Can you get too much exercise? It seems that may be the case for some people.
Two new scientific studies have shown that physical activity may have a dark side.
The first study, published in the June issue of the journal Mayo Clinic Proceedings, reviewed several large-scale studies looking at the duration and intensity of exercise and on heart health. The researchers found that while exercise is great for cardiovascular health, exercising for too long might be bad for you.
They argue that extreme endurance training, like for marathons, can cause damage.
"Physical exercise, though not a drug, possesses many traits of a powerful pharmacological agent," the authors write in the study. "As with any pharmacological agent, a safe upper-dose limit potentially exists, beyond which the adverse effects (musculoskeletal trauma, metabolic derangements, cardiovascular stress, etc) of physical exercise training may outweigh its benefits."
The study explains that this extreme training can cause volume overload in the atria and right ventricle. While this is only temporary, repeat trainings over a long period of time might be causing adverse effects, such as atrial fibrillation.
In fact, long-term vigorous exercise training, like for marathon running or professional cycling, has been linked to a five-fold increase in atrial fibrillation prevalence.
"When people come to me as a cardiologist and say they want to run a marathon, I say, 'OK, do one and cross it off your bucket list and then let's focus on an exercise pattern that's more ideal to producing long-term health benefits and improving your longevity,'" Dr. James O'Keefe, lead author and clinician at Saint Luke's Hospital in Kansas City, tells CBC News.
Beyond one hour of intense exercise, the study shows, there's not much added benefit to your health. And for some people, it could even be harmful.
But University of Toronto physical education professor Jack Goodman stresses that this area of research is new, and we shouldn't jump to conclusions.
"Most people who have a long-standing history of endurance activity are much healthier than the average population," he says. "But there is a greater risk for them developing some arrhythmias later in life."
He has studied the short-term strain on the heart after intense exercise and is planning to do more research on how the heart is affected over the long-term as well.
The second study, published in the journal PLoS One, reviewed six previous studies involving 1,687 adults. While researchers noted that physical activity is usually linked with better health, they found that not everyone got the same benefit from working out.
For around 10 per cent of the people studied, exercise had a negative affect on at least one of the heart risks looked at: blood pressure, levels of insulin, HDL cholesterol, and triglycerides. Seven per cent of the participants did worse on two counts.
"It is bizarre," lead author and genetics and nutrition professor at Louisiana State University, Claude Bouchard, tells the New York Times. "It is not possible yet to make more specific recommendations because we do not understand why this is happening."
So while exercise is certainly beneficial for most, beware of over-doing it.
Watch the video below about three must-do machine exercises at the gym.