continuous flow of Diet Coke, burgers, whatever," Gordon says, "in this very dynamic ecosystem, called our gut."
Gordon and Ruth Ley, a member of his lab, compared the gut bacteria of fat and skinny mice by testing mice that had a gene for obesity — and their siblings that lacked the gene.
It turned out that the obese mice had a smaller proportion of a kind of bacteria known as Bacteroidetes.
Next, Ley looked at twelve obese people. The results, she says, were "just like the mice." And as the 12 people lost weight over a year, their gut populations changed, becoming more and more like those in skinny mice.
The issue, then was to determine which came first: the fat, or the bacteria. To find out, the lab took mice that had never been exposed to any bacteria, whose guts were totally germ-free. Half of them got bacteria taken from skinny mice. The other half got bacteria from fat mice.
Both groups put on body fat. But the mice that received bacteria from obese donors gained more fat over the course of the experiment.
It remains unknown how much of a difference microbes might make to obesity and fat levels when compared to, say, exercise.
Gordon says it's too early to try to use the new ideas to treat obesity. "I am worried that people will start trying to treat themselves," he says, noting that the possibility is "very dangerous and very premature."
He says it will take much more work to learn whether changing a person's existing gut bacteria can make them lose or gain weight.