Say Hello to the Bugs in Your Gut

One implication of this work is that the energy content of food isn't a fixed quantity. Consider the 110 calories per cup listed on a box of Cheerios. Some people may get that much, others may get less, depending on their gut microbiota. A difference of just 25 calories a day—that's half a rice cake or one chocolate kiss—between what you take in and what you burn could mean a gain or loss of more than two pounds in a year and 20 pounds over a decade.

On a more practical note, this work suggests that somehow altering the microbial populations in the gut could be one way to modify weight. If Gordon's work continues to pan out, it may be possible someday to use probiotics—dietary supplements containing potentially beneficial microbes—or other microbe-manipulating strategies to aid weight loss by nudging the gut microbiota to be less efficient at extracting energy.

Probiotics are already on the market, most of them containing some form of Lactobacillus, best known for its yogurt-making abilities. They're used to fight allergies, diarrhea and a variety of other conditions, although the evidence for their use remains spotty. Major companies such as General Mills and food-ingredient supplier Danisco are exploring links between probiotics and weight control.

It's a bit early to do such microbial gardening for weight loss, cautions Randy Seeley, associate director of the Obesity Research Center at the University of Cincinnati College of Medicine. Though he's impressed with the work Gordon's team has done, he isn't sure the results make sense from an evolutionary perspective. As the obese volunteers lost weight, their gut microbes shifted toward a community that would extract less energy from its food supply. That doesn't make sense from a survival point of view: "If I saw myself getting leaner, I'd want my body to say to my microbes, 'Guys, help me out here,' and make the extra more calories, not fewer," says Seeley.

Gordon is the first to acknowledge that there's a lot of work to be done before anyone can point to gut microbes as a cause of obesity or start manipulating them as a way to lose weight. Even if this line of research doesn't pan out, the convergence of microbiology, molecular biology and a host of other disciplines will shine new light on how we process what we eat and what causes obesity.

While scientists have known for more than a century that we humans live with a huge community of permanent tiny neighbors, it is only recently that research like Gordon's has suggested that these neighbors may have unexpected effects on our health. In response, the National Institutes of Health has launched the Human Microbiome Project, in order to learn more about our gut bugs—starting with their genes. Sequencing of bacterial genes could also help researchers prospect for hitherto unknown chemicals made by our microbes that protect our health. What is making the Human Microbiome Project feasible is the recent development of superfast gene sequencing technologies.