by Glenn Wadley, Associate Professor, Institute for Physical Activity and Nutrition (IPAN), Deakin University, Australia.
Endurance exercise and healthy hearts
A wealth of evidence shows that physical activity helps prevent heart disease and all causes of mortality (1) and has benefits for the heart at any age (2): aerobic exercise – often referred to as ‘cardio’ – and in particular endurance-training, is beneficial to the heart. But these effects – in adults – are only temporary and lost soon after training is stopped (3-5). Because of this, it has been assumed until now that the beneficial effects of exercise for the heart are also temporary for young and adolescent mammals, including humans.
The athlete’s heart – when big is beautiful
Moderate levels of endurance exercise training improve the structure and function of the heart, and makes it grow larger, resulting in what is called athlete’s heart, or physiological cardiac hypertrophy (3-5). This larger heart is beneficial and quite distinct from the enlarged hearts observed in disease, which display reduced function, and increased scarring and molecular and structural differences, in addition to heart failure and increased mortality (6). In contrast, athlete’s heart can improve quality of life, since people with a physiologically healthy, bigger heart will pump more blood and thus can train harder at any given age (7). This effect is of particular benefit as adults get older.
The workhorse cells of the body – cardiomyocytes
In the heart, specialised muscle cells do the heavy lifting: they are called cardiomyocytes, and are highly resistant to fatigue. The adult human heart contains several billion cardiomyocytes and their coordinated contraction produces around 100,000 heartbeats per day, every day.
Until recently, we thought that the number of cardiomyocytes in mammals’ hearts was fixed shortly after birth. Adult cardiomyocytes don’t get renewed or multiply much, so we thought that the heart grew larger in response to training because the existing cardiomyocytes grew larger, rather than because there were more of them.
However, our recent study has established that an increase in cell number also plays a considerable role in cardiac growth in response to just four weeks of moderate intensity exercise, if the training is conducted during juvenile life, which is 5-9 weeks of age for a rat. This training period would be equivalent to late childhood and puberty in humans. We also found that this effect of exercise on cell number diminishes with age and is lost by adulthood. Indeed, when the same exercise training program is conducted in adolescent rats (11-15 weeks of age, or around the time of late puberty and reproductive maturation in humans), there is a much smaller impact on cardiomyocytes multiplying. In adult rats, heart mass and cardiomyocyte size still increase following exercise training, but without any increase in cardiomyocyte number. Clearly, endurance exercise is beneficial for the heart at any age, but it appears that a window of time exists in the younger heart whereby exercise might be able to grow more cardiomyocytes.
With regards to the benefits of juvenile exercise for the heart, perhaps the most compelling finding is that the increased heart mass and around 40% increase in cardiomyocyte number remain well into adulthood. What’s more, this increase in cell number is sustained despite the rats being couch potatoes for a prolonged period of time – the equivalent of 10 years in humans! Having more cardiomyocytes potentially makes the heart better equipped for the structural and functional challenges of adult life. For example, in the UK, the 915,000 survivors of heart attack (8) are left with a heart containing up to 25% fewer cardiomyocytes (9) that are not replaced, along with a large degree of scarring and fibrosis. Thus, having more cardiomyocytes saved up for a rainy day could be a handy reserve if you are unfortunate to suffer a cardiac event.
There are already plenty of good reasons to exercise regularly and we know exercise is beneficial for heart health at any stage of life. However, should these recent findings translate to humans they would provide a new reason to ensure there are sufficient opportunities for children to engage in regular physical activity in school curricula. Importantly, our research suggests that there may be long-term cardiac benefits of physical activity for all children, even if the children do not continue with regular exercise in adulthood. Unfortunately, we know the majority of children do not meet physical activity recommendations (10) and therefore their hearts may be missing out on the best start to life.
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- Australian Bureau of Statistics, Australian Health Survey: Physical Activity, 2011-12. 2013. http://www.abs.gov.au/AUSSTATS