Gut Bugs Determine Health Benefits of Sourdough Whole-Grain Bread | Glycemia Improves W/ White Bread in Some!

Yes, white bread can be the healthier choice. Whether your post-prandial glycemic response will improve or deteriorate with white vs. whole-grain sourdough bread appears to be fully dependent on your gut bugs.
Even though it has gotten a bad rep over the past 20 years, bread is (still?) consumed daily by billions of people, worldwide. I guess that's also because the scientific evidence of which the Internet gurus will tell you that it would "prove that bread is the devil" is by no means unambiguous.

It was thus high time for scientists to perform a randomized crossover trial like the one by Talk Korem and colleagues from the Weizmann Institute of Science - a study that used data from two 1-week-long dietary interventions comprising consumption of either traditionally made sourdough-leavened whole-grain bread or industrially made white bread.
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A study that found no significant differential effects of bread type on multiple clinical parameters. Following a 3-day run-in period, all participants who had been logging their regular food intake with a smartphone application for quite some time, subjects consumed either white bread made from mostly refined wheat flour (locally known as “achid”) from a major brand common in Israel, or a sourdough-leavened bread made from whole-grain wheat flour with traditional methods in an artisanal bakery.

Figure 1: Illustration of our experimental design, a 2x2 randomized crossover trial comparing the effects of 1-week consumption of industrial white bread and sourdough-leavened whole-grain bread (Korem 2017).
As you would expect it from any well-designed study, the subjects were randomly assigned to two sequence groups: white bread followed by sourdough bread (WS), or sourdough bread followed by white bread (SW) - with surprisingly little differences between (a) the groups and (b) the white bread vs. sourdough feeding during the intervention periods.
  • Figure 2: (A) No significant difference between the treatment effects of the two types of bread on clinical parameters measured in this study. (B and C) The effect of white bread (left panel) or sourdough bread (right panel) on the glucose metabolism of subjects who consumed white bread first (dark blue lines) or sourdough bread first (yellow lines | Korem 2017).
    The gut microbiota composition remained person specific throughout this trial and was generally resilient to the intervention. Practically speaking: there's nothing of the sort of a general benefit of whole-grain sourdough bread as it is propagated in the health literature.
  • There was a statistically significant interpersonal variability in the glycemic response to different bread types, which did not change over the course of the study period; an observation that suggests that the lack of phenotypic difference between the bread types stems from a person-specific effect. 
To get to the bottom of these initially surprising results, the researchers analyzed the significant interpersonal variability and sought to predict from baseline measures in advance whether it is white or sourdough bread that will induce lower glycemic responses for each individual.
Note: The study at hand highlights (once again) the futility of the glycemic index and a low GI is not the only benefit of whole-grain bread. As the authors point out "the published glycemic index for similar bread types was 70 and 54 for white bread and sourdough bread, respectively" and highlight that, since the "glycemic index inherently ignores inter-individual differences, it [results] in wrong classification for the ten subjects who had lower glycemic responses to white bread than to sourdough bread" (Korem 2017).

Furthermore, the higher content of vitamins and fiber in whole-grain vs. white bread have been associated with extra-benefits ranging from gastrointestinal and metabolic health to heart health - effects that will not surface within a few days, but can be observed only in response to long-term (months or years) consumption of whole-grain vs. white bread; and these effects may be partly mediated by changes of the microbiome in response to long-term whole-grain consumption. How you react to white vs. whole-grain bread could thus well change over time.
Korem et al. used a classification algorithm based on gradient boosting regression (Friedman 2001) to model the complex non-linear relationships between the subjects individual baseline microbiome samples and the observed reaction to white vs. sourdough bread.
"To reduce the immense feature space of microbial gene relative abundances, [the scientists] performed a principal component analysis (PCA) on the training set of each cross-variation and selected four principal components. For species, pathways, and modules abundances, we removed sparse features and those correlated to other features across the training set of each CV and then retained only the top six features best correlated with the outcome in the training set of each CV fold" (Korem 2017).
The scientists model performed very well and predicted the glycemic response of the subjects with an accuracy of 83% - irrespective of the exact numbers, the scientists were thus able to demonstrate "that the glycemic-response-inducing bread can be accurately classified for each subject using only microbiome data" (Korem 2017) - in that, informative features were the MetaPhlAn-derived relative abundances of Coprobacter fastidiosus, a Gram-negative, obligately anaerobic, rod-shaped, non-spore-forming and non-motil bacterium, and Lachnospiraceae bacterium 3_1_46FAA, a member of the anaerobic Clostridiales family that has been associated with colon cancer protection in humans.
Glycemia is not only about the type of floor and presence of bacteria during the baking process: Baking Bread With ~100g Extra-Fat Reduces the Glycemic Response: Coconut Oil Beats Butter, Grapeseed & Olive Oil | reread this SuppVersity Classic from April 2016.
What are the implications? It's important to note that scientists rightly point out that the "exact mechanisms for the effect of the gut microbiota on glycemic control are yet to be elucidated," it is also unquestionable that their ability to "highly accurate predict" the glycemic response to bread based solely on data derived from stool samples, opens up new doors to personalized nutrition advice, of which the study at hand confirms once again that it cannot be based on 'one-size-fits-it-all' paradigms like "sourdough is better for your health than cheap white bread" (Korem 2017).

Unfortunately, it will probably take 2-3 years before you will find reliable test kits at the doctors' office or online. Those that are currently available lack scientific back-up (plus: glycemia is not all, see red box) | Comment!
  • Friedman, Jerome H. "Greedy function approximation: a gradient boosting machine." Annals of statistics (2001): 1189-1232.
  • Korem, Tal, et al. "Bread Affects Clinical Parameters and Induces Gut Microbiome-Associated Personal Glycemic Responses." Cell Metabolism 25.6 (2017): 1243-1253.
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