Before you spend a week gulping green juice, you might want to consider its impact on your health.

A new study has found that just a three-day juice cleanse can significantly alter the beneficial bacteria in your mouth and gut, leading to potentially harmful consequences.

“Unhealthy juice cleanse” might sound odd, but modern juicing techniques strip fruits and vegetables of some of their health benefits. Juices often don’t include the insoluble fiber that is key to gut health, and they also contain highly concentrated natural sugars.

Researchers from Northwestern University studied a small group of healthy adults ranging in age from 18 to 35. Each participant was randomly placed on one of three diets for three days. The diets included a juice-only fast, a diet with a combination of juice and regular food, and a plant-based, whole food diet. The researchers collected saliva, inner cheek and stool samples four times during the study to track changes in bacteria.

They found that those on a juice-only diet saw a significant increase in oral bacteria associated with inflammation and a decrease in bacteria that benefit oral health. Their gut bacteria did not change much, but they did see a slight increase in bacteria linked to leaky gut and inflammation. These participants also saw an increase in bacteria linked to cognitive decline.

On the other hand, those who followed the plant-based, whole food diet saw beneficial bacterial changes in their guts, including increases in bacteria that maintain gut health and reduce inflammation.

So, the next time you want to grind your fruits and veggies into liquid form, consider skipping the juicer and grinding them with your teeth instead.