Can eating the same foods make your gut bacteria look like everyone else’s? Not necessarily, according to new research, and that has big implications for how we think about dieting.
In a study published in eGastroenterology, researchers at KU Leuven had 18 healthy adults eat only oats, milk and water for six days. They wanted to see if reducing food variety to an extreme would make participants’ gut microbiomes more alike.
It didn’t.
Instead, each person’s gut bacteria remained stubbornly unique, with some microbiomes even becoming more different from each other during the intervention. Overall microbial diversity also dropped, and researchers observed a temporary rise in a bacterial community type (known as Bacteroides2) that has been linked with inflammation in other studies.
The study followed participants across three phases: a week of their normal diets, the six-day “oatmeal-only” phase and then a return to regular eating. During the restricted phase, calorie intake fell by more than 30%, fiber intake rose and food variety nearly disappeared, but the gut microbiome didn’t converge.
So why does this matter? It challenges a long-held assumption that simplified diets lead to predictable gut health outcomes. Instead, it points to personalized nutrition as the future of dietary interventions, where your baseline gut health, genetics and other individual factors shape how your body responds.
There was good news: when participants returned to their regular eating habits, their gut bacteria largely bounced back, showing how resilient these microbial communities can be.
So even when everyone eats the same thing, their gut bacteria still play by their own rules.
This research was supported by KU Leuven, the Research Foundation Flanders, the ReALity Innovation Fund in Germany and the PRIMA project by Novo Nordisk.