Exercise is said to foster better health and well-being as well as increasing life expectancy. But you can have too much of a good thing, it seems, as evidence shows that there may an optimal level of exercise and exceeding it may be harmful.
Even 15 minutes of exercise a day is said to increase life expectancy by three years. So it seems odd that research published in PLOS ONE last week shows that less active female mice have a longer life expectancy.
In the study, researchers examined a genetic variant in mice (the t haplotype) that they predicted would express a personality trait, characterised by a shy, less explorative and less active nature. Previously, mice with this genetic variant have been shown to live longer.
They found the mice were indeed less active, less exploratory and tended to consume less food.
So should you hang up your training shoes in favour of a less active lifestyle? Well, no. But there is a growing body of evidence suggesting there may be an optimal amount of exercise for improvements in health and life expectancy.
Wearing yourself out
Research shows that people who exercise vigorously can reduce their risk of early death by 40% with up to approximately 50 minutes a day of activity. Beyond this, there appears to be little additional benefit from exercise.
Similar findings reported elsewhere from large-scale studies suggest people who exercise at moderate intensity and duration may experience greater health and survival benefits than keen athletes who exercise daily at high intensities for much longer.
If you are following exercise guidelines (30 minutes a day, five days a week), it’s unlikely that you will see any negative consequences.
But excessive exercise (such as completing a marathon or ultra-endurance event) places a significant load on the heart that can result in temporary reductions in function.
While this decline is often reversed within one week, the long-term effects of repetitive high-intensity exercise may counteract the benefits of moderate physical activity for your heart.
Studies have shown a high incidence of cardiac fibrosis in older, life-long elite athletes, compared to their younger or non-athletic counterparts.
And the prevalence of a common disturbance in heart rhythm (atrial fibrillation), is considerably higher in athletes, particularly in those who have taken part in more than 1,500 lifetime hours of endurance sport practice.
A greater danger
Despite these findings, the advantages of participating in physical activity are considerable for the majority of the population who choose not to participate in regular extreme endurance events, or in any exercise at all.
It is well documented that sedentary time, whether it be the number of hours sitting or watching television, is independently associated with poorer life expectancy.
The World Health Organization predicts that physical inactivity is responsible for the same number of global deaths per year (over five million) as smoking.
An intriguing series of recent studies has examined the potential metabolic and cardiovascular benefits of interrupting prolonged sitting time (such as during work hours) with short (approximately two minutes) bursts of mild exercise.
They draw attention to more achievable improvements in physical activity levels in the more sedentary parts of the population.
What about the mice?
But what should we understand from the PLOS ONE study on mice? Well, it aimed to evaluate how animal personality relates to behavioural differences, which is quite a different concept to examining how humans approach health and longevity in the 21st century.
While decreasing activity and saving energy may be beneficial for rodents in the face of potential predators or a reduction in food availability (or both), neither tends to be an issue for the modern human in a developed country.
And while the study suggests there’s greater life expectancy with the genetic variant that made the mice timid and sedentary, it wasn’t actually measured.
Finally, a number of additional behavioural differences (lower food consumption, for instance, and a less explorative nature) potentially contributed to improvements in their life expectancy – less movement and activity may reduce the chances of a rodent being detected by a predator.
Associations are one thing, cause and effect is quite another. Just because two things are observed together, doesn’t mean one causes the other or vice versa. So the lower levels of activity in mice may not be the cause of their increased longevity.
Increasing physical activity for both healthy people and those suffering from a chronic disease is undoubtedly beneficial, providing the approach is sensible and within recommended guidelines.
Although there’s a growing trend for greater participation in endurance events such as the marathon, you should consider how prolonged engagement in such activities may negatively impact your health.
For now, stick with the wise words of Dr James O’Keefe and Dr Carl Lavie:
run for your life….at a comfortable speed and not too far.