Which is released into the bloodstream during exercise?

insulin
endorphin
serotonin
melatonin

Endorphin

Explanation:

Endorphins are a type of endogenous neuropeptide, that is, protein chains made by the body itself, which are responsible for stimulating brain areas that produce pleasure for the body. These substances are also known as endogenous opioids, because their chemical composition and performance is very similar to that of opium derivatives, such as heroin and morphine. These are molecules that generally do not produce a nerve signal by themselves, but that modulate and alter neuronal sensitivity to other substances.

Their absence or a low level of them induce depressive and anxious symptoms, making it more difficult to overcome aversive situations and traumas

When intense and regular physical activity is performed, at least three times a week for 60 minutes, endorphins are released.