220 Triathlon (June 2008, p. 65) on breathing while riding your tri bike:
It’s no coincidence that the very best racing cyclists occasionally look fat in photos: they’re using their diaphragms to pull air into the bottom of their lungs, making their stomach expand. Breathing deeply can be improved. And it’s worth it because becoming more efficient with each breath will increase the amount of oxygen available, lowering your heart rate for the same intensity. This allows you to work harder by removing carbon dioxide effectively.