An enlarged heart as a result of moderate exercise could be misdiagnosed as heart disease.
The physical demands placed on the body can trigger the heart of an athlete to grow - a condition called "athlete's heart," which is non-fatal.
The heart becomes enlarged with slightly thicker muscle walls, bigger chambers, and lower heart function due to high endurance sporting activities.
Athlete's heart can often be confused with a potentially dangerous condition called hypertrophic cardiomyopathy (HCM), and it can be difficult to differentiate between them.
"It's well known that the hearts of endurance athletes adapt in response to exercise, a phenomenon called 'athlete's heart.' This study is the first to show that healthy adults who do regular exercise may also develop enlarged hearts. As a result, there's a risk that some active adults could be misdiagnosed with heart disease," says Declan O'Regan, of the MRC Clinical Sciences Centre, based at Imperial College London, and one of the lead scientists on the research.
Previously, the extent to which hearts of healthy people adapt to moderate exercise has been unknown. The new study analyzed the activity levels of 1,000 people over the past year by asking them to select one of four categories to identify how many hours of exercise they achieved each week.
The findings, published in Circulation: Cardiovascular Imaging, showed that one third of participants reported 3-5 hours of exercise per week, and 1 in 5 of these individuals developed an enlarged heart as a result.
The same heart adaptations were observed in almost half of the participants who reported completing more than 5 hours of exercise per week.
Researchers suggest that doctors should consider a person's activity level before diagnosing common heart conditions.
Frequenting the gym could increase heart muscle thickness
Above a threshold of 3 hours, the more activity that the person does, the more the heart is likely to adapt, and the more the exercise, the more pronounced the changes, the research team suggests.
"Going to the gym frequently increases the thickness of your heart muscle and the volume of your heart chambers, particularly the right ventricle. It's a completely normal, healthy response. It shouldn't be misdiagnosed as being heart disease," explains O'Regan.
Changes in the heart allow for more blood to be pumped, which helps supply exercising muscles with the oxygen and nutrients they need to function. Changes to the heart's thickness and volume occur in tandem, which distinguishes them from changes seen in disease, which occur in isolation.
According to O'Regan, doctors worldwide use a standard set of values - the data that underpins these ranges comes from a relatively small study with people who were mainly sedentary - to see if the thickness and volume of a person's heart fall into the healthy or abnormal range, to ensure consistency between various hospitals.
"In this latest study, we looked at a much larger and broader group of people. We found that more people reported being active than had done in previous studies. Our recommendations reflect this growing participation in exercise," he says.
Healthy or abnormal heart ranges should account for activity levels
O'Regan notes that updating the ranges to account for activity should be straightforward. An individual's height, age, and gender - factors that are known to change the shape and structure of a person's heart - are already accounted for in analysis.
People who exercise regularly may develop hearts that are larger (right), with thicker muscle and bigger chambers, than those who are more sedentary (left).
Image credit: MRC Clinical Sciences Centre
A person's activity level could be established in the clinic in the same way as the present factors.
"This study was based on an everyday clinical problem. We frequently look at cardiac scans where people have enlarged hearts, and whilst they may have a family history of heart disease, they also do regular exercise. So the question is, have they actually got inherited heart disease, or is it just that they're active?" says O'Regan.
Knowledge of a person's exercise level can be used in combination with a heart scan and electrