One Major Side Effect of Eating a Low-Calorie Diet, Says New Study

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If you're looking to lose weight, slashing your calories for a brief time might seem tempting—especially if you start dropping pounds almost right away—but you could be causing a negative ripple effect in your gut, according to new research in Nature.

To determine what kind of link existed between caloric intake and gut bacteria changes, researchers studied 80 older women whose weight ranged from slightly overweight to severely obese. For 16 weeks, half of the participants followed a medically supervised meal replacement plan that consisted of shakes that totaled fewer than 800 calories per day, and the other half served as a control group by maintaining their usual habits and weight for those months.

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Gut bacteria analysis was done for all participants before and after the study period. Not surprisingly, the number and type of microorganisms in the control group didn't change. But there were significant shifts for the low-calorie diet group.

For them, the bacteria adapted in order to absorb more sugar molecules in order to survive. That caused an imbalance in the type of bacteria that thrived and caused a considerable increase of harmful bacteria—most notably, it prompted a surge of Clostridioides difficile, also known as C. diff. This is a notoriously problematic type of bacteria, according to the Centers for Disease Control. It tends to cause diarrhea and colitis and can become chronic even when treated regularly.

These results aren't surprising, according to Kristin Gillespie, RD, a nutritional counselor. Diet plays a significant role in gut health and other research has shown a relationship not only with the quality of our diet, but also the quantity of food consumed when it comes to keeping a good balance of beneficial gut bacteria, she says.

"Changes in diet, such as calorie restriction or shift in macronutrient intake, like going keto or low-carb, can impact the diversity, amount, and overall composition of the gut microbiome," says Gillespie.

The good news is that previous research also shows that these changes can be short-lived if you transition back to adequate calorie levels. A better strategy may be to skip the very low-carb plan and focus instead on diet quality, she adds. You might not see the dramatic changes in the short term, but it's likely your gut will be happier in the long run.

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