Trendy juice cleansing can harm your health in just three days, warns new research. The trendy fibre-free form of dieting favoured by celebrities may increase inflammation-linked bacteria in the gut and mouth, say scientists.
A new American study, published in the journal Nutrients, found that a vegetable and fruit juice-only diet — even for just three days — can trigger shifts in gut and oral bacteria linked to inflammation and cognitive decline. Researcher at Northwestern University, Illinois, studied three groups of healthy adults.
One group consumed only juice, another had juice with whole foods and a third ate only whole plant-based foods. The research team collected saliva, cheek swabs and stool samples before, during and after the diets to analyse bacterial changes using gene-sequencing techniques.
The team found that the “juice-only” group showed the most significant increase in bacteria associated with inflammation and gut permeability, while the plant-based whole food group saw more favourable microbial changes. The “juice plus food” group had some bacterial shifts but less severe than the juice-only group.
The researchers say their findings suggest that juicing without fibre may disrupt the microbiome, potentially leading to long-term health consequences. Study senior author Dr. Melinda Ring, director of the Osher Centre for Integrative Health at Northwestern University Feinberg School of Medicine,said: “Most people think of juicing as a healthy cleanse, but this study offers a reality check.
“Consuming large amounts of juice with little fibre may lead to microbiome imbalances that could have negative consequences, such as inflammation and reduced gut health.”
She explained that juicing strips away much of the fibre in whole fruits and vegetables, which feeds beneficial bacteria that produce anti-inflammatory compounds such as butyrate. But without fibre, sugar-loving bacteria can multiply.
Dr Ring says the high sugar content in juice further fuels harmful bacteria, disrupting the gut and oral microbiome. The study also suggests that reduced fibre intake may impact metabolism, immunity and even mental health.The research team found that, unlike the gut microbiota, which remained relatively stable, the oral microbiome showed “dramatic” changes during the juice-only diet.
Scientists found a reduction in beneficial Firmicutes bacteria and an increase in Proteobacteria, a bacterial group associated with inflammation. Dr Ring said: “This highlights how quickly dietary choices can influence health-related bacterial populations.
“The oral microbiome appears to be a rapid barometer of dietary impact.”
She says the findings highlight the need for more research on how juice and other diets impact the microbiome, especially in children, who often consume juice as a fruit substitute. Study first author Professor Maria Luisa Savo Sardaro, of Northwestern University and San Raffaele University, Italy, said: “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.”
Dr Ring says the findings also highlight the importance of prioritising fibre in dietary guidelines and food production.
She added: “If you love juicing, consider blending instead to keep the fibre intact, or pair juices with whole foods to balance the impact on your microbiome.”