I would say the best popular/scientific books are Breath by Nestor (released this year), and Jaws by Kahn (2018). It's coming into public consciousness, although the books show that this knowledge was available in the 1930's and forgotten, and in the 1700's and forgotten, etc.

If you prefer video, here's an in-depth interview with Nestor: https://www.youtube.com/watch?v=U5o9b2RVC2E

The shorter clips extracted from this interview are compelling.

For more specific medical advice, there is "Sleep, Interrupted" by Park, and "The 8 hour sleep paradox" by Burhenne.

It's a big area because breath affects so many parts of your physiology, but all these people are saying overlapping things, coming from their own professional viewpoint. There is a particular focus on the relation to teeth, and that is one of the most common correlations.

But there are also correlations with sinus problems, acid reflux, anxiety, ADD, high blood pressure, weight gain and obesity, and bad sleep. That is, extremely common things that most people just live with for years. But they don't have to.

Also, if you have children, you have to answer the question: why do so many children need orthodontic work? What happened 1000 years ago when there were no orthodontists?

The answer is that our diets changed drastically in the last 200 years, and also ~10,000 years ago with the advent of agriculture. If you don't believe that, the question still needs an answer!

Same with the causes of all these minor chronic problems. Most people just live with the symptoms, and for some reason don't ask for the fundamental reasons why. Like food, breath is at the the base of your physiology, so it makes sense as a candidate.

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If you want a whole bunch of studies, I have this book (recommended by my dentist) and there are 140 scientific papers cited in the back.

https://www.amazon.com/Airway-Management-Solution-Health-Cri...

"Airway dentistry" is actually its own separate subfield, which is weird because a lot of practicing dentists seem only dimly aware of it! But at least in the United States you can find "sleep dentists" or "airway dentists" in most major cities. But somehow I went 20+ years being completely unaware of their existence.

The evidence that's cited over and over for the agriculture thesis is the collection of skulls at U Penn. Modern humans have drastically different skulls than humans before agriculture. That is described in "Breath" and "Jaws".

I don't recall a primary source for the speech thesis, but it's mentioned in "Sleep, Interrupted", "Jaws", and "Breath". Although actually I just googled and it was easy to find a reference:

https://dentalsleeppractice.com/ce-articles/evolution-human-...

Significant evolutionary changes to the human head are flat face, smaller chin, shorter oral cavity, changes in jaw function, repositioning of ears behind jaws, ascent of the uvula and descent of the epiglottis, right angle bend in tongue, creation of compliant, combined, flexible airway-foodway, and speech.9

9. “Reconstructing Human Origins”, Conroy, GC; 2nd ed. WW Norton, NewYork, 2005.

I haven't heard of this book but it seems legit: https://www.amazon.com/Reconstructing-Human-Origins-Modern-S...

Anyway I think there is plenty of scientific evidence. What really matters is that this knowledge makes its way into popular culture and common medical practice. That appears to be happening, but it's surprisingly uneven!

Hope that helps, and feel free to follow up with questions... I'm thinking of starting a blog on this subject, based on reading several books, and talking to many doctors who seem completely unaware of this.

Nestor's book explains why doctors are unaware of it, and why most people get poor treatment for these issues: because doctors are specialists, and they are used to fixing severe problems. They are trained to see their part of the equation. Breathing is more holistic because it affects everything: your teeth, mouth, eyes, brain, heart, etc. And bad breathing kills you over 20 years. It doesn't cause an emergency in most cases.