A mushroom a day may keep the doctor away. The active ingredients in a Asian mushroom can change bacteria in your stomach for the better, researchers have found. By altering gut bacteria, the Reishi mushroom (Ganoderma lucidum) prevents and reverses symptoms of obesity in mice, including weight gain and insulin resistance.

“Mice kept on a high-fat diet gained up to 25 percent more than mice kept on the same diet with extracts from the Reishi mushroom,” said David Ojcius, a microbiologist at the University of the Pacific School of Dentistry in San Francisco who participated in the study, published Tuesday in the journal Nature Communications. Consumption of Reishi mushroom extract with high-fat food prevented the development of fat tissue, gut inflammation and buildup of harmful bacteria in the bloodstream — all symptoms of obesity in both mice and humans.

Ojcius and colleagues from Chang Gung University and Chang Gung Biotechnology in Taiwan treated mice kept on a high-fat diet with various compounds from Reishi mushrooms, called polysaccharides, to see which ingredient played the biggest role. The ratio of two different types of gut bacteria, firmicutes and bacteroidetes, can indicate gut health, Ojcius said. In this case, a subset of the polysaccharides from the Reishi mushroom altered the bacterial ratio in favor of a healthier gut.

If the Reishi mushroom works by changing gut bacteria, then could introducing healthier bacteria into the stomach be enough to save a fat mouse?

Only the poop can tell.

Doctors have been known to use fecal transplants to alter the gut bacteria of patients with a variety of metabolic disorders, such as Crohn’s disease. Ojcius and collaborators transplanted fecal matter from mice that were fed high-fat diets “seasoned” with Reishi mushroom into already obese mice. These transplants reduced obesity symptoms more than ones from mice kept on a high-fat diet without Reishi seasoning.

The findings in this study suggest that Reishi can promote stomach health in people already experiencing symptoms of obesity, Ojcius said.

But don’t rush to buy out mushroom markets just yet. Reishi extract only helped to slim fat mice; it doesn’t have a strong effect on mice eating regular chow. Having a healthy gut is what prevents metabolic disease, said Ojcius, as evidenced by the observation that fecal transplants from healthy, lean mice, regardless of Reishi mushroom consumption, also reduced body weight in obese mice.

The technology to identify multiple bacteria in a stool sample has emerged only within the last decade. As we know more about the gut’s role in health, Ojcius said, technology that analyzes poop could help doctors or patients monitor gut bacteria and prevent oncoming issues.

Exercise, overall diet and genetics also help define the body’s internal bacteria, Ojcius points out.

Reishi, like many other natural medicinal products, isn’t regulated by the Food and Drug Administration. Because anyone can sell Reishi extract, Ojcius suggests doing research before purchasing.

Correction: An earlier version of this story misstated David Ojcius’ role in the study published June 22 in the journal Nature Communications. He participated in, but did not lead the study.