The "runner's high" effect is the result of the brain naturally producing opioids after exercise.
The new study, which is published in the journal Brain Plasticity, investigates the effect of acute exercise - defined as a single session of physical activity - on human mood and cognition.
The research was carried out by scientists at the Center for Neural Science at New York University (NYU) in New York City.
As the authors of the new study explain, although research has already been carried out on the effects of acute exercise on cognition and mood, fewer studies have focused on the neurobiological underpinnings of these effects.
Focusing on the effects of a single session of exercise is important because it helps researchers to gain a better understanding of how sustained exercise over a longer period of time may add up to create long-term changes in the brain.
Besides summing up the existing findings, the new review also focuses on the cognitive and behavioral changes that take place in both humans and rodents after an intense single session of exercise. The review also points out the strengths and limitations of the current literature, identifying directions for future research.
Lead investigator Wendy A. Suzuki, Ph.D., a professor of neural science and psychology in the Center for Neural Science at NYU, explains the context of the new study:
"Exercise interventions are currently being used to help address everything from cognitive impairments in normal aging [...] to motor deficits in Parkinson's disease and mood states in depression. Our review highlights the neural mechanisms and pathways by which exercise might produce these clinically relevant effects."
The review mainly examines the effects of a single session of aerobic exercise, lasting approximately 1 hour. Prof. Suzuki and her team reviewed information available from brain imaging and electrophysiological studies.
Acute exercise improves mood, focus, and reduces stress
The existing research shows that physical activity may protect against neurodegeneration and other aging-related forms of cognitive impairment. The authors report several key findings.
Firstly, across all of the studies reviewed, acute exercise consistently resulted in three main effects: better executive function (the mental processes that help us to plan, focus, and multitask); better mood; and lower stress levels.
Additionally, the studies revealed that acute exercise activates several extended brain areas. "One of the most dramatic effects," the authors write, "is the change in neurochemical levels."
This includes neurotransmitters; exercise was shown to increase levels of dopamine, epinephrine, norepinephrine, and serotonin. Dopamine helps the brain to learn and is involved in the rewarding circuits.
A certain level of physical activity seems to be able to increase serotonin, which relieves anxiety and depression, in both humans and rodents.
Furthermore, a single exercise session seemed to increase levels of so-called neuromodulators. These include endogenous opioids and endocannabinoids - that is, "feel good" chemical substances that are naturally produced by the brain when we exercise. These account for the runner's high effect and exercise-induced states of euphoria.
Endogenous opioids are involved in the brain's response to pain and stress, as well as in self-control and reward.
One of the most challenging areas for future research is understanding the relationship between the post-exercise neurochemical changes found in rodents, and the behavioral ones found in humans.
This is due to a "gap" in the current literature signaled by the researchers, where an insufficient number of studies have investigated the cognitive and behavioral effects of acute exercise in rodents.
"The studies presented in this review clearly demonstrate that acute exercise has profound effects on brain chemistry and physiology, which has important implications for cognitive enhancements in healthy populations and symptom remediation in clinical populations.
Study co-author Julia C. Basso, Ph.D., Center for Neural Science at NYU