New research published in Nutrients in January reveals that three-day “juice cleanses” may not offer the health benefits often claimed, instead potentially leading to higher markers of inflammation in the oral and gut microbiomes. This study adds to a growing body of expert caution against juicing as a primary health strategy, with many suggesting its popularity is driven more by diet culture, celebrity influence, and social media trends rather than scientific evidence of superior health benefits. While fruits and vegetables are undoubtedly vital for a balanced diet, consuming them whole is generally recommended over juicing to preserve essential nutrients and support a healthy microbiome.

The oral and gut microbiomes are complex ecosystems of bacteria, fungi, and viruses that play crucial roles in numerous aspects of human health. The oral microbiome has been linked to conditions such as Alzheimer’s disease, obesity, and autoimmune disorders, while the gut microbiome is extensively researched for its impact on mental health, cancer, diabetes, and various neurological conditions. Given their importance, the effects of dietary practices like juicing on these microbiomes are a significant area of scientific inquiry.

Prior to the January 2025 Nutrients study, other research had explored juicing’s impact. A 2018 study, for instance, indicated improvements in fecal microbiota after three weeks when a juice regimen was added to a normal diet. Conversely, a 2022 study found that participants on a three-day juice fast experienced decreased diversity of gut microbes; however, a group that supplemented their juice fast with fiber showed an increase in bacterial diversity, highlighting the critical role of fiber.

The recent Nutrients study built upon this existing knowledge with a refined design. Researchers divided fourteen adults, aged 18 to 35, into three distinct groups for three days. One group consumed only a “cleanse” of juiced fruits and vegetables. The second group consumed the same juice but was permitted to add whole foods as desired. The third group followed a plant-based diet with a calorie content equivalent to the juice-only group. Throughout the study, researchers meticulously collected saliva, cheek swabs, and stool samples to track changes in participants’ oral and gut microbiota. DNA extraction and bacterial gene sequencing allowed for the identification of specific bacteria, similar to using a barcode scanner, as explained by study author Maria Luisa Savo Sardaro, PhD. Statistical analysis then quantified the shifts in bacterial populations.

The findings were striking: “We found that the exclusive juice group had the most significant increase in bacteria linked to inflammation and gut issues,” Savo Sardaro reported. The juice-plus-food group also exhibited changes, though they were less pronounced. In stark contrast, the plant-based whole food group showed beneficial changes, including an increase in bacteria associated with gut health and inflammation control. This stark difference underscores the impact of whole food consumption versus juice-only intake on microbial balance.

The primary reason juicing may negatively affect the microbiome lies in its processing: the removal of fiber. Amy Moyer, RD, director of culinary medicine at the University of North Carolina, Greensboro, explains that juicing can result in the loss of approximately 90% of fiber. As fiber is extracted, the sugar content in the remaining liquid becomes proportionally higher. This combination of low fiber and high sugar is detrimental to gut bacteria. Savo Sardaro clarified, “Fiber is essential for feeding beneficial bacteria that produce anti-inflammatory compounds like butyrate. Without fiber, the bacteria that thrive on simple sugars—often linked to inflammation—can multiply.” Furthermore, Lisa Froechtenigt, RD, noted that the drastic dietary changes involved in a juice-only cleanse can inherently disrupt microbial balance, as the human body requires a balance of macronutrients, including fiber, and may enter a