If you under breathe you will have insufficient blood oxygen, and that forces your enzymes to do extra work. We can think of enzymes as the labor work force of our body chemistry system. Without them, chemical reactions slow to a halt. Most of our metabolic enzymes are manufactured by our bodies. Relatively few are needed for the digestion of the food we eat if the food is live and has its own enzymes intact.
Most people breathe the hard way due to stress and mild or severe breathing blocks.
That's a little or a lot like swimming upstream. Environmental, immune system and food issues aside, asthmatics largely breathe in an over stimulating way. People who under breathe are hard to motivate. This is why I talk so much about properly assessing your breathing. This brings the blocks up to the level of conscious awareness so we can work on their removal.