Endurance athletes have long appreciated the exercise-induced altered state of consciousness. Runner’s high has been described as pure happiness, elation, a feeling of unity with one’s self and/or nature, endless peacefulness, inner harmony, boundless energy and a reduction in pain sensation. However, some runners never experience these feelings in any measure and others experience them only occasionally.
Finding a biochemical basis for this has intrigued scientists since the 1970’s. The first candidates to explain the analgesic (pain reducing) and euphoric mental states were adrenaline and noradrenaline. However, research soon showed that their rise and fall in the blood stream did not correlate well with the sensations felt by the runner. The next candidates were opioids (endorphins), which are made within our bodies and produce effects similar to externally injected opiates (like morphine). However, some observations do not quite fit this hypothesis. One is that opiates depress respiration, contract pupils and inhibit gastrointestinal mobility. An experimental issue that might be confounding measurements is that opiates are very similar to another active compound, adrenocorticotrophic hormone, whose concentration increases with stress (such as exercise). Another conceptual issue is that opiates/endorphins are too large and of the wrong chemical type (peptides) to easily cross the blood-brain barrier and therefore seem unlikely to be able to produce the euphoric aspects of a runner’s high.
What Dietrich and McDaniel (1) proposed recently is that the exercise-induced analgesia (pain reduction), euphoria, addiction (just ask a marathoner to stop training for two weeks!!), reduced anxiety, impaired short-term memory (how long have we been running??) and silent introspection are really similar to the effects of smoking marijuana (!!!). They got this idea from measurements in their lab that showed that exercise activates the endocannabinoid system (endo meaning within our body). Within our bodies there are receptors for cannabinoids (like the active ingredients in marijuana) and a mechanism for making them. There are cannabinoid receptors in muscle, skin, endothelial cells and lungs.
Their ideas were based on the following observations. Using trained male college students running on a treadmill or cycling on a stationary bike at 70-80% of maximum heart rate, they found that the concentration of anandamide in the blood increased dramatically. Because endocannabinoids are lipophilic (fat liking), they can easily cross the blood-brain barrier (and therefore affect what is happening in our brains). Unlike opiates (endorphins), canabinoids do depress our breathing or inhibit gastrointestinal motility. In fact cannabinioids, in general, act as brochodilators and help in breathing. Other researchers have found that tissue damage (mechanical, thermal, chemical, etc.) activate the endocannabinoid system. Thus the “pounding of the pavement” by our feet may produce micro-damage that activates the endocannabinoid system…. So, who’s up for a run?
(1)A. Dietrich and W.F. McDaniel, Endocannabinoids and exercise, British Journal of
Sports Medicine, Vol. 38, p. 536-541, 2004.
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