What Type of Bread Is Best For Your Glycemic Index?

MedicalResearch.com Interview with:

Prof. Avraham A. Levy Department of Plant and Environmental Sciences Weizmann Institute of Science Rehovot Israel

Prof. Levy

Prof. Avraham A. Levy
Department of Plant and Environmental Sciences


Prof. Elinav

Prof. Eran Elinav
Department of Immunology


Prof. Segal

Prof. Eran Segal
Department of Computer Science And Applied Math

Weizmann Institute of Science, Rehovot Israel

MedicalResearch.com: What is the background for this study? 

Response: We performed a type of clinical trial that is very powerful in comparing short term effects of interventions – a crossover trial. In this trial, each subject is compared to themselves; in our case, we compared increased short-term (1 week) consumption of industrial white bread vs. matched consumption of artisanal sourdough-leavened whole-wheat bread – which we originally viewed as radical opposites in terms of their health benefits. We measured various clinical end points – weight, blood pressure, various blood tests – and also the gut microbiome.

To our great surprise, we found no difference between the effects those two breads had on the various end points that we measured. This does not mean that bread consumption had no effect – but that this effect was generally similar for its two types. In fact, when we analyzed our data when pooling together the two bread types (i.e., testing whether bread of any type had an effect), we found that just one week of bread consumption resulted in statistically significant changes to multiple clinical parameters – on the one hand, we saw a reduction in essential minerals in the blood (calcium, magnesium, iron) and an increase in LDH (marker of tissue damage); on the other hand, we saw an improvement in markers of liver and kidney function, inflammation markers and cholesterol levels.

In terms of the microbiome, we have found only a minimal difference between the effects of the two bread (two microbial taxa that were increased with white bread) – but in general, we saw that the microbiome was very resilient to this intervention. This is surprising as the current paradigm in the field is that a change in nutrition rapidly changes the makeup of the microbiome. We say that this is probably dependent on the kind of change – as we had a nutritional change here which was significant enough to change clinical parameters, which we tend to think of as very stable, and yet had a minimal effect on the microbiome.

At this point, there were two possible explanations to what we saw:
The first is that bread had an effect in our intervention, but it was very similar between those two very distinct types.
The second is that these two distinct types indeed had different effects, but they were different for each subject – and thus cancel out when we look at the entire population.

MedicalResearch.com: What are the main findings?

Response: We set out to check that in terms of one type of response to foods – the glycemic response. What we found is that indeed people had widely different and opposite responses to these two bread types. In fact, half the people had higher glycemic responses to white bread, and the other half had higher responses to sourdough bread. We also proved rigorously that this was statistically significant and not a result of random fluctuations.

So having very personal, often opposite responses to the same kind of bread poses a problem – how would we know, in advance, which type of food is better for each person? So here we proposed a solution in terms of a predictor. We showed that we could have predicted, with fairly good accuracy, which bread induces lower glycemic responses for each subject personally – and do that based on their initial microbiome configurations. We need more research to establish that, but we envision a future where each of us would have their microbiome profiled and then receive personal nutrition advice.

MedicalResearch.com: What should readers take away from your report?

Response: Let’s start with what readers shouldn’t take away from our study. We’ve studied just two types of bread (even if very “extreme opposites”), with a short term intervention, and limited to a certain population. So we do not claim that “white bread is good for you” or that “white bread is as good as whole-wheat/sourdough/artisanal bread”. What we do claim is that the common belief that “wholewheat / sourdough bread is always better than white bread” is wrong.

More generally, our study challenges the common belief that types of food that are considered healthy are indeed healthy for everybody. We know that people are different in their genetic makeup and lifestyle, and here we show that people have rather stable and person-specific microbiomes. These differences between people affect how they respond to even the same meals which means that foods which are considered healthy may not be healthy for everyone, and vice-versa.

MedicalResearch.com: What recommendations do you have for future research as a result of this study?

Response: Many interesting questions are raised by this study: Which mechanisms drive differences between people? What biological mechanisms in the microbiome drive differences between people? If ‘one-size-fits-all’ diets do not work, how can we better personalize diets? We are currently conducting research to answer some of these questions.

MedicalResearch.com: Thank you for your contribution to the MedicalResearch.com community.


Bread Affects Clinical Parameters and Induces Gut Microbiome-Associated Personal Glycemic Responses Korem, Tal et al.
Cell Metabolism , Volume 25 , Issue 6 , 1243 – 1253.e5
DOI: http://dx.doi.org/10.1016/j.cmet.2017.05.002

Note: Content is Not intended as medical advice. Please consult your health care provider regarding your specific medical condition and questions.

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