The state of euphoria induced by prolonged exercise was known first as ‘second wind’ and more recently as ‘runner’s high’. Scientists originally attempted to explain the experience in terms of the effects of the ‘stress hormones’ adrenaline and noradrenaline. Then came the ‘endorphin hypothesis’. And now we have the ‘endocannabinoid hypothesis’: a suggestion that the physical and psychological wellbeing experienced by many endurance athletes is due to the exercise-induced activation of endogenous cannabinoids – lipids whose actions in the body resemble those of the active constitutent of cannabis.

This theory, supported by scientific evidence that exercise boosts blood concentrations of endocannabinoids, is given a thorough airing in a review by US researchers published in the British Journal of Sports Medicine.

Their first point is that the endorphin hypothesis – that the runner’s high is induced by the release of endogenous opioids in response to exercise – doesn’t hold water because, among other lesser reasons, these chemicals are simply too large to cross the blood-brain barrier and exert the central effects that are claimed for them.

The endocannabinoid hypothesis, on the other hand, is supported by the following observations:

• Unlike opioids, endocannabinoids can suppress pain at peripheral sites as well as centrally;
• Unlike opioids, they do not produce such side effects as severe respiratory depression, pinpoint pupils and constipation;
• Endocannabinoids inhibit swelling and inflammation and reduce pain caused by the release of chemicals (such as lactic acid);
• The intense psychological experiences reported by users of cannabis – sedation, reduced anxiety, distortions of time estimation, euphoria, enhanced sensory perception and feelings of wellbeing – are strikingly similar to the experience of runner’s high;
• Research on animals has suggested that one of the principal roles of the endocannabinoid system may be the refinement of movements needed for coordinated locomotion;
• Activation of endogenous cannabinoids through exercise could account for the phenomenon of exercise addiction;
• Endocannabinoids act as vasodilators and bronchodilators, which should make exercise feel easier.

As the authors of the review point out: ‘Further research is necessary to characterise the precise nature of this endocannabinoid response to exercise, specifically the relative importance of factors such as the nature of the activity, exercise duration, exercise intensity, sex and age.’

But in the meantime they suggest that the endocannabinoid hypothesis is a feasible alternative to the endorphin theory and should be investigated as such.

Br J Sports Med 2004;38:536-541