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In a nutshell
- Northwestern University researchers found that just three days of juice cleansing can significantly alter oral bacteria, increasing potentially harmful species while decreasing beneficial ones
- While gut bacteria proved more resilient, juice-based diets increased bacterial species associated with inflammation and gut permeability issues
- In contrast, eating whole fruits and vegetables during the study’s elimination diet phase increased beneficial gut bacteria that produce important compounds for digestive health
Study shows concerning bacterial changes triggered by detox diets
CHICAGO — Juice cleanses have become increasingly popular, with 26% of consumers trying these programs for perceived benefits like detoxification and improved digestion. But new research suggests these trendy cleanses may have unexpected effects on the trillions of beneficial bacteria living in our mouths and guts.
Northwestern University researchers conducted a study examining how different types of juice-based diets affect our microbiome, the vast community of microorganisms that play crucial roles in our health. Their findings, published in the journal Nutrients, reveal that even short-term juice cleanses can rapidly alter our bacterial communities, potentially in concerning ways.
Modern juicing removes most of the insoluble fiber found in whole fruits and vegetables while concentrating their natural sugars. This transformation might diminish some of the health benefits we expect from produce, particularly when it comes to feeding our beneficial gut bacteria, which thrive on dietary fiber.
“Most people think of juicing as a healthy cleanse, but this study offers a reality check,” said senior author Dr. Melinda Ring, director of the Osher Center for Integrative Health at Northwestern University Feinberg School of Medicine, in a statement. “Consuming large amounts of juice with little fiber may lead to microbiome imbalances that could have negative consequences, such as inflammation and reduced gut health.”
To investigate these effects, researchers recruited 14 healthy adults between ages 18-35 and randomly assigned them to follow one of three different three-day dietary protocols: an exclusive juice fast, a diet combining juice with regular food, or a plant-based whole food diet. The scientists collected samples from participants’ saliva, inner cheek, and stool at four different timepoints to track changes in their bacterial communities.
Surprisingly, the most dramatic shifts occurred not in the gut, but in the mouth. Participants following juice-based diets showed rapid changes in their oral bacteria, with increases in species associated with inflammation and decreases in beneficial bacteria that help maintain oral health. The exclusive juice diet led to particularly notable changes, including significant increases in Proteobacteria, a group of microbes often linked to inflammation when present in high numbers.
While gut bacteria proved more resilient to short-term dietary changes, the researchers observed subtle shifts that raised potential concerns. Participants on juice-based diets showed increases in bacterial species associated with increased gut permeability (sometimes called “leaky gut“) and inflammation. They also experienced increases in bacteria that previous research has linked to cognitive decline.
In contrast, when participants followed a pre-study “elimination diet” that emphasized whole plant foods while avoiding processed foods, alcohol, and caffeine, their gut bacteria shifted in potentially beneficial ways. This phase increased bacteria known to produce beneficial compounds called short-chain fatty acids, particularly butyrate, which helps maintain gut health and reduce inflammation.
So while juicing may seem like a healthy choice, we might be better served by simply eating whole fruits and vegetables. The fiber removed during juicing appears to play crucial roles in maintaining healthy bacterial communities throughout our digestive system.
“If you love juicing, consider blending instead to keep the fiber intact, or pair juices with whole foods to balance the impact on your microbiome,” says Ring.
Paper Summary
Methodology
Participants were carefully screened to ensure they were healthy adults without conditions that might affect results. Before starting their assigned diets, all participants followed a three-day elimination diet to establish a baseline. The researchers collected biological samples at four key timepoints: before starting (baseline), after the elimination diet, immediately after the three-day diet intervention, and two weeks later. They used advanced DNA sequencing techniques to identify and measure the different types of bacteria present in each sample.
Results
The most significant changes occurred in participants’ oral bacteria, particularly in their saliva. Juice-based diets increased potentially problematic bacteria while decreasing beneficial species. While gut bacteria showed fewer changes, there were concerning trends in the types of bacteria that increased during juice-based diets. Most changes began returning to normal after participants resumed their regular diets.
Limitations
The relatively small sample size (14 participants) and short duration (three days) limit the ability to draw broad conclusions about long-term effects. Additionally, participants’ regular diets before the study might have influenced their responses to the interventions. The researchers note that a larger study with longer intervention periods would help confirm their findings.
Discussion and Takeaways
These results suggest that juice cleanses might not deliver all their promised benefits and could potentially have unintended negative effects on our bacterial communities. The study highlights the importance of dietary fiber in maintaining healthy bacterial populations and suggests that eating whole fruits and vegetables might be more beneficial than juicing them.
Funding and Disclosures
The research was funded by the Osher Center for Integrative Health at Northwestern University through their IM AWARE Advisory Council Research Pilot Grants program. The authors declared no conflicts of interest.
Publication Information
This study was published in Nutrients (Volume 17, Issue 458) on January 27, 2025, under the title “Effects of Vegetable and Fruit Juicing on Gut and Oral Microbiome Composition” by researchers from Northwestern University, San Raffaele University, and the University of Illinois Urbana-Champaign.
