Bacteria and our daily bread

As someone who spends a lot of time in coffee shops achieving immortality, I partake of the occasional piece of bread. Cinnamon raisin toast (even dry) with dark coffee (preferably Sumatra) is part of my ideal breakfast, with a couple of eggs and delusions of grandeur, extra salt. On the grandeur, not on the eggs. I’m not crass. Anyway, I’ve mentioned quite a few times in sci.casual that human research is horrendously complicated mainly because we’re just so diverse. We’re not built the same way across countries and cultures – we’re not even built the same even if we lived in the same street.

All that weird, winding lead-up was supposed to lead into diets – different people are going to take to diets differently by virtue of being different. This doesn’t mean that there’s no point in doing any research into what we eat, neither does this mean we should disregard everything we know about eating healthy. Kind of like this week’s Featured Article, which tackles a little bit of both. Korem et al. wanted to look at the effects of white versus sourdough bread on the gut microbiome.

Before we move on, I’m sure you’ve heard people use the term gut flora to describe the various goings on in the intestines – that’s actually the gut microbiome that I mentioned earlier (it is its own environment, after all), but let’s not split hairs here – and when you think flora, you think plants.

Public Domain (Author: Lisbeth Ekelof, 2015)

(wow, just how much fiber did you eat!?)

Anyway, your gut flora is made up of a lot of organisms, often bacteria, both good and bad that you’ve picked up along the way. Some of them can cause disease (like E. coli, if it ends up somewhere else on your body, or C. difficile, which is just plain nasty), but others help you with your digestion. ‘Probiotics‘ ring a bell? (by the way, the link goes to an highly informative open-access article in a peer-reviewed journal – don’t fear it because it’s an academic paper, give reading a shot). Our respective diets can have an impact on our gut flora, and this is where sourdough bread comes in.

Sourdough bread is made from fermented dough, and the starting material includes Lactobacillus bacteria, which gives it its unique texture and baking techniques, but it REALLY should be noted that none of the bacteria survive the baking process, so it’s not exactly probiotic. So then – between industrial, mass-produced white bread and artisanal, traditionally-made sourdough, will it make any changes to the gut microbiome, now that they’re even when it comes to the bacteria?

Flickr/Public Domain (Author: pixellaphoto, 2014)

Korem et al. ran their study on two groups of ten people – one group eating sourdough then white, the other group the other way around – with statistically similar ages, body mass index, waist-to-hip circumference, and so on (remember, you want to reduce the number of variables here so you don’t end up with alternate explanations for why things happen). The participants ate one type of bread at least for breakfast for a week, didn’t have either type of bread for the next 2 weeks, then ate the other type of bread for a week. They measured several things from each subject at the start and end of each bread-eating period, including blood tests (itself including several things including cholesterol, iron, creatinine, and so on), blood pressure, weight, basal metabolic rate, and blood glucose. To be sure that only the type of bread mattered, the researchers made sure that the participants took in the same amount of nutrients over the course of the study. The data gathering took a little over a month to complete.

A quick aside: if you read the Featured Article (and if not, it IS open-access!!!!!), they have the methods at the very end of the paper, past the citations, under ‘Bread Preparation’ – it only sounds complicated, but that’s just the language of the paper and it’s probably simpler than it sounds. The things you got to do to get published…

Anyway, the results: turns out there’s no difference. Hey, sometimes the lack of a result is a meaningful result, OK? In this case, the type of bread itself didn’t affect the gut microbiome. It didn’t even make a difference between the testing groups – both had changes in their blood glucose anyway because they ate some kind of bread. Well, as you would expect from breaking down carbohydrates. What else they found was more interesting – there’s variability between people when it comes to their body’s response to eating bread after all. Remember – these people were very similar among several measurements. These differences had to do with their gut microbiomes, and the authors assert that the results could predict what kind of bread can give a greater glycemic response just by looking at what’s in your guts.

Flickr/Public Domain (Author: Internet Archive Book Images, original work by Todd, 1859, p. 382)

The take-home? We still have to pay attention to what we ourselves eat and not focus too much on what everyone else is eating. What works for you may not work well for your friends and/or family (and vice versa), so take all non-professional diet advice with a grain of salt, although a low-salt diet may not be a bad thing for those with a tendency for hypertension. Anyway, again, in terms of making changes to your diet to promote better health, you really should be speaking to a professional, not a guy whose daily diet includes black coffee and scientific research.

I do prefer sourdough, though.

Featured Article: Korem T, Zeevi D, Zmora N, Weissbrod O, Bar N, Lotan-Pompan M, Avnit-Sagi T, Kosower N, Malka G, Rein M, Suez J, Goldberg BZ, Weinberger A, Levy AA, Elinav E, Segal E. (2017) Bread Affects Clinical Parameters and Induces Gut Microbiome-Associated Personal Glycemic Responses. Cell Metabolism 25(6): 1243-1253. DOI: 10.1016/j.cmet.2017.05.002

Featured Image: Public Domain (Author: Ulrike Mai, 2016)


  1. I love bread (I even did a science project with sourdough starters my freshman year of high school) and my stepmom happens to be a sourdough baker, so this article immediately interested me! You did a great job explaining the science behind it all while keeping it fun and fascinating. Great post!

    Liked by 1 person

  2. No difference really? Quite a shock. :/
    However bread is always popular in discussion because it has so many theories. However I preffer pizza bread with organic baking powder (not yeast). 🙂 Even if it’s not scientificly proven some people notice the difference.

    Liked by 1 person

    1. We all have our preferences. I like whole-grain, sourdough, or rye bread. I’ve been really digging this sourdough rye I’ve recently found, and if I *ever* find sourdough marble rye bread, I think my search is over – I’ve found *my* bread.

      Liked by 1 person

  3. I prefer the taste of sourdough too. It makes sense that the baking process kills bacteria. Does this research assert that sourdough is not better for you in any way? I’ve read that the flour is altered by the process (pre-digested in some way buy the bacteria) and therefore easier to digest.

    I’m trying to learn to bake sourdough bread. One issue I can think of is that my overall bread consumption might go up if I had a good supply of sourdough!

    Liked by 1 person

  4. Useful info. Fortunate me I discovered your web site by accident, and I am shocked why this coincidence did not came about in advance! I bookmarked it.

    Liked by 1 person

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