When you over-exercise you stress out the body and emphasize the secretion of stress hormones,
namely cortisol. It should be clear why this is a problem at this stage. Just remember our equation:
Excess Calories + Excess Cortisol + Excess Insulin = Weight Gain.
Even if you are in a caloric deficit, if you over-exercise the excess cortisol you produce is likely to
minimize or undo your result. This is a key reason the whole “eat less, exercise more” paradigm can’t work