Just Three Days of Juicing Could Wreak Havoc on Your Gut, Scientists Warn

Your next juice cleanse might be riskier than you think. Recent research has found that as little as three days of juicing can lead to noticeable and possibly harmful changes in our native gut and mouth bacteria.

Scientists at Northwestern University conducted the study, which compared the short-term effects of three different diets on the microbiome. Compared to people on the other diets, people who only drank juice showed an increase in bacteria associated with inflammation. The results indicate that juicing can have its negatives, at least if people aren’t making sure to eat enough fiber elsewhere, the researchers say.

Juicing is seen as a tastier and more convenient way to get our daily intake of fruits and vegetables. But it’s well known that the juicing process strips away much of the important fiber naturally found in these foods. Because a diet low in fiber can lead to problems like constipation and negatively affect the microbiome (the neighborhood of bacteria that live in and on our bodies), the Northwestern University researchers wanted to see if a juicing-only diet could possibly do the same.

The small study involved 14 people who were assigned to one of three diets for three days: a diet where people only ate whole plants, a typical “juice-fast” diet with no other foods allowed, and a diet where people both drank juices and ate whole foods. The volunteers had their oral and gut microbiomes measured before, during, and after the study.

Juicing-only volunteers experienced a clear shift in the composition of their mouth microbiome, the researchers found, characterized by a reduction of helpful Firmicutes bacteria and an increase in Proteobacteria, which have been linked to inflammation. The researchers also found more subtle gut changes in people who only juiced; they tended to experience a relative increase in bacteria linked to gut permeability, inflammation, and cognitive decline. People on a juicing/whole food diet experienced smaller shifts in their microbiomes compared to the juicing-only group, while those on a plant-based diet experienced potentially helpful changes.

“These findings suggest that short-term juice consumption may negatively affect the microbiota,” the researchers wrote in their paper, published last month in the journal Nutrients. Importantly, the changes tied to a juicing-only diet may have been caused not just by a lack of fiber, but by the high sugar content found in these drinks.

The study is based on a small sample of volunteers, so it will take more research to replicate and truly confirm the team’s results here. But fiber is already known to be an essential part of a healthy diet. The researchers say their work demonstrates the importance of not sidelining fiber, even if you think you’re doing your body a favor with a juice-fast.

“Most people think of juicing as a healthy cleanse, but this study offers a reality check,” said senior study researcher Melinda Ring, director of the Osher Center for Integrative Health at Northwestern University’s Feinberg School of Medicine, in a statement from the university. “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.”