The study found that three-day juicing regimens increased bacteria linked to gut permeability (sometimes referred to as “leaky gut”) and inflammatory conditions, raising concerns about long-term or repeated use of fruit and juice-only cleanses.
Researchers were surprised by “how rapidly and significantly the oral microbiome changed,” says the senior author, Melinda Ring, MD, a physician at Northwestern Medicine and the director of the Osher Center for Integrative Health at Northwestern University Feinberg School of Medicine in Chicago.
“Within just three days, we observed noticeable shifts in bacteria known to thrive on simple sugars and contribute to inflammation,” she says.
Juicing Increased Inflammation in the Oral Microbiome
To explore how juicing impacts the gut and oral microbiomes, researchers recruited people between 18 and 35 years old with a BMI between 18.5 and 30. People were excluded if they had a condition or took medication that was likely to impact systemic inflammation, blood sugar control, or the microbiome.
A total of 23 people were included, and 14 people (7 women and 7 men) were included in the final microbiome analysis. Participants were placed into one of three groups:
- Cold-pressed juice only
- Cold-pressed juice with whole foods
- Whole plant-based foods only — no juicing
Scientists collected saliva, cheek swabs, and stool samples before, during, and two weeks after the three-day diet intervention to analyze bacterial changes using gene-sequencing techniques.
Key findings included:
- The juice-only group showed the most significant increase in bacteria associated with inflammation and gut permeability.
- The plant-based whole food group had favorable microbial changes.
- The juice-plus-food group had some bacterial shifts but they were less severe than the juice-only group.
“Most people think of juicing as a healthy cleanse, but this study offers a reality check,” says Ring.
Juicing Increased ‘Bad’ Bacteria in the Mouth
Juicing mainly affected the makeup of the oral microbiome — the “good” and “bad” microbes in the mouth. Assessing the oral microbiome, scientists found a reduction in beneficial Firmicutes bacteria and an increase in Proteobacteria, a bacterial group associated with inflammation.
“The oral microbiome is the body’s first line of microbial exposure, playing a crucial role in the breakdown of food and interaction with the immune system. Changes in the oral microbiome can have cascading effects throughout the body,” Dr. Ring says.
Harmful bacteria in the mouth can travel to the gut, contributing to imbalances and inflammation there, she says.
“Research shows that poor oral health is linked to systemic diseases, including heart disease, diabetes, and inflammatory bowel conditions. The connection between the oral and gut microbiomes suggests that maintaining oral health is integral to overall digestive and systemic health,” says Ring.
Short-Term Juicing Isn’t Likely to Harm Health
While it’s true that fiber supports a healthy microbiome, three days of juicing isn’t likely to be detrimental to overall health, says Jacquelin Danielle (J.D.) Fryer, RD, a performance dietitian-nutritionist at Banner Sports Medicine High Performance Center in Scottsdale, Arizona.
In some situations, juicing could even be helpful, says Fryer.
“For instance, a temporary fresh juice diet might be advantageous for stool impaction, autoimmune disorders, or skin conditions that require large boluses [single doses] of vitamins and minerals. Or someone who has just completed chemotherapy and lacks appetite and finds it challenging to eat enough of the nutrients that juicing can provide,” she says.
The intervention in the study is too short to make any generalizations about long-term health impacts, says Fryer.
Why Does Fiber Matter?
The lack of fiber in the juicing diet may be driving the disruption, the authors wrote.
Juicing strips away much of the fiber in whole fruits and vegetables, which feeds beneficial bacteria that produce anti-inflammatory compounds such as butyrate.
Without fiber, sugar-loving bacteria can multiply. The high sugar content in juice may fuel harmful bacteria, according to the authors.
Juice Cleanse With Caution
“I advise patients to approach juice cleanses with caution, especially if they are prolonged or frequent,” says Ring.
While occasional juicing can supplement fruit and vegetable intake, the lack of fiber in a “juice-only” cleanse can lead to imbalances in the microbiome and promote inflammation, she says.
“Instead of juice cleanses, I recommend whole-food diets rich in fiber, or if you enjoy juicing, consider blending rather than juicing to retain fiber. For those seeking “cleansing” benefits, focusing on fiber-rich whole foods, hydration, and minimizing added sugars can be more effective and sustainable,” she says.
If weight loss is the goal, juicing isn’t likely to help, says Alyssa Kwan, RD, of Stanford Health Care in California. “The weight ‘lost’ is simply water weight that can easily be gained back,” she says.
“I don’t usually recommend juicing, as it does not ‘detox’ the body; only the liver can do that. Not only does juice strip away fiber, it also lacks calories and protein important for muscle development,” says Kwan.