It isn’t true. At the very least it is misleading.
Our nervous system forces us to breathe at the necessary rate to fully oxygenate our blood at all times. This is true when we’re resting as well as when we’re running a marathon. The body’s oxygen requirements are different depending on our activity, and the nervous system automatically adjusts to oxygenate the blood fully. Normal blood oxygenation is 95-100%.
When we are at rest, we breathe slower since the body’s oxygen demands are less. Breathing more or deeper while the body is at rest does not “oxygenate” the blood. It hyper-ventilates, which is just a way of saying breathing (ventilating) more than necessary.
Anytime we breathe more than the body requires, the big change we are causing is the removal of carbon dioxide, one of the body’s metabolic wastes. Hyperventilation removes more carbon dioxide than the body produces, and this leads to a whole host of physiological effects. Most significantly, the blood vessels in the brain constrict, limiting blood flow to the brain. This makes us light-headed.
So that light-headed feeling you get when breathing deeply isn’t energization or “oxygenation,” but constricted blood vessels in the brain. Your brain is actually getting less oxygen than if you were to breathe normally!
