Not being able to eat after heartbreak is caused by the "fight" mode your body enters after the breakup, according to ter Horst. Your sympathetic nervous system kicks in, whaich is the system that allows your body to respond quickly in the event of an emergency. Your pupils dilate, the pulmonary alveoli widen, and your heart starts to beat faster. Long story short: You're going into survival mode. Having a bite to eat becomes a secondary concern. To help out, the body has found a way to suppress the inevitable hunger pangs: There's fewer constrictions and relaxations of the muscles in your stomach and bowels, which consequently slows down the digestion of food.
In case you were wondering.