A recent study from Northwestern University has raised concerns about the health effects of juice-only diets. Published in Nutrients, the research indicates that consuming only vegetable and fruit juices for as little as three days can lead to changes in gut and oral bacteria associated with inflammation and cognitive decline.
The study involved three groups of healthy adults: one group consumed only juice, another combined juice with whole foods, and a third ate exclusively plant-based whole foods. Researchers collected saliva, cheek swabs, and stool samples before, during, and after the dietary interventions to assess bacterial changes using gene-sequencing techniques.
Results showed that the juice-only group experienced significant increases in bacteria linked to inflammation and gut permeability. In contrast, those on a plant-based whole food diet exhibited more favorable microbial changes. The group consuming both juice and whole foods saw some bacterial shifts but less severe than the juice-only participants. These outcomes suggest that juicing without fiber may disrupt the microbiome, potentially leading to long-term health issues.
"Most people think of juicing as a healthy cleanse, but this study offers a reality check," stated Dr. Melinda Ring, senior author of the study and director of the Osher Center for Integrative Health at Northwestern University Feinberg School of Medicine.
Dr. Ring further explained that "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."
Juicing removes much of the fiber found in whole fruits and vegetables. This fiber is essential for feeding beneficial bacteria that produce anti-inflammatory compounds like butyrate. Without it, sugar-loving bacteria can proliferate due to the high sugar content in juices, disrupting both gut and oral microbiomes. Reduced fiber intake may also affect metabolism, immunity, and mental health.
Unlike gut microbiota which remained relatively stable during the study period, dramatic changes were observed in the oral microbiome among those on a juice-only diet. A reduction in beneficial Firmicutes bacteria was noted alongside an increase in Proteobacteria—bacteria associated with inflammation.
"This highlights how quickly dietary choices can influence health-related bacterial populations," said Dr. Ring. "The oral microbiome appears to be a rapid barometer of dietary impact."
The findings emphasize the need for further research into how juices and other diets affect microbiomes—particularly concerning children who often consume juice instead of whole fruits.
Maria Luisa Savo Sardaro emphasized that "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."
This study underscores prioritizing fiber within dietary guidelines and food production processes. For now, Dr. Ring advises those who enjoy juicing: "Consider blending instead to keep the fiber intact or pair juices with whole foods to balance impacts on your microbiome."