Researchers suggests that the release of endorphins varies by exercise intensity.
Researchers found that adults who engaged in an hour of high-intensity interval training (HIIT) experienced a significant increase in endorphin release compared with those who engaged in an hour of less demanding physical activity.
Study co-author Tiina Saanijoki, of the Turku PET Centre at the University of Turku in Finland, and colleagues recently reported their results in the journal Neuropsychopharmacology.
Current guidelines suggest that adults should engage in at least 150 minutes of moderate-intensity aerobic activity or 75 minutes of vigorous-intensity aerobic activity every week in order to improve or maintain physical health.
But the benefits of exercise are not just physical. A number of studies have shown that exercise can boost mood and help to alleviate anxiety and depression. Such effects have been attributed in part to the release of endorphins triggered by exercise.
Endorphins - often referred to as "feel-good hormones" - are peptides produced by the brain that bind to the brain's opiate receptors, reducing the perception of pain and triggering feelings of euphoria.
"The level of plasma β-endorphin [beta-endorphin] is usually elevated during intense exercise," say Saanijoki and colleagues, "but a plausible link between circulating endorphin concentrations and mood responses to acute exercise has not been established."
Moderate vs. intense exercise
For their study, the researchers sought to determine whether or not there are differences in endorphin release in response to conventional aerobic exercise and HIIT, which is a form of exercise comprising short bursts of highly intense activity, broken up by brief periods of less demanding activity.
The team enrolled 22 healthy men, all of whom were aged between 21 and 36 years, to the study.
On two separate days, the men completed 1 hour of moderate-intensity aerobic exercise and 1 hour of HIIT. Using positron emission tomography (PET), the researchers measured the endorphin release of each subject after each session, as well as after a rest period.
The mood of each participant after the completion of each exercise session was also assessed.
The researchers found that HIIT led to a significant rise in the release of endorphins in the men. This occurred in areas of the brain associated with pain, reward, and emotion, including the thalamus, insula, orbitofrontal cortex, hippocampus, and anterior cingulate cortex.
Furthermore, the team found that HIIT caused negative feelings in the men, which was also associated with an increase in endorphin release.
"At very high exercise intensities the release of endorphins appears to be linked to increased negative feelings and pain, and may be needed to manage the emotionally and physically demanding challenge," explains Saanijoki. "However, such negative feelings may discourage further exercise."
After moderate-intensity aerobic activity, the men reported feelings of pleasure and euphoria, which they found corresponded with endorphin release.
"At moderate training intensities, the pleasurable sensations caused by the possible release of endorphins may promote habitual exercise," notes Saanijoki.
Overall, the researchers believe that their study sheds light on how different intensities of exercise influence endorphin release.
"Our results highlight that exercise intensity affects endorphin release and that the brain opioid system is involved in both positive and negative feelings caused by physical exercise performed at different intensities," says Saanijoki.
"Exercise-induced endorphin release may be an important mechanism which supports exercise motivation and maintenance of regular exercise. [...] Exercise intensity should be taken into account when starting new exercise routines."