This is a very interesting question and it’s shaping a lot of my research at the moment. I haven’t much in the way of definitive results yet as it’s a tricky one to measure. Mouth breathing in a breathwork session, for example, has tremendous benefits and has been proven to be great for weight control. But in day to day it has major drawbacks. So, it’s hard to be conclusive.

All I know is that mouth breathing can cause inflammation, a slower digestive system, and a sympathetic nervous system response which, over time, adds weight because toxins aren’t properly removed.

I hope that answers your questions a little bit. If I find more scientific papers tackling this directly, I’ll let you know :)