Researchers in the Republic of Korea and the United States conducted the randomized, double-blind, placebo-controlled trial to evaluate two probiotic–phytonutrient formulations with different probiotic complexity to assess their effects on bowel function, microbial metabolites, zonulin and gut microbiome composition.
Both formulations contained strains known to support microbial homeostasis and produce beneficial metabolites, while standardized citrus bioflavonoids and dandelion extracts were included to enhance microbial metabolism and potentially act synergistically with the probiotics.
“Findings suggest that probiotic–phytonutrient supplementation may influence gut microbial activity and gut barrier-related biomarkers through microbiome-mediated metabolic modulation,” the researchers wrote in the journal Nutrients.
Effects of probiotics and phytonutrients on gut microbiome function
The gut microbiome plays a central role in maintaining digestive health by supporting nutrient metabolism, producing beneficial metabolites such as short-chain fatty acids (SCFAs) and regulating immune function.
Probiotics are widely used to support gut health and may improve bowel function, stool consistency, and microbial balance. They can influence intestinal barrier function by altering microbial metabolism, promoting the production SCFAs like butyrate which strengthens the intestinal barrier by providing energy to colonocytes, increasing tight junction protein expression and supporting mucin production.
They also enhance microbial conversion of tryptophan into indole derivatives that activate the aryl hydrocarbon receptor (AhR) and help maintain mucosal integrity. These pathways regulate intestinal permeability, making biomarkers such as zonulin valuable indicators of barrier function.
Plant-derived phytonutrients may further support gut health by modulating microbial composition and metabolism, potentially providing prebiotic-like effects, the researchers noted.
They hypothesised that combining probiotics with phytonutrients may produce complementary benefits by enhancing microbial activity, metabolite production and intestinal barrier function.
Study design, methods and results
The researchers recruited 60 healthy adults who were randomly assigned in equal numbers to placebo, PBP1, or PBP2 groups. Participants received one capsule daily for 8 weeks while maintaining their usual diet and lifestyle and avoiding probiotics, prebiotics and related supplements.
The researchers formulated PBP1 with two Lacticaseibacillus strains and phytonutrients (dandelion root powder and citrus bioflavonoids), and PBP2 with a five-strain probiotic blend plus the same phytonutrients. The placebo contained only corn starch. All capsules were administered once daily under fasting conditions.
The researchers assessed outcomes at baseline, week 4, and week 8, evaluating bowel habits and quality of life using a validated survey. They collected fecal samples at baseline and week 8 to analyze SCFAs, zonulin, tryptophan metabolites and gut microbiota composition.
Results showed that both probiotic–phytonutrient blends improved bowel function by increasing normal stool patterns. They also changed gut metabolism, raising SCFA (especially butyrate and acetate) and shifting tryptophan metabolism toward more indole production. The researchers noted that the changes suggest stronger microbial activity, especially in the PBP2 group.
Both interventions slightly reduced the gut permeability marker zonulin, particularly in participants with higher BMI, suggesting improved intestinal barrier function in people who may have weaker baseline gut health.
Furthermore, both supplements increased beneficial microbes such as Lactobacillus and other SCFA-producing bacteria, while overall microbiome diversity stayed stable, with PBP2 showing broader microbial changes than PBP1.
Higher indole levels and lower tryptophan levels were linked with lower zonulin, suggesting that microbial tryptophan metabolism may help support gut barrier integrity. According to the researchers, butyrate likely contributed to strengthening the intestinal lining.
“Supplementation with PBP1 and PBP2 improved bowel function and was associated with changes in microbiome-derived metabolites, including SCFAs and tryptophan–indole metabolism, with BMI-dependent changes in barrier markers,” the researchers concluded. “These findings suggest a potential role of microbiome-mediated metabolic modulation in intestinal health.”
They added that larger, longer-term studies are needed to confirm these findings.
Source: Nutrients; doi: https://doi.org/10.3390/nu18132085; “Effects of Probiotic–Phytonutrient Blends on Defecation, Intestinal Barrier Function, and Gut Microbiota: A Randomized, Placebo-Controlled Trial.” Authors: Hwang, A. Y. Et al.