New! Sign up for our email newsletter on Substack.

Study Links 3-Day Juice Diet to Unhealthy Gut Bacteria Changes

The allure of a juice cleanse, often touted as a fast track to better health, may be more fiction than fact, according to a new Northwestern University study. A mere three days on a juice-only diet can significantly alter the bacteria in your gut and mouth, potentially paving the way for inflammation and even impacting cognitive function, the study reveals.

Published in the journal Nutrients, the research throws a wrench into the popular notion of juicing as a healthy detox. “Most people think of juicing as a healthy cleanse, but this study offers a reality check,” says Dr. Melinda Ring, senior author of the study and director of the Osher Center for Integrative Health at Northwestern University Feinberg School of Medicine and a Northwestern Medicine physician.

The study divided healthy adult participants into three groups: one consumed only juice, another combined juice with whole foods, and the third stuck to a whole plant-based diet. Researchers meticulously collected saliva, cheek swabs, and stool samples before, during, and after the dietary interventions, employing advanced gene-sequencing techniques to analyze bacterial changes.

The results were striking. The juice-only group experienced the most dramatic shift in bacteria linked to inflammation and increased gut permeability. In contrast, the whole plant-based food group exhibited more favorable microbial changes. The juice-plus-food group fell somewhere in between, showing some bacterial changes, though less severe than the juice-only group.

So, what’s the culprit? Fiber. Juicing removes the majority of fiber found in whole fruits and vegetables. Fiber acts as a crucial food source for beneficial gut bacteria, which in turn produce anti-inflammatory compounds like butyrate. Without fiber, sugar-loving bacteria can thrive. And with juices often packed with sugar, these harmful bacteria get an extra boost, disrupting the delicate balance of the gut and oral microbiome. The study also suggests that this reduced fiber intake could have broader implications for metabolism, immunity, and even mental well-being.

Interestingly, the oral microbiome seemed to react more quickly than the gut. The juice-only diet led to a decline in beneficial Firmicutes bacteria in the mouth and a surge in Proteobacteria, a group associated with inflammation. “This highlights how quickly dietary choices can influence health-related bacterial populations,” Ring explains. “The oral microbiome appears to be a rapid barometer of dietary impact.”

Maria Luisa Savo Sardaro, first author of the study and a research associate in the Amato Lab at Northwestern University and a professor of food microbiology at San Raffaele University in Rome, emphasizes the importance of the nutritional content of juice diets. “The nutritional composition of juice diets — specifically their sugar and carbohydrate levels — plays a key role in shaping microbial dynamics in both the gut and oral cavity and should be carefully considered,” Sardaro notes.

The research team stresses the need for further investigation into the effects of juice and other specialized diets on the microbiome, particularly in children, who often consume juice as a substitute for whole fruit. They also point to the broader implications for dietary guidelines and food production, advocating for the prioritization of fiber.

For those who enjoy juicing, Ring suggests a simple tweak: “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.”

This study, while raising important questions, is not without its limitations. The researchers acknowledge the small sample size and short duration of the dietary interventions, which may have limited their ability to detect more subtle changes in the microbiome. They also point out that the study focused on healthy adults, and the results may not be generalizable to other populations. Future research with larger and more diverse groups of participants is needed to confirm these findings and explore the long-term effects of juicing on health.

Despite these limitations, the study provides valuable insights into the complex relationship between diet, the microbiome, and health. It serves as a reminder that even seemingly healthy dietary choices can have unintended consequences, and that a balanced diet rich in fiber remains a cornerstone of good health.

The study, titled “Effects of Vegetable and Fruit Juicing on Gut and Oral Microbiome Composition,” was published online on January 27, 2025, in the journal Nutrients. The research was funded by the Osher Center for Integrative Health at Northwestern University.


Did this article help you?

If you found this piece useful, please consider supporting our work with a small, one-time or monthly donation. Your contribution enables us to continue bringing you accurate, thought-provoking science and medical news that you can trust. Independent reporting takes time, effort, and resources, and your support makes it possible for us to keep exploring the stories that matter to you. Together, we can ensure that important discoveries and developments reach the people who need them most.