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Previous studies have found that regular exercisers tend to have lower levels of anxiety and depression compared with less active people.Getty Images/iStockphoto

Exercise seems a bit like cilantro: some people love it, others hate it. But what accounts for the chasm between those who dread the gym, and those who dread missing even a single session there?

A new study of the brain’s signalling networks in the journal Medicine & Science in Sports & Exercise offers an optimistic perspective on the prospects for bridging this gap. Stick with your exercise routine through those initially unpleasant weeks, the results suggest, and you too can learn to love the gym, thanks to long-term adaptations in how your brain processes mood-altering chemicals.

Previous studies have found that regular exercisers tend to have lower levels of anxiety and depression compared with less active people, and they also get a bigger mood boost after a single workout. It’s a self-reinforcing cycle: you’re more likely to be motivated for your next workout if the last one made you feel great.

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But it’s not clear how this cycle gets started, or what changes in brain chemistry make it possible. To find out, researchers at the University of Turku in Finland recruited 64 volunteers to complete a series of exercise tests and questionnaires. They used a medical imaging technique called positron emission tomography to measure the activity of mu-opioid receptors, or MORs, in the brain.

These MORs respond to the presence of endogenous opioids, the body’s own version of opioid drugs, and play a role in processing reward, pain, motivation, stress, and emotions.

“It is possible that some people are born with a more responsive MOR system, and it helps them to tolerate and like exercise, and that’s why it’s easy for them to engage in higher levels of exercise,” explains Tiina Saanijoki, the study’s lead author. “Or it can be the other way round, so a better-functioning MOR system has developed through regular exercise habits.”

Saanijoki put her volunteers through a test in which they cycled to the point of exhaustion to assess their aerobic fitness. On another day, some of them then did an hour of continuous moderate cycling, and others did a session of high-intensity interval training on the bike, in order to determine how different types of exercise affected opioid signalling.

Sure enough, the fittest subjects (as measured both by the cycling test and by their self-reported levels of weekly exercise) saw the biggest change in MOR activity after the continuous moderate workout. The same was true for the heaviest exercisers after the high-intensity interval workout. The more exercise you do, it seems, the more neurochemically satisfying it gets.

This still doesn’t prove that regular exercise leads to a more responsive opioid system, as opposed to the other way around. But studies in rats offers some suggestive evidence for the former explanation. For example, rats that exercise for five to eight weeks have higher levels of endogenous opioids such as endorphins circulating in their brains.

There’s an important caveat here. The talk of opioids and endorphins may suggest that exercise triggers some sort of euphoric bliss in responders. That’s the impression that emerged from early research into what’s known as runner’s high in the 1970s, but subsequent studies have found that such experiences are exceedingly rare.

“Runner’s high is quite a mythical thing, and maybe not worth focusing on too much,” Saanijoki says.

Instead, the sensations and emotions triggered by exercise tend to be subtle, sometimes operating below the level of consciousness, and are likely mediated by several different sets of brain chemicals in addition to opioids, including endocannabinoids, the brain’s own version of cannabis.

Still, the findings suggest two key takeaways. The first is that, if you don’t enjoy exercise, that doesn’t necessarily mean you’re doing something wrong. “I think we need to acknowledge that there’s a huge variation between individuals in these responses, and not all people find exercise pleasant or rewarding,” Saanijoki says.

The second is that this can change. Just as your body adapts to a new exercise routine, so too does your brain – and, if Saanijoki’s hypothesis is correct, you may eventually wonder how you ever lived without it.

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.

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