I have been reading articles claiming that serotonin, endorphins and BDNF are produced during exercise, but they never mention the intensity or duration of exercise for this to happen.

To not make this question too broad, I want to narrow it down to, lets say, a 4-5k run over 20 minutes. Also, I want to restrict it to the body's response during these 20 minutes.

How will this affect the brain? Which chemicals do the body produce/release into the blood stream that could have an effect on the brain? And, how much?

I am asking because I am always curious about the mechanisms underlying the effects of exercise on the brain and because it's always difficult to find information regarding results of exercise; Googling mostly results in content marketing articles (like this one for Buffer).


In this article The Effects of Acute Exercise on Mood, Cognition, Neurophysiology, and Neurochemical Pathways: A Review (Brain Plasticity, 2017), there's a comprehensive table The Time Course of Behavioral, Functional, Physiological, and Neurochemical Effects of Acute Exercise (scroll to Fig 1 and zoom it) that shows the duration and intensity of exercise needed for the effective release of cortisol, neurotrophins (BDNF, IGF-1), neurotransmitters (dopamine, norepinephrine, serotonin, acetylcholine, GABA and gltamate) and endogenous opioids.

Examples of effects of exercise on the cognitive function and mood:

  • Prefrontal-dependent cognitive function (learning, memory): 50 min exercise at 85% of age-predicted mean heart rate
  • Enhancement of positive mood: low to high intensity exercise 7-70 min
  • Decrease in negative mood: 60-120 min exercise at ~50% intensity


In humans, acute exercise stimulates the HPA axis in an intensity-dependent fashion, with increases in cortisol occurring after a threshold amount of exercise that equates to a duration of 10 minutes or more at approximately 60% of VO2 max or greater...After exercise cessation, peripheral cortisol levels peak around 30 minutes and remain elevated for up to two hours.

...cortisol strongly influences learning and memory...The extent to which cortisol influences learning and memory processes depends on several factors including the intensity of cortisol elevation. For example, moderate increases in cortisol enhance working memory...Cortisol elevation over a certain threshold, however, serves to impair memory...

acute exercise also helps mitigate the debilitating effects of stress.

Brain derived neurotrophic factor (BDNF):

Moderate exercise for 30 minutes can release BDNF.

...studies have shown a positive association between acute exercise-induced peripheral BDNF levels and short-term memory as well as motor skill memory.

But, this does not work in everyone and not in all circumstances...


Twelve, healthy, regular exercisers underwent PET scans after either rest or 30 minutes of treadmill running at 85% of age-predicted MHR. Surprisingly, dopamine was not found to increase after acute exercise in these humans.


Acute exercise significantly increased plasma serotonin, with a positive linear correlation found between serotonin concentration and exercise intensity.

As serotonin regulates mood, emotion, sleep, and appetite, and dopamine regulates motivation memory, reward, and attention, the central fatigue hypothesis claims that the interaction between these two neurotransmitters contributes to exercise-induced fatigue, with a high serotonin to dopamine ratio supporting a low activity, exhausted state and a low ratio supporting a high activity, activated state.

The effects of exercise-induced increase of norepinephrine and acetylcholine on cognitive function are not well known.

Endogenous opioids (ß-endorphins, enkephalins and dynorphins) and endocannabinoids:

Neuromodulators are involved in a variety of processes including pain modulation, reward, response to stress, and autonomic control.

In humans, acute exercise causes significant increases in peripheral levels of endogenous opioids; this effect is intensity-dependent, corresponds to acute exercise-induced changes in HPA axis hormones, and is linked to improvements in mood.

Though the endogenous opioids have received much attention in terms of their involvement in the “runner’s high”, scientists are beginning to understand that endocannabinoids may be equally or perhaps more involved.

Other sources:

Effects of physical exercise on anxiety, depression...(Clinical Psychology Review, 2001):

Acutely, emotional effects of exercise remain confusing, both positive and negative effects being reported.

...or as Alex Corb, PhD says in Boosting Your Serotonin Activity (Psychology Today):

Interestingly, if you try to do too much exercise, or feel forced into doing it, it may not have the right effect. Recognizing that you are choosing to exercise changes its neurochemical effect. That may be a result of your ancient instincts — the difference between running because you're hunting something, and running because it's hunting you.

How to increase serotonin in the human brain without drugs (Journal of Psychiatry & Neuroscience, 2007):

Exercise improves mood in subclinical populations as well as in patients. The most consistent effect is seen when regular exercisers undertake aerobic exercise at a level with which they are familiar.

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