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As hunter-​gatherers human­be­ings have been long-​distance endurance ath­letes. We were wired to run, accord­ing a team of researchers led by David Raichlen, with the release of endo­cannabi­noids in the brain as the ulti­mate reward, the effect called ‘runners-​high’. In the last cen­tury some­thing unex­pected hap­pened: humans became sedentary.

We traded in our active lifestyles for a more immo­bile existence.

But these were not the con­di­tions under which we evolved. David Raichlen from the Uni­ver­sity of Ari­zona, USA, explains that our hunter-​gatherer pre­de­ces­sors were long-​distance endurance ath­letes. ‘Aer­o­bic activ­ity has played a role in the evo­lu­tion of lots of dif­fer­ent sys­tems in the human body, which may explain why aer­o­bic exer­cise seems to be so good for us’, says Raichlen. How­ever, he points out that test­ing the hypoth­e­sis that we evolved for high-​endurance per­for­mance is prob­lem­atic, because most other mam­malian endurance ath­letes are quadrupedal. ‘So we got inter­ested in the brain as a way to look at whether evo­lu­tion gen­er­ated exer­cise behav­iours in humans through moti­va­tion path­ways’, says Raichlen.

Explain­ing that most human ath­letes expe­ri­ence the infa­mous ‘runner’s high’ after exer­tion, which is caused by endo­cannabi­noid sig­nalling in the so-​called ‘reward cen­tres’ of the brain, Raichlen adds that the reward sys­tem is also able to evolve, becom­ing more potent in mice that increase their exer­cise lev­els over gen­er­a­tions. How­ever, lit­tle was known about the role of endo­cannabi­noids in the other aer­o­bi­cally active mam­mals, so Raichlen, Gre­gory Gerde­man and their col­leagues decided to find out how exer­cise influ­enced the endo­cannabi­noid lev­els of two mam­malian nat­ural ath­letes — humans and dogs — and a low activ­ity species — fer­rets. Recruit­ing recre­ational run­ners and pet dogs from the local com­mu­nity, Raichlen and Adam Fos­ter trained the par­tic­i­pants to run and walk on a tread­mill and col­lected blood sam­ples from the par­tic­i­pants before and after the exer­cise. Unfor­tu­nately, the fer­rets were less coop­er­a­tive, so the team col­lected the fer­rets’ blood sam­ples after exer­cise and dur­ing rest. Next, Andrea Giuf­frida and Alexan­dre Seil­lier analysed the endo­cannabi­noid lev­els in the blood sam­ples and found that the con­cen­tra­tion of one endo­cannabi­noid — anan­damide — rock­eted in the blood of the dogs and humans after a brisk run. And when the team tested the human run­ners’ state of mind, they found that the ath­letes were much hap­pier after the exer­cise. How­ever, when the team analysed the fer­rets’ blood sam­ples, the animal’s anan­damide lev­els did not increase dur­ing exer­cise. They did not pro­duce endo­cannabi­noids in response to high– inten­sity exercise.

This leads Raichlen to sug­gest that nat­ural selec­tion used the endo­cannabi­noid sys­tem to moti­vate endurance exer­cise in humans and other ani­mals that walk and run over long dis­tances. He says, ‘These results sug­gest that nat­ural selec­tion may have been moti­vat­ing higher rather than low– inten­sity activ­i­ties in groups of mam­mals that evolved to engage in these types of aer­o­bic activities.’

Hav­ing found that exer­cis­ing mam­mals release plea­sur­able endo­cannabi­noids in response to exer­cise, could these brain chem­i­cals be the magic bul­let that solves the obe­sity cri­sis? Sadly not, says Raichlen, who explains that couch pota­toes are not about to leap sud­denly out of their comfy chairs and expe­ri­ence the plea­sur­able effects of exer­cise, because they prob­a­bly can­not pro­duce enough endo­cannabi­noids. He says, ‘Inac­tive peo­ple may not be fit enough to hit the exer­cise inten­sity that leads to this sort of reward­ing sen­sa­tion.’ How­ever, he is opti­mistic that inac­tive indi­vid­u­als can be helped to build up their exer­cise tol­er­ance until they cross the thresh­old where they become moti­vated to exer­cise by endo­cannabi­noids. Raichlen also sug­gests that exer­cise could be a cheap solu­tion to many med­ical con­di­tions, improv­ing our men­tal state through the endo­cannabi­noids and our car­dio­vas­cu­lar and pul­monary con­di­tion through good old– fash­ioned exertion.

(Sources: The Jour­nal of Exper­i­men­tal Biol­ogy, 15.04.2012 (online March 2012))