May 3, 2012
Early human history necessitated a very physically active lifestyle: our ancestors hurried while hunting for food, ran to escape from predators and traveled cross-country to look for resources. "Survival of the fittest" evolution caused our bodies to adjust to high levels of exercise and reap benefits from being active. Over time, however, we seem to be getting less and less active while ignoring our evolutionarily built-up strengths and needs. Why is this?
David Raichlen of the University of Arizona theorizes that evolution generated motivation pathways in the brain that encouraged us to exercise. This motivation takes the form of the "runner's high," which inspires a lot of runners to keep pushing through the pain of laborious breath and dripping sweat. This sensation is most likely caused by endocanabinoid signaling in the brain. While naturally produced in the human body, this chemical is also an active ingredient in marijuana that causes good moods and lightened spirits. When we run, the endocanabinoid signaling activates the "reward center" in our brains, reinforcing that exercise is a good behavior to continue in the future.
Raichlen conducted a study on the role of endocanabinoids in other mammals that are physically active to examine the issue further. He and his team found that the concentration of anandamide, a type of endocanabinoid, shot up in the dogs and humans after exertion, but stayed the same in ferrets. The humans reported experiencing a rush of happiness after exercise, which corresponded to the increase in the chemical production. The team concluded that animals evolved for high levels of activity and exercise experience benefits from endocanabinoids, whereas animals that didn't evolve in this way do not.
This led them to believe that natural selection used endocanabinoids to motivate humans to exercise and participate in endurance activities. The fact that these chemicals are released after high-intensity rather than low-intensity exercise suggests that we are meant to participate in intense aerobic activity for maximum health.
Why, then, are we ignoring this positive chemical reinforcement and opting for a more sedentary lifestyle? Raichlen suggests that the physically unfit cannot produce enough endocanabinoids to experience the pleasurable effect of the runner's high, and therefore are less motivated to exercise. They are typically just not fit enough to work out at the intensity required to initiate the runner's high. They then continue to opt for their lazy lifestyle partially because they have no chemical stimulants to push them during exercise.
Expanding upon Raichlen's suggestions, I'd argue that the endocanabinoids present in junk food are being used as a substitute for those created during exercise, leading the inactive to stay inactive. Why work out if you can get the same rush from eating sweets as you can from running sprints?
Of course, if the inactive force themselves to become active, they will eventually reach the fitness level needed for chemical stimulation and will become more motivated and healthy as a result. Replacing the pleasure from exercise with the pleasure of gluttony is not the right answer. So get out there and run.