Juice Fasts Can Wreck Your Health in Just 72 Hours, According to a New Study

A 2025 study from Northwestern University might give you pause the next time you consider swapping a few meals for a juice cleanse.

In February, researchers at the university published their study on the “Effects of Vegetable and Fruit Juicing on Gut and Oral Microbiome” in the journal Nutrients, finding that just three days on a juice-only diet was enough to “trigger shifts in gut and oral bacteria linked to inflammation and cognitive decline.” 

To reach this conclusion, the team assembled a cohort of healthy adult volunteers and assigned each to one of three diets: an exclusive juice cleanse, a juice-plus-food plan, or a fiber-rich plant-based diet for three days. The participants had saliva and stool samples collected before, during, and after the study and also had their cheeks swabbed to analyze changes in the microbiota, also known as the microbial communities that live in our mouths and play a significant role in digestion, immune systems, and even affect our moods. 

And while the team found that the gut microbiome remained relatively stable across the diets, the oral microbiomes told a different story.

As the authors concluded, participants on the juice-only program experienced a measurable increase in Proteobacteria, a bacterial group linked to inflammation, and a decrease in Firmicutes, which are generally considered beneficial bacteria for humans. The most significant shifts in these particular bacteria were observed in the saliva samples, and the changes, the authors noted, persisted for up to two weeks after the cleanse ended.

“This highlights how quickly dietary choices can influence health-related bacterial populations,” Dr. Melinda Ring, director of the Osher Center for Integrative Health at Northwestern University Feinberg School of Medicine, shared in a statement. “The oral microbiome appears to be a rapid barometer of dietary impact.”

The authors explained that this is likely due to the juices lacking the fiber found in whole fruits and vegetables.

Additionally, the cleanses provided participants with a 20-gram dose of sugar per serving. The low fiber, high sugar content, the researchers noted, could encourage the growth of bad bacteria associated with “gut permeability” and cognitive decline. Notably, while the changes in the gut microbiota were smaller than those observed in the mouth, researchers still detected increases in potentially pro-inflammatory strains like Porphyromonadaceae among those on the juice-only diet, which is a family of bacteria linked to markers of anxiety. 

“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,” first author Maria Luisa Savo Sardaro noted.

The plant-based group fared much better. The researchers found that participants in this group had higher levels of Faecalibacterium prausnitzii, a butyrate producer associated with reduced inflammation and enhanced gut health. 

Fortunately, the researchers also found that the negative effects from the cleanses appeared to reverse after 14 days once the diet ended and participants returned to their normal habits. While the researchers note that this was both a small and short study, they hope it sparks a conversation about products marketed with “healthy” claims and encourages people to consider their fiber content.

“Most people think of juicing as a healthy cleanse, but this study offers a reality check,” Ring said. “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.”