Story highlights Thanks to gut bacteria, obesity may be detectable as a gas

A study shows heavier people have higher levels of methane on their breath

Controlling bacteria with antibiotics could help slow weight gain in some people

According to the latest research, it may be on your breath.

It turns out that obesity may be detectable as a gas, thanks to organisms that inhabit our gut. In a study published in the Journal of Clinical Endocrinology & Metabolism, researchers extend our knowledge about the hidden universe of the microbes that live within us to show that obesity is associated with certain populations of microbes that give off a distinctive gas.

To be more specific, obesity may smell a lot like ... methane, which is to say, like not much at all, since methane in its naturally-occurring state is actually odorless.

In the study, Dr. Ruchi Mathur, director of diabetes in the department of medicine at Cedars-Sinai Medical Center, and her colleagues analyzed the breath of 792 men and women of various ages.

Mathur focused on detecting methane in the breath, since animal studies found that the presence of a certain family of organisms called archaea, which are older than bacteria and colonize the gut, was linked with weight gain and conveniently released small amounts of methane gas.

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Mathur also knew from her own work analyzing the gas makeup of the breath from bariatric surgery patients that those releasing higher levels of methane in their breath tended to have a body mass index (BMI) nearly seven points higher on average than those with lower levels.

And sure enough, Mathur found that among the nearly 800 participants she tested, those with higher levels of methane (three or more parts per million over 90 minutes) and hydrogen gases (20 or more parts per million) in their breath also tended to be heavier, with a BMI about 2.4 points greater than those with normal levels of the gases and about 6% more body fat on average.

"Our hope is that this is one piece of the complex puzzle that is obesity," says Mathur, "and that by identifying people who are obese because they have this microorganism, we can manipulate and work with the gut microbiome to lead to benefits in weight loss in that subgroup."

The culprit, she believes, is a member of archaea known as Methanobrevibacter smithii, which is present in the intestinal tract of about 70% of people, but elevated in about 30%. It's that smaller group of individuals who might be genetically predisposed to harboring levels of M. smithii that might put them at higher risk of developing obesity.

M. smithii harvests hydrogen molecules from neighboring microbes in the gut, which it then transforms into methane gas. The more it scavenges hydrogen from its environment, the more other microbes produce. But all of that activity is focused on extracting energy and nutrients from food, so along with the hydrogen gas, the microbes are also packing in more calories for the host, which can lead to weight gain.

It's also possible, says Mathur, that the release of methane slows the transit of digested food through the intestinal tract, and that could increase the time for additional calories from digested food to be absorbed and added to the body's tally.

In order for M. smithii to thrive, it needs the hydrogen from surrounding microbes, and that may be why people with higher levels of both hydrogen and methane gases in their breath were heavier than those with elevated levels of methane or hydrogen alone.

So how does this help control the obesity epidemic? For those whose weight gain may be due in part to the activity of M. smithii, controlling the organisms with antibiotics or other medications could slow down the rate at which they pack on the pounds, and these individuals could easily be identified with a relatively simple breath test.

Mathur and her colleagues are also working with the American Diabetes Association to test a group of people with prediabetes who are overweight or obese and have elevated levels of methane in their breath. The researchers will test the participants' glucose tolerance, the time it takes for digested food to transit through the intestinal tract, and the amount of calories in the patients' stool.

Then the volunteers will be given an antibiotic to essentially wipe out the population of M. smithii and the same parameters will be measured again, to see if eliminating the microbes will help change the patient's weight profile and alter their trajectory toward diabetes.

She is also studying a group of children to see how early M. smithii buildup occurs, and how soon in development it starts to set up a pattern of weight gain that might then be interrupted by changing the composition of the gut microbial world.

"From an evolutionary perspective, our relationship with the microorganisms that live in us has basically been symbiotic, and we have evolved together," she says. "We've had that relationship for millennia, but it is just now being explored and discovered in more detail." And, when it comes to controlling the obesity epidemic, could lead to the (sweet?) smell of success.