For six weeks, hundreds of adults with low-fiber diets added either a powder made from more than 30 whole plant ingredients, a probiotic capsule, or simple bread croutons to their normal meals. By the end, the group eating the plant blend had experienced a far broader reshaping of their gut microbiome than either comparison group, while also reporting improvements in several common digestive complaints and higher energy—even though many other health measures remained unchanged.
For years, public health advice has emphasized eating more fruits, vegetables, and whole grains. Yet the researchers behind the ZOE BIOME trial argue that another aspect of plant-based eating has received far less attention: diversity.
Rather than asking whether people simply eat enough plant foods, the study explored whether consuming a wider variety of whole plant ingredients could influence the community of microbes living in the human gut and, in turn, affect how people feel in everyday life.
To test that idea, investigators designed a randomized controlled trial centered on a commercially available whole-food blend containing more than 30 plant ingredients. The mixture combined fruits, vegetables, mushrooms, herbs, spices, nuts, seeds, and whole grains chosen for their fiber, polyphenols, micronutrients, and other compounds considered capable of supporting beneficial gut microbes.
Testing a simple dietary change
The study enrolled healthy adults in the United Kingdom between 35 and 65 years old whose daily fiber intake was below 20 grams. Participants also had to meet other eligibility requirements, including not already following vegetarian or vegan diets and not having gastrointestinal diseases or other conditions that could interfere with the results.
More than 8,000 volunteers were initially screened. After eligibility assessments, 399 people were randomly assigned to one of three groups.
One group consumed 30 grams per day of the diverse plant blend. A second group received a daily probiotic capsule containing Lacticaseibacillus rhamnosus GG. The third group served as a functional control by eating 28 grams of bread croutons matched for energy content.
The intervention lasted six weeks and was conducted entirely remotely, with participants collecting stool samples, blood samples, dietary records, and questionnaire responses from home.
After exclusions during secondary screening, 349 participants were included in the modified intention-to-treat analysis. The average participant was about 51 years old, roughly three-quarters were women, and baseline characteristics—including body mass index, physical activity, diet quality, and fiber intake—were similar across all three groups.
Self-reported adherence exceeded 98% in every treatment arm, and participants generally maintained their usual diets throughout the study.
The biggest difference appeared inside the gut microbiome
The study’s primary goal was not weight loss, cholesterol reduction, or blood sugar control. Instead, it focused on whether the interventions altered the composition of the gut microbiome.
Researchers analyzed stool samples using metagenomic sequencing, examining changes in bacterial species before and after the six-week intervention.
The contrast between groups was striking.
Participants consuming the plant blend experienced significant changes in the relative abundance of 57 microbial species. By comparison, only 14 species changed significantly in the bread-crouton group, while just four changed in the probiotic group.
Those differences between the plant blend and both comparison groups were statistically significant.
The researchers also evaluated these microbial species using previously developed ZOE Microbiome Health Rankings, which classify species according to their associations with health markers.
When directly comparing groups, they did not find statistically significant differences in the health rankings of the microbial species that increased or decreased.
However, additional within-group analyses painted a more nuanced picture.
Within the plant-blend group, microbial species that increased during the intervention tended to have more favorable health rankings than those that decreased. This pattern did not appear in either the probiotic or control groups.
The researchers observed a similar pattern using their separate microbiome diet ranking system, in which increasing species within the plant-blend group were more favorably ranked than decreasing species.
The authors emphasized that these analyses do not establish a causal relationship between the microbial rankings and cardiometabolic health. Instead, they interpret the findings as indicating a shift toward a microbiome composition associated with more favorable dietary health markers.
The microbiome changed in ways the probiotic did not
Beyond counting changing species, the researchers also examined the overall structure of participants’ gut microbial communities.
At the start of the trial, microbiome composition did not differ significantly between groups.
Six weeks later, however, the overall microbial communities differed significantly across treatments. When researchers compared each group’s microbiome before and after the intervention, significant changes appeared only among participants consuming the diverse plant blend.
Interestingly, one commonly used measure of microbial diversity—species richness—actually decreased modestly in the plant-blend group relative to the control group, while other diversity measurements did not differ significantly.
Rather than interpreting diversity alone as a marker of improvement, the authors focused on the broader changes in microbial composition and the specific bacterial species that shifted during the intervention.
