The paper shows the fundamental flaw and short-sightedness that characterizes life sciences in general. 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, o…
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.