New York, March 13 (IANS): Breathing dirty air takes a heavy toll on gut bacteria, boosting risk of obesity, diabetes, gastrointestinal disorders and other chronic illnesses, a new study revealed.

The study, published in the journal Environment International, is the first to link air pollution to changes in the structure and function of the human gut microbiome - the collection of trillions of microorganisms residing within us.

The research found, with young adults exposed to higher levels of ozone showing less microbial diversity and more of certain species associated with obesity and disease.

"We know from previous research that air pollutants can have a whole host of adverse health effects," said study senior author Tanya Alderete, Assistant Professor at University of Colorado Boulder in the US.

"The takeaway from this paper is that some of those effects might be due to changes in the gut," Alderete added.

Worldwide, according to research published this month, air pollution kills 8.8 million people annually - more than smoking or war.

While much attention has been paid to respiratory health, Alderete's previous studies have shown pollution can also impair the body's ability to regulate blood sugar and influence risk for obesity.

Other research has shown visits to emergency rooms for gastrointestinal problems spike on high pollution days, and youth with high exposure to traffic exhaust have greater risk of developing Crohn's disease.

To investigate just what might be going on inside the gut, the research team used cutting-edge whole-genome sequencing to analyse fecal samples from 101 young adults in Southern California.

The researchers looked at data from air-monitoring stations near the subjects' addresses to calculate their previous-year exposure to ozone (which forms when emissions from vehicles are exposed to sunlight), particulate matter (hazardous particles suspended in the air), and nitrous oxide (a toxic byproduct of burning fossil fuel).

Of all the pollutants measured, ozone had the greatest impact on the gut by far, accounting for about 11 per cent of the variation seen between study subjects - more of an impact than gender, ethnicity or even diet.

Those with higher exposure to ozone also had less variety of bacteria living in their gut, according to the study.

"This is important since lower (bacteria) diversity has been linked with obesity and Type 2 diabetes," noted Alderete.

Subjects with higher exposure to ozone also had a greater abundance of a specific species called Bacteroides caecimuris.

That's important, because some studies have associated high levels of Bacteroides with obesity.

In all, the researchers identified 128 bacterial species influenced by increased ozone exposure.

Some may impact the release of insulin, the hormone responsible for ushering sugar into the muscles for energy.

Other species can produce metabolites, including fatty acids, which help maintain gut barrier integrity and ward off inflammation, the researchers said.

"Ozone is likely changing the environment of your gut to favour some bacteria over others, and that can have health consequences," Alderete concluded.