Feb. 10, 2025 — They may sound delicious and colorful, but there’s new evidence that juice cleanses can have negative health consequences — particularly due to a lack of an important kind of fiber.
Researchers at Northwestern University found significant differences in gut and mouth bacteria between people following a juice-only diet versus diets that included whole foods. They suggested that the changes may be due to a lack of insoluble fiber, which adds bulk to the stool and helps move it through the body.
The changes in the body they documented during juice cleansing are linked to inflammation, cognitive impacts, and gut permeability (which refers to the ability of the body to absorb nutrients into the bloodstream while keeping harmful substances out). Published in the journal Nutrients, the study didn’t conclusively link the bacterial changes to specific health outcomes. Instead, the researchers analyzed important types of bacteria from inside the cheek, from saliva, and from stool samples.
“Most people think of juicing as a healthy cleanse, but this study offers a reality check,” said senior author Melinda Ring, MD, director of the Osher Center for Integrative Health at Northwestern University Feinberg School of Medicine, in a news release. “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.”
The study was small and included just 23 people who were recruited from the university via flyers. They ranged in age from 18 to 35 and didn’t have major health problems. They followed one of three diets for three weeks. Those assigned to the juice-only group got 800 to 900 calories daily exclusively from cold-pressed juice. Another group got the same amount of juice plus foods from their regular diet. And the third group ate a plant-based whole-food diet.
All participants followed what is known as a three-day “elimination diet” typically used at the start of a juice cleanse, focusing on organic fresh fruit, vegetables, gluten-free whole grains, eggs, and eight glasses of water per day while avoiding things like caffeine and alcohol. They repeated that diet the last three days, too. The researchers took the saliva, cheek swab, and stool samples before, during, and after the diet period.
The elimination diet was shown to have significant impact on bacteria found in the saliva, and the researchers reported that the changes could potentially be positive for health because one observed change has been linked in the past to cognitive benefits.
In the juice-only group, the researchers found significant bacterial changes pointing toward an increase in inflammation and gut permeability. The researchers suggested that the increased inflammation signals may be linked to the high sugar content and low fiber content of the juice. The juice-plus-food group had similar changes but to a lesser extent. The plant-based whole food group experienced what the researchers considered beneficial bacterial changes.
Overall, the researchers concluded that bacteria that thrive on a high-sugar diet may multiply during a juice cleanse, and the bacteria that are fed by fiber and have anti-inflammatory effects may be diminished. This could lead to impacts on metabolism, immunity, and mental health, the study authors concluded.
