We know small changes in diet can have a large effect on the microbiome. For example, adding seaweed to a mouse diet can select for microbes with the ability to digest particular polysaccharides within seaweed (https://www.nature.com/articles/s41586-018-0092-4). A company spun out of the Sonnenburg lab at Stanford is using this tech for microbiome therapeutics in humans (Novome). Those mice were given 10% seaweed by weight, so that serving of sourdough is right in the same range.
My point is, the researchers designing these experiments know what they're doing, and they're well thought out. You can't criticize something if you don't know the details of the field.
Also, look at the comparison. It is seaweed! Wouldn't you expect that eating 10% of your weight in seaweed changes a microbiome? It is a massively different food than what we (or mice) typically eat - thus the microbiome has to adapt. I am not saying the discovery is not important - it is the right question, set up correctly.
Compare that to a human eating two slices of different breads for a week. The differences between bread are too subtle. The amount ingested is too little, the time is too short. Your microbiome is already set up for digesting bread. That's all I am saying -
I will stick to my story that there is very little chance that this would ever be sufficient to detect any change.
even different sugars would digest through different pathways.
Every single fermented food is treated as if it were a high potency drug.
For example, in one study, for each day of a week people in the groups would eat either 110g of sourdough bread or 110g of white bread. Really, is that your experiment? You are trying to detect differences between eating 110g of two breads, once a day for one week?
Why would anybody in their right mind expect any change in the microbial composition of the gut? Or anything really...
Thus the reality is that not only there is almost no research in the field - that little that is performed is absurdly badly done.