Packing on the pounds? Blame that red-eye flight. (Reuters/Toby Melville)

All living organisms -- including the bacteria that populate your body -- have biological clocks that can get thrown off by international travel. In humans, the result is jet lag. But at the microbial level, the result could be a gut full of the wrong kinds of bacteria -- including species that promote obesity.

In a study published Thursday in Cell, researchers looked at the microbiome -- or bacterial populations --  in the fecal matter of rodent and human subjects. The abundance of different bacteria (and their activities) changed based on the time of day the sample was taken.

The researchers then messed with their subjects' biological rhythm. For the two human subjects, this meant a flight from the United States to Israel. The mice were kept at home, but the researchers manipulated their light sources and meal times to simulate a trip across time zones. In both cases, the gut microbiome was affected.

The jet-lagged mice started putting on more weight than those that hadn't "traveled," even though they were on the same diet. When the researchers transferred gut bacteria from jet-lagged mice to healthy ones, these new mice saw the same effect.

Humans were similarly affected: The two participants had an increase in bacteria that have been linked to obesity. But their microbiome shifted back to a healthy baseline after only a couple of weeks.

"These findings provide an explanation for a long-standing and mysterious observation, namely that people with chronically disturbed day-night cycles due to repetitive jet lag or shift work have a tendency to develop obesity and other metabolic complications," senior author Eran Elinav of the Weizmann Institute of Science said in a statement. "These surprising findings may enable us to devise preventive treatments for these people to lower their risk for these complications."

But the actual health implications remain to be seen, Elinav told Nati0nal Geographic. It may not be that traveling across time zones causes weight gain, and even if it does, the microbiome might just be one piece of the puzzle.