Downside of fruit and vegetable juices for gut health

When it comes to nutrition, fruit and vegetable juices provide a vitamin, mineral, and antioxidant boost. However, according to a clinical trial published in the journal Nutrients, “detox juice cleanses” may not be the best option when it comes to the recommended “five portions of fruit and vegetables a day”. ¹

Students on a diet

The authors put 14 students on three different diets for three days:

• five followed a diet consisting exclusively of cold-pressed fruit and vegetable juices (800 to 900 kcal per day, i.e. about 10 glasses); 
• four followed a “normal” diet accompanied by fruit and vegetable juices; while
• five followed a low-calorie diet rich in “whole” plant foods containing fiber (800 to 900 kcal).

Before starting their respective diets, in order to assess the influence of different foods on microbial composition, all participants followed a three-day “elimination diet” consisting of organic fruit, vegetables, whole grains, and eggs, with very little or no red meat, dairy products, processed foods, gluten, alcohol, coffee, or sugar.

To analyze how the different diets impacted the microorganisms in the volunteers’ gut, oral, and salivary microbiota, the researchers collected samples from their stool, saliva, and from the inside of their cheeks before the experiment, between the two diets, and afterwards.

Increased risk of periodontitis and tooth decay

They found that fruit and vegetable juices, whether consumed alone or with food, significantly alter the oral microbiota by increasing pro-inflammatory bacteria, even if these changes are temporary.

The researchers noted a reduction in Veillonellaceae, a bacterium capable of converting salivary nitrates into nitrites. However, fewer nitrites means less effective inhibition of the bacteria responsible for periodontitis and an increased risk of tooth decay.

For the gut microbiota, the impact of juices was less pronounced, but the researchers noted an increase in bacteria associated with inflammation, intestinal permeability, and cognitive decline.

Too much sugar, too little fiber

According to the authors, juices are higher in sugar and carbohydrates, and lower in fiber than whole fruits and vegetables, which adversely impacts both the oral and gut microbial flora.

The elimination diet had the most positive impact on the gut microbiota: by providing more fiber, it promoted the growth of bacteria that produce highly beneficial short-chain fatty acids (sidenote: Short chain fatty acids (SCFA) Short chain fatty acids (SCFA) are a source of energy (fuel) for an individual’s cells. They interact with the immune system and are involved in communication between the intestine and the brain. Silva YP, Bernardi A, Frozza RL. The Role of Short-Chain Fatty Acids From Gut Microbiota in Gut-Brain Communication. Front Endocrinol (Lausanne). 2020;11:25. ) (SCFAs) such as butyrate. 

While the scope of this study is limited by its small size and short duration, it confirms that the priority for microbiota health should be the consumption of whole fruits and vegetables in order to fully benefit from their fiber.