The study's researchers (at the University of Oviedo in Spain) examined 38 adult subjects who were free of cancer, had no autoimmune or digestive diseases, and had not taken any pre-, pro-, or antibiotic products in the month prior. Subjects were asked detailed questions about their eating habits over the previous year; then, researchers calculated the nutrients each subject consumed. These included dietary fibers such as pectin (which is found in citrus) and cellulose, as well as simple carbohydrates (from things like white bread and pasta). After taking the survey, the participants provided a fecal sample, which was analyzed to determine bacterial levels.
The researchers discovered a number of significant relationships between the subjects' diet and their gut bacteria. In particular, consumption of white bread was found to be associated with increased levels of the bacteria Lactobacillus. This was attributed to hemicellulose, an insoluble dietary fiber found in white bread.
While previous research has found that dietary fibers play an important prebiotic role (that is, they foster the generation of healthy bacteria in the gut), this is the first study to look at the interplay between polyphenols and fiber. The research team attributes their findings on white bread to this relationship, suggesting that previous beliefs about dietary fiber and processed wheat — such as the idea that white bread has no health benefits — simply don't tell the whole story.
Of course, there are many, many caveats to consider here. First, this is a very small study, the results of which may or may not be replicable in future research. And, considering the many different wheat processing methods in Europe versus the U.S., it's hard to say whether these findings would hold with American subjects. Another potentially confounding factor is chewing; the degree to which you break down your food before swallowing could affect the way the food's fiber and polyphenol content affect your body's microbiome.
And, obviously, these findings don't change anything for those who are allergic to gluten and/or wheat. Still, for those who have (begrudgingly) cut out white bread for speculative health reasons, the idea that processed carbs could boast digestive benefits may be a good enough reason to get the burrito instead of the bowl.