Gut bacteria affect our eating habits, a new study suggests.

Trillions of bacteria live in our guts and research has already shown that gut microbes help with digestion. Now, researchers at the UC San Francisco, the Arizona State University and the University of New Mexico, have found that these gut-dwellers might be influencing our food habits including cravings.

For the study, the team conducted a review of all the studies published on the subject. They found that these bacteria aren't some passive guests that live-off random nutrients in the gut, but actually influence the body to seek unique blend of nutrients that they thrive on.

Certain bacteria prefer fats, while others sugars - researchers say. Each bacterial community sends molecular signals to the immune and the nervous system to influence food choices.

"Bacteria within the gut are manipulative," said Carlo Maley, PhD, director of the UCSF Center for Evolution and Cancer and corresponding author on the paper, according to a news release. "There is a diversity of interests represented in the microbiome, some aligned with our own dietary goals, and others not."

The good news is that the microbiome in the gut can be altered to suit our choices. The researchers say that eating certain foods or following a certain diet, even for just 24 hours, can dramatically shift the population of microbes in the guts.

"Our diets have a huge impact on microbial populations in the gut," Maley said. "It's a whole ecosystem, and it's evolving on the time scale of minutes."

The vagus nerve, the researchers say, might be helping bacteria alter our food preferences. This vagus nerve connects 100 million nerve cells from the digestive tract to the base of the brain. Microbes shift our dietary preferences by changing the signals in the vagus nerve. For example, bacteria communities might produce toxins, which make us feel bad. Similarly, these bacteria can even send feel-good signals to the brain.

Study authors believe that altering bacterial communities in the gut - via diet, ingestion of probiotics or using antibiotics to kill unwanted bacteria - can all change our dietary habits as well as moods.

The researchers said that they were funded by the National Institutes of Health, the American Cancer Society, the Bonnie D. Addario Lung Cancer Foundation and the Institute for Advanced Study, in Berlin. Their article is published in the journal BioEssays.