Exercise makes us feel good – certainly after the fact, if not always during it. The explanation has seeped its way into popular culture: endorphins. The name is a portmanteau of “endogenous”, meaning produced within the body, and “morphine”, a drug that, like heroin, is derived from the opium poppy. Our brains produce endorphins in response to injury, illness or stress and, just like morphine, they relieve pain and generate a sense of well-being. Exercise also provokes an endorphin release, hence the post-exertional high – and the reason habitual athletes suffer withdrawal if forced to stop sport for some reason.
A growing body of research is beginning to make it clear that endorphins are far from the only compounds our bodies produce when we exercise. Scientists are identifying an ever increasing array of molecules they bundle under the collective term “myokines”. Some of these have unsurprising local effects, stimulating muscle fibre growth – the reason athletes bulk up with training – and new blood vessel formation, which increases the supply of oxygen and nutrients. But there are also many signalling molecules pumped into the circulation by working muscles that influence far-distant tissues and organs, and which help explain the myriad health benefits from being physically active.
An obvious requirement when exercising is for fuel, and myokines optimise the body’s ability to metabolise glucose. In doing so, they can help counter the insulin resistance that drives much Type-2 diabetes. Glucose is our bodies’ instant-energy currency and while we hold reserves in our muscles and liver, these will typically be depleted after 20 minutes of aerobic exercise. Myokines also prime fat cells to release their energy stores once glycogen runs low. Somewhat counter-intuitively, they also seem to stimulate fat cells to “burn” calories purely to generate heat. This may be an evolutionary hangover from the millennia before central heating – shivering helps maintain body temperature. The net effect is to gear our physiology to mobilise rather than accumulate fat, a useful adjunct to weight control.
Several myokines turn out to be molecules that have long been identified as regulators of the immune system, influencing white blood cell populations, modulating the inflammatory response and protecting the linings of blood vessels. In our evolutionary past, running will have been associated with injury risk – whether hunting or fleeing – and a body primed to repair damage and fight infection in wounds will have conferred survival advantage. In less danger-fraught times, these effects help explain the protective effect of regular exercise against conditions linked to chronic inflammation, such as heart disease and stroke.
Intriguingly, at least one myokine identified to date has cancer-inhibiting properties. This may reflect a consistent feature of biology: that molecules have diverse functions in different tissues and organs. However serendipitous, the finding offers a tantalising glimpse of the mechanisms behind the protective effect of regular exercise against many forms of cancer.
Physical activity also helps prevent the two commonest types of dementia. Multi-infarct dementia has many features in common with heart disease and stroke, so this is unsurprising. But physical activity also causes the brain regions affected by Alzheimer’s disease to produce large quantities of a protective molecule called BDNF, which is a myokine.
One of the concerning issues with the current enthusiasm for weight-loss injections is that they don’t just reduce body fat, they cause loss of muscle mass too. And when a drug promises to take care of obesity, the motivation to exercise may be substantially reduced as well. The drive to change the numbers of the bathroom scales through pharmaceutical means might in the long run prove damaging, not beneficial, to health. The unfolding story of myokines shows just how complex biology is. By focusing on one single aspect of health, we risk neglecting and losing far, far more.
This column will be taking a short break while I move to Canada, where I am going to practise as a family physician (as GPs are known) in British Columbia. The column will return later in the spring. The medicine I write about will be broadly the same, but the system will be different – not least due to Canadians’ determination to strengthen general practice as the cornerstone of their healthcare.
This transatlantic perspective will, I hope, prove a valuable lens through which to continue to examine what is happening to medicine and the NHS back home in the UK.
[See also: Why Britain isn’t working]
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This article appears in the 12 Mar 2025 issue of the New Statesman, Why Britain isn’t working