Are juice cleanses really healthy? New research challenges growing trend

Juice cleanses are often marketed as a way to detoxify the body and boost health, but new research from Northwestern University suggests they may do more harm than good. According to the findings, a vegetable and fruit juice-only diet — even for as little as three days — can lead to microbial shifts associated with inflammation and cognitive decline.

Published in Nutrients, the study analyzed three groups of healthy adults following different dietary patterns. One group consumed only juice, a second group supplemented juice with whole foods, and a third group ate only whole, plant-based foods.

Scientists collected saliva, cheek swabs, and stool samples before, during, and after the diets to analyze bacterial changes using advanced gene-sequencing techniques.

The juice-only group exhibited the most dramatic changes in their microbiome, including an increase in bacteria linked to inflammation and gut permeability. In contrast, the whole plant-based food group saw more favorable microbial changes. The juice-plus-food group experienced some bacterial shifts but to a lesser extent than the juice-only group.

“Most people think of juicing as a healthy cleanse, but this study offers a reality check,” says senior author Dr. Melinda Ring. “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.”

“This highlights how quickly dietary choices can influence health-related bacterial populations. The oral microbiome appears to be a rapid barometer of dietary impact.”

Reduced fiber

The researchers spotlight that juicing removes much of the fiber found in whole fruits and vegetables. 

Fiber plays a crucial role in feeding beneficial gut bacteria that produce anti-inflammatory compounds like butyrate. Without fiber, sugar-loving bacteria can flourish, disrupting the balance of the gut microbiome.

Additionally, the study shows that high sugar content in juice can further fuel harmful bacteria, potentially impacting metabolism, immunity, and even mental health.

Role of the oral microbiome

The study also revealed that the oral microbiome responds quickly to dietary changes. The juice-only group experienced a decline in beneficial Firmicutes bacteria and an increase in Proteobacteria, a bacterial group linked to inflammation. 

Unlike the gut microbiota, which remained relatively stable, the researchers say that the oral microbiome exhibited significant shifts during the juice-only cleanse.

Since the findings suggest fiber plays a crucial role in maintaining a healthy microbiome and that juice cleanses may disrupt this balance, researchers suggest further studies must be conducted — particularly in children, who frequently consume juice as a fruit substitute.

“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,” explains co-author Maria Luisa Savo Sardaro, Ph.D.

“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,” Ring concludes.