A bug, or microorganism, whose presence appears on breath tests may help identify if a person is susceptible to obesity.

The obesity bug is a microorganism living in the human digestive tract. A study from Cedars-Sinai in Los Angeles found that a breath test may help doctors determine if a person has the obesity bug living in the digestive system, and thus is more likely to be obese, according to LiveScience.

The doctors maintain that overeating and lack of activity are the primary causes of obesity. However, other factors can contribute to excessive weight gain.

The obesity bug is called Methanobrevibacter smithii. It is identified by high methane and hydrogen levels in the breath. People with high levels of methane and hydrogen are more likely to have higher Body Mass Indexes and higher percentages of body fat than those with more normal levels of the gasses, according to the study.

"Usually, the microorganisms living in the digestive tract benefit us by helping convert food into energy. 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," explained Ruchi Mathur, the lead author of the study.

Cedar-Sinai researchers measured how quickly food moved through the bodies of 792 people, and how much caloric value was extracted from the food before and after antibiotics were used to remove some of the M. smithii from their digestive tracts.

The study suggests that M. smithii, which scavenges hydrogen from other digestive tract microbes and turns it into methane, makes other bacteria more efficient. Mathur explained, "Essentially, it could allow a person to harvest more calories from their food."

The study will be published in the April issue of the Journal of Clinical Endocrinology & Metabolism, to be released on March 28.