Obesity has its obvious manifestations; it's a disease that is difficult to conceal. And now, doctors say they can even smell it on your breath.

Doctors from Cedars-Sinai Medical Center in Los Angeles say certain gas-emitting microbes living in the human gut might determine one's propensity for packing on too many pounds; and the presence of methane and hydrogen on one's breath from these microbes is closely related to excess body weight and body fat.

These doctors concede that overeating and a lack of activity are the primary causes of obesity. Yet other factors — namely, the abundance or reduction of certain microbes that line the intestines, detectable on the breath — also may contribute to excessive weight gain.

Their work will appear online March 28 in the Journal of Clinical Endocrinology & Metabolism.

Hungry microbes

The concept that gut microbes are linked to obesity is not new. Dozens of papers have been written on the topic in the last few years alone. Scientists don't understand, however, which microbes are the chief culprits. [Why We're Fat: 8 Surprising Causes of Obesity]

Doctors led by Ruchi Mathur, director of the Cedars-Sinai Diabetes Outpatient Treatment and Education Center, focused on a methane-producing microbe called Methanobrevibacter smithii. The theory is that M. smithii eats the hydrogen produced by other microorganisms in the gut. Lower hydrogen levels, in turn, increase fermentation in the gut, allowing the body to absorb more nutrients and more energy, or calories, from food.

"Usually, the microorganisms living in the digestive tract benefit us by helping convert food into energy," said Mathur. "However, when this particular organism, M. smithii, becomes overabundant, it may alter this balance in a way that causes someone to be more likely to gain weight."

Mathur's previous study on rats, published last year, seemed to indicate that M. smithii promoted weight gain. An abundance of M. smithii could leave detectable levels of methane on the breath of obese human subjects as smoking-gun evidence of this microbe's role in obesity, Mathur said.

Breath test

So, Mathur's group analyzed the breath of 792 people. They found that the subjects either had normal breath content, higher concentrations of methane, higher levels of hydrogen, or higher levels of both gases. Those who tested positive for high concentrations of both gases had significantly higher body mass indexes and higher percentages of body fat.

But the presence of hydrogen with methane on the breath of obese subjects may indicate that more than the methane-producing M. smithii is to blame. Or, at a minimum, the picture is complex.

Indeed, French researchers publishing a study in June 2012 in the International Journal of Obesity found that the gut microbe Lactobacillus reuteri was most associated with obese subjects in conjunction with lower, not higher, levels of M. smithii. An earlier study by some of these same French researchers, published in the journal PLOS ONE in 2009, found excess M. smithii associated with anorexia.

Mathur's group also could not ascertain cause and effect: Is obesity caused by a certain imbalance of microbes, or does a diet associated with obesity — for example, highly processed foods rich in calories and low in nutrients — change the gut microbe flora in such a way that promotes obesity. That is, the obesity bug is not something one catches randomly like the flu. The microbes most recently associated with obesity are in all human guts, along with trillions of other microbes.

What is increasingly clear, however, is that some people put on weight more easily than others. Is this entirely genetics, or gut microbes, or both? Further studies of the gut microbe flora may lead to new insights … and therapies.

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