Glycemic responses to bread may depend more on the composition of the gut microbiome than the composition of bread (white vs. whole wheat).
A small trial including 20 people were given either sourdough whole-grain bread or refined white bread to eat for one week. After a two-week break, each participant switched bread types for another week.
The study found very surprising results. Consuming either bread type improved cholesterol levels and improved markers of inflammation. The glycemic response was also dependent on the person’s gut microbiome composition and not bread type. This was surprising considering that fiber slows digestion and normally lowers the glycemic response. The bacterial strains that affected the glycemic response were Coprobacter fastidiosus and Lachnospiraceae bacterium, the latter of which has previously been associated with the development of type 2 diabetes.
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More research needs to be done before any definitive conclusions can be made but this study just highlights the potential importance of the gut microbiome in the glycemic response to food. Here is a link to the full study.