Juice cleanses are a popular way to kickstart a health journey, but these diets may do more harm than good in as little as three days.

A study published recently in the journal Nutrients found that consuming vegetable and fruit juice-only diets for three days triggered detrimental gut and oral bacteria changes linked to inflammation, memory and thinking problems.

Special: Doctors Offer New Hope for Immune Health, Stable Mood, and Overall Well-Being

Exactly how juice-only cleanses trigger such changes is not fully understood, but researchers suggest it may owe to the lack of fiber in juice. 

“Most people think of juicing as a healthy cleanse, but this study offers a reality check,” said study author Dr. Melinda Ring, director of the Osher Center for Integrative Health at Northwestern University Feinberg School of Medicine in Chicago. 

"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," she said in a news release.

When fruits or veggies are juiced, much of the fiber is stripped away. Fiber feeds good bacteria that produce anti-inflammatory compounds. 

Without fiber, sugar-loving or bad bacteria can multiply, disrupting the balance of good and bad bacteria — known as the microbiome — in the gut and mouth. 

To see how juicing affects the gut and oral microbiomes, researchers studied three groups of healthy adults. 

One group consumed only cold-pressed fruit and veggie juice, another had juice with whole foods, and a third group ate only whole plant-based foods.  

They collected saliva, cheek swabs and stool samples before, during and after the diets to analyze any bacterial changes. 

Folks in the juice-only group showed the most significant increase in bacteria associated with inflammation and gut permeability or leaky gut. By contrast, folks in the plant-based whole food group saw more favorable changes to their gut bacteria. 

The juice-plus-food group had some changes in their gut bacteria, but these shifts were less severe than those seen in the juice-only group, the study showed.

People who consumed juice-only diets showed dramatic changes in their mouth bacteria including in beneficial Firmicutes bacteria and an increase in Proteobacteria, a bacterial group linked to inflammation.

“This highlights how quickly dietary choices can influence health-related bacterial populations,” Ring explained. 

“The oral microbiome appears to be a rapid barometer of dietary impact,” she added.

Researchers said the findings point to a need for a deeper look at how juice and other diets affect the microbiome, especially in kids who often drink juice in lieu of eating fruit.