The vast, diverse community of microbes inhabiting the intestines, scientists are finding, can influence metabolism and weight.
The story in humans is far from certain, though, and scientists say it's too soon to concoct microbe-filled "stay-slim" beverages — a fact that has not prevented some companies from promoting their bacteria-laden products as helpful for weight loss.
Bacteria that draw the maximum calories from our food would have been useful to our hunter-gatherer ancestors but are less beneficial for modern people eating an American fast-food diet. In addition to our ready access to high-calorie eats, the bacteria we carry around have changed, says Andrew Gewirtz, an immunologist at Emory University in Atlanta, through antibiotic use, improved hygiene and cleanliness in the food supply.
This, he believes, could be one environmental cause for obesity and related conditions such as diabetes.
On the whole, our gut bacteria are beneficial, says Ruth Ley, a microbial ecologist at Cornell University in Ithaca, N.Y. They prevent disease-causing bacteria from taking hold in our body simply by filling up all the available space. And they help us digest foods, such as some starches, that we cannot break down ourselves, producing vitamins and energy sources we can use.
"You might just generally be sicklier without them," Ley says.
Babies are born bacteria-free but start to pick up bacteria during and after birth. Infants mostly collect bacteria from their mothers and others around them; in a sense, the gut community is inherited from family members. If the gut-obesity theory proved correct, that would suggest obesity risk could be passed along with them.
"If a person has changes in their gut bacteria — and that could be due to anything, to diet, to antibiotic use — if that person has kids, then they can transfer those gut bacteria and maybe transfer the problem," Gewirtz says.
Because people pick up different bacteria from their environments, people have different gut communities. For example, in a study published in the journal Nature in April, scientists reported that some Japanese people could digest compounds in nori, the seaweed in sushi, because they hosted the right bacteria for the job.
Every person carries at least 160 different kinds of gut bacteria, scientists estimated in another Nature study published in March. Most fall into two divisions, or phyla: the bacteroidetes and the firmicutes. Both of these groups are found in soil and water as well as in animals. Some cause disease, but many in the intestine are beneficial. The firmicutes, in particular, are good at digesting our food. The more firmicutes in a person's intestines, the more calories they can collect from a meal.
This is where the obesity link comes in.
In 2005, Ley and colleagues at the Washington University School of Medicine in St. Louis, where she worked at the time, studied the gut bacteria found in mice, hoping to use them as a model to study obesity. They compared normal, lean mice with ones that were genetically obese because they had a mutation in the hormone leptin, which normally controls appetite and metabolism.
As in people, the main intestinal inhabitants of mice were bacteroidetes and firmicutes. But the researchers discovered that obese and lean mice had different proportions of each. In particular, fat mice tended to have more firmicutes, and fewer bacteroidetes, in their guts than lean mice.
In another study, Ley and colleagues worked with sterile mice that have no gut bacteria. These mice eat a lot, but don't get fat, presumably because they don't have the bacteria to extract the full complement of calories from their food. But when the scientists transferred the bacteria from fat mice to bacteria-free mice, the recipient mice gained weight. This result, reported in a 2006 Nature paper, directly suggests there's something about the bacterial community in the obese mice that contributes to weight gain.
What about human beings? Ley and co-workers examined the proportion of firmicutes and bacteroidetes in 12 people, in a study they also reported in Nature in 2006. They examined bacteria samples from obese people as they followed a yearlong diet program. Before the diet, the subjects had more firmicutes and fewer bacteroidetes than healthy-weight people. As the year progressed and the dieting continued, the bacteroidetes numbers went up, and the firmicutes numbers went down.
In addition to squeezing every last available calorie out of the food we eat, bacteria can also influence metabolism by signaling the body to store fat or burn less of it, and slow down food moving through the intestine so there is more time to collect calories.