A popular diet regimen can cause permanent changes to both your brain and the bacteria in your gut, research has found.

This Weight Loss method, known as “intermittent energy restriction” (IER), involves switching between days of eating normally and days of fasting. Researchers found that this diet can affect the interplay between brain activity and the gut microbiome—the collection of good bacteria living in your gut.

Understanding these relationships, the team said, could shine a light on the factors involved in successful dieting and maintaining a healthy weight. The full findings of the study were published in the journal Frontiers in Cellular and Infection Microbiology.

“The composition of the gut microbiome is greatly influenced by dietary choices,” Professor Qiang Zeng, a researcher at the Health Management Institute of the PLA General Hospital in Beijing, told Newsweek. “Altering one’s dietary habits leads to changes in the microbiome’s composition. Other studies have also demonstrated that intermittent fasting can modify both the diversity and composition of the gut microbiome.”

Zeng and colleagues on the team monitored the gut microbiome, blood, and brain activity of 25 Chinese adults with an average age of 27 years. All the participants were obese, with a body mass index between 28 and 45. A person is classified as obese if the BMI is over 30.

Obesity is a major risk factor for several diseases, including cardiovascular disease and diabetes. Over 1 billion people worldwide are obese, and many try to lose weight using various diets, including IER.

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In the study, each participant underwent two dietary phases: a “high-controlled” fasting phase and a “low-controlled” fasting phase.

During the high-controlled phase, participants spent 32 days receiving meals from dietitians that slowly decreased in calorie count down to around one-quarter of their basic energy needs.

For 30 more days after this, they underwent the low-controlled fasting phase, where they were given a list of recommended foods designed to give the women only 500 calories each day and the men 600 calories per day.

Following these fasts, the participants were found to have lost an average of around 17 pounds—roughly 7.8 percent of their average body weight.

The authors also found that the patients’ microbiomes changed, with some bacterial species increasing sharply in numbers while others, including Escherichia coli, fell.

E. coli is a common bacterium, and some strains cause such sicknesses as diarrhea, urinary tract infections, respiratory illness and pneumonia.

“Significant alterations were observed in certain gut bacteria abundances due to the IER diet, including pathogenic E.coli as well as obesity-related probiotics Faecalibacterium prausnitzii, Parabacteroides distasonis, and Bacterokles uniformis. However, it should be noted that data regarding the gut microbiome is extensive; therefore further research is required to explore specific microbial populations involved in Weight Loss,” Zeng said.

The researchers reported that certain regions of the brain associated with the regulation of appetite and addiction saw decreased activity levels after the fasts and said this was a result of the microbiome changes.

The team found that a high abundance of some bacteria, including E. coli, was associated with reduced activity in the region of the brain that plays a role in willpower when losing weight.

They also found that an abundance of other species of bacteria was positively correlated with increased activity in brain regions associated with attention, motor inhibition, emotion and learning.

“The gut microbiome is thought to communicate with the brain in a complex, two-directional way,” said paper co-author Xiaoning Wang of the Institute of Geriatrics at the PLA General Hospital in a statement.

“The microbiome produces neurotransmitters and neurotoxins which access the brain through nerves and the blood circulation. In return the brain controls eating behavior, while nutrients from our diet change the composition of the gut microbiome,” Xiaoning said.

This implies that changes in the brain and changes in the microbiome after Weight Loss affect each other.

However, because the study looked only at correlations, not causation, the researchers still aren’t sure exactly what drives these changes or why the brain and microbiome have such a degree of interplay.

“The next question to be answered is the precise mechanism by which the gut microbiome and the brain communicate in obese people, including during Weight Loss,” said Liming Wang, a paper co-author and a researcher at Beijing’s Health Management Institute.

He asked: “What specific gut microbiome and brain regions are critical for successful Weight Loss and maintaining a healthy weight?”

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Update 12/20/23, 1:16 p.m. ET: This article was updated with comment from Qiang Zeng.