A few days of a juice only diet is viewed by many as healthy, and a good way to cleanse the body. But... a recent study found that this is not true for the gut microbiome (the community of fungi, bacteria, and viruses), which is so important to our health. A vegetable and fruit juice only diet, even if it's only for three days, is unhealthier than juice plus whole foods, or only whole-plant foods diet.
The juice only diet promoted the growth of bacterial species in the gut that are linked to gut permeability, inflammation, and cognitive decline. The juice only diet also resulted in changes in the saliva and cheek microbiota (microbiome), especially an increase in pro-inflammatory bacteria. The researchers thought these changes were due to the low fiber intake of the juice only diet.
This is because fiber feeds the beneficial bacteria (linked to good health) in the gut. Juices are low in fiber, but whole fruits and vegetables are high in fiber (as are whole grains, seeds, nuts, legumes).
Participants in the study were divide into 3 groups. Each group followed one diet (juice only, juice + food, or whole plant based foods only) for three days. Bacteria in the mouth (saliva and cheek swabs) and gut (through stool samples) were analyzed before, during, and after.
Before participants followed one of the 3 diets, they first followed an elimination diet for 3 days: organic fresh fruits, vegetables, gluten-free whole grains, eggs, and 8 glasses of water a day. No alcohol, caffeine, sugar, processed foods, dairy, red meat, and gluten (e.g., wheat, rye, barley, spelt). This diet also had beneficial changes in the gut microbiome with increased numbers of bacteria linked to good health, for example, an increase in the gut bacteria Faecalibacterium prausnitzii.
From Medical Xpress: Juicing may harm your health in just 3 days, study finds
Think your juice cleanse is making you healthier? A new Northwestern University study suggests it might be doing the opposite. The study, recently published in 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.
Northwestern scientists 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. Scientists collected saliva, cheek swabs and stool samples before, during and after the diets to analyze bacterial changes using gene-sequencing techniques.
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 favorable microbial changes. The juice plus food group had some bacterial shifts but less severe than the juice-only group. These findings suggest that juicing without fiber may disrupt the microbiome, potentially leading to long-term health consequences.
"Most people think of juicing as a healthy cleanse, but this study offers a reality check," said senior author Dr. Melinda Ring, director of the Osher Center for Integrative Health at Northwestern University Feinberg School of Medicine and a Northwestern Medicine physician.
"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," Ring said.
Why fiber matters - Juicing strips away much of the fiber in whole fruits and vegetables, which feeds beneficial bacteria that produce anti-inflammatory compounds such as butyrate.
Without fiber, sugar-loving bacteria can multiply. The high sugar content in juice further fuels these harmful bacteria, disrupting the gut and oral microbiome. The study also suggests that reduced fiber intake may impact metabolism, immunity and even mental health.
The oral microbiome responds quickly - 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.