The feeling of butterflies in your stomach is chiefly associated with adrenaline, also called epinephrine. This hormone is released during excitement, anxiety or perceived danger as part of the body’s fight-or-flight response.
Adrenaline is produced mainly by the adrenal medulla, the inner part of the adrenal glands above the kidneys. It increases heart rate, breathing and blood flow to skeletal muscles. At the same time, the sympathetic nervous system changes activity in the digestive tract, where altered muscle contractions and reduced digestive activity can create a fluttering sensation.
The phrase does not mean that literal movement is occurring in the stomach. It is a vivid description of signals travelling through the brain–gut connection. Related chemicals, including norepinephrine and cortisol, can also participate in the broader stress response, but adrenaline is the standard single-answer response to this quiz question.
Butterflies can arise from positive excitement as well as fear. A first date, public performance, examination or major competition may all trigger a similar physical reaction.