Deep breathing, especially through the nose, is a foundational pillar of health and poor posture affects your breathing pattern.
Diaphragmatic breathing is difficult because of the restrictions around the chest.
Every breath you take sends “Interoceptive” signals to your brain, from within the lungs, heart, and other organs. All report on the quality of your breath to the insula (the brain's interoceptive cortex).
Now, if you have poor posture, what happens?
The intercostal muscles and diaphragm can't contract well to create a good vacuum--with the abdomen compressed; your organs flat out can't get out of the way. As a result, you don't take in nearly as much Air as your lungs can. Your weak abs also do a sucky job pushing what air you get back out.
A 2006 American Academy of Physical Medicine and Rehabilitation study showed that poor posture (slumping) produced the worst lung capacity and expiratory flow compared to regular sitting and a posture designed to mimic standing spinal alignment.
Without enough O2, you're at risk for issues like fatigue and even stress on your heart.
That can activate a more overall stress response through the body, elevating cortisol.
Elevated cortisol is associated with a host of conditions, such as weight gain, mood swings, and trouble sleeping.
It also links to a shutdown of the brain's executive functions, meaning you can forget about great decision-making or real focus.