Not sure as it would depend on what kind of deep breathing exercises you're doing - there are so many - and your age, state of health and other factors too.
Deep breathing does burn calories though by increasing your oxygen intake and your metabolism. And of course there are many other related benefits, such as increased energy levels, craving control, better sleep and improved emotional balance.
Each morning I combine a breathing exercise called the Bellows with the Ab Lift and have no doubt it helps firm and tone my belly.
