I just finished an audio book Brain Maker. CO and I used to discuss how the brain repairs itself, I find brain science interesting. This is more how the microbiome in the gut influences the brain but is part of brain science. Te author, Dr Perlmutter, is a practicing neurologist.
First, there are some indications that our gut bacteria does influence our brains. But this book carries that way beyond where the science is. Dr Perlmutter had a bad habit of saying things like, "I frequently treat Alzheimer's patients with an out of balance gut microbiome" or "correlation does not equal causation but I have no doubt when more studies are done we will discover that ...". Neither had any scientific validity.
It is clear the dr has decided the microbiome is the solution to many cognitive problems as well as diabetes and obesity. He is unwilling to actually wait on on the science to prove it. And sadly for him, the studies that have been done since the book was written in 2014 are not showing probiotics, prebiotics, and fecal transplant to be silver bullets.
One other problem he runs into is that there are numerous strains of many of his touted gut bacteria. He tells the reader/listener that it is important to make sure your probiotic contains bacillus whateverus (he has several he recommends). When one Googles bacillus whateverus, one discovers there are a couple hundred strains of it. Just like there are many strains of Coronavirus, but only one causes the problems we see today. Taking the wrong strains probably provides no benefit (though again research would be needed), and there isn't good research on what strains are the key ones.
There are some studies that do show a correlation between gut biomes and various conditions. We don't know what triggers the correlation, but it is certainly possible the gut biome comes first and triggers the condition. It is also certainly possible it goes the other way. And it is possible a 3rd factor triggers both. And it just might be statistical noise.
Some of the ideas he presents are worth scientists continuing to look into. Some, his total hatred of grain for example, appear problematic. Others may yet prove an important step for health. But given the way he totally oversteps his science I am very skeptical he is anything more than the Sham Wow spokesman with a degree.