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One Surprising Side Effect of Cutting Calories, New Study Finds

It may be more than just weight loss you experience when you scale back your caloric intake.

For many people trying to lose weight, counting—and cutting—calories is a tried-and-true method of shedding pounds without having to adhere to a specialized diet. However, new research suggests that it's not just your weight that may change when you stick to a calorie-restricted eating plan.

According to a June 2021 study published in the journal Nature, calorie restriction can have a significant effect on a person's gut microbiome, as well. To conduct their study, a group led by researchers from the University of California San Francisco (UCSF) monitored a group of 80 overweight and obese women over a 16-week period. During this time, half of the study population went on a liquid diet consisting of 800 calories a day, while the other half of the study subjects maintained their weight.

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Among the group adhering to the 800-calorie diet, researchers found that "this very-low-calorie diet profoundly altered the gut microbiome, including an overall decrease in gut bacteria," explained Peter Turnbaugh, Ph.D., an associate professor of microbiology and immunology at UCSF and a senior author on the study, in a statement.

Fecal samples from study subjects were then transplanted into mice raised in sterile conditions. What researchers discovered was that the mice whose transplants included samples of dieter's post-calorie-restriction fecal bacteria lost weight. The main difference between the gut bacteria of those on the diet and those who weren't? The amount of C. difficile, a bacterium that is associated with diarrhea and inflammation of the digestive tract.

woman's abdomen and belly button, she is touching her slim stomach with two hands

"Ordinarily we would predict increased inflammation or even colitis following an increase in C. difficile," explained Turnbaugh. In this case, however, the mice with increased C. difficile counts showed only a mild increase in inflammation.

While Turnbaugh explained that the study's findings in no way suggest that the voluntary introduction of C. difficile should become a go-to strategy for weight loss, he noted that the surprising findings certainly merit further study.

"We want to better understand how common weight loss diets might impact and the microbiome and what the downstream consequences are for health and disease," said Turnbaugh of the study's results.

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Sarah Crow
Sarah Crow is a senior editor at Eat This, Not That!, where she focuses on celebrity news and health coverage. Read more about Sarah