As expected, the probiotic successfully introduced its intended bacterial strain. The number of participants in whom L. rhamnosus could be detected rose substantially only in the probiotic group.
Participants also noticed changes in how they felt
The biological measurements were accompanied by several changes that participants themselves reported experiencing.
Compared with the bread-crouton group, those consuming the plant blend reported greater improvements in energy levels over the six weeks.
They also reported reductions in several gastrointestinal symptoms, including indigestion, constipation, heartburn, and flatulence. When symptoms were combined into broader gastrointestinal domains, the plant blend was associated with lower scores for indigestion, constipation, and overall digestive symptoms.
Although the average improvements were relatively modest in this generally healthy population, the researchers also looked at how many individuals experienced improvements.
Compared with the control group, a larger proportion of participants consuming the plant blend reported increased energy and happiness, along with reductions in digestive symptoms including indigestion, constipation, flatulence, heartburn, and overall gastrointestinal discomfort.
Compared with the probiotic group, the plant blend also produced greater reductions in constipation severity and feelings of hunger.
Participants consuming the plant blend additionally showed differences in reported bowel movement frequency compared with the control group, although stool consistency itself did not differ significantly.
Many other health measures stayed the same
The study did not find evidence that the plant blend broadly changed blood metabolites related to lipids, fatty acids, glucose regulation, or inflammation.
Researchers measured 106 blood metabolites but reported no clinically relevant differences between treatment groups after six weeks.
Similarly, they found no significant differences in body weight, waist circumference, stool consistency, acne, sleep quantity, or most mood measures.
Several exploratory subgroup analyses were also conducted.
Among participants who already had elevated lipid or inflammatory markers at baseline, no significant treatment differences emerged for those measures.
Among participants who entered the study with more noticeable gastrointestinal symptoms, the plant blend was associated with larger reductions in rumbling stomach and constipation than the control treatment.
The researchers also noted that a greater proportion of participants in the plant-blend group reported improvements in sleep quality compared with both comparison groups, although overall sleep-quality scores did not differ significantly between treatments.
The blend also affected how people felt after a single meal
The study included a separate postprandial crossover experiment involving participants from the control arm of the main trial.
Thirty-four participants completed this portion of the research.
They consumed a standardized high-carbohydrate breakfast on separate occasions, once by itself and once with the plant blend added.
The primary outcome was the post-meal glucose response.
The plant blend did not significantly alter peak glucose concentrations after the meal.
However, participants reported feeling less hungry, more full, and more energetic over the three hours following the meal when the plant blend accompanied breakfast.
These immediate effects on appetite and energy occurred without measurable changes in post-meal glucose.
Convenience was part of the research question
The investigators framed the trial around a practical challenge rather than simply a nutritional one.
They note that many people struggle to consume sufficient fiber or prepare a wide variety of fresh plant foods because of modern lifestyles and food environments that favor convenience.
The whole-food plant blend was designed as a simple way to increase exposure to diverse fibers, polyphenols, micronutrients, and plant ingredients without requiring major dietary changes.
The researchers instructed participants to add the blend to meals while otherwise maintaining their habitual diets, allowing the study to evaluate whether this relatively straightforward addition could influence the gut microbiome.
The findings come with important boundaries
The authors emphasize that the trial involved healthy adults with relatively low fiber intake and lasted only six weeks.
Many outcomes—including blood metabolite measurements, body composition, and several other health indicators—did not change during that period.
The improvements in digestive symptoms and energy were based on participants’ self-reports, and while statistically significant, the researchers describe the effect sizes as small and of uncertain clinical significance in this healthy population.
They also stress that associations between shifts in microbial species and health rankings should not be interpreted as proof that the microbiome changes themselves caused health improvements.
Even so, the trial demonstrated that adding a diverse blend of more than 30 whole plant ingredients to everyday meals produced substantially broader changes in gut microbial composition than either an energy-matched food control or a single-strain probiotic, while coinciding with improvements in several aspects of digestive comfort and subjective energy.
Publication details
Alice C. Creedon et al, Does a diverse whole-food plant-based dietary intervention improve gut microbiome composition, gut symptoms, energy and hunger in healthy adults? A randomised controlled trial, British Journal of Nutrition (2026). DOI: 10.1017/s0007114526107703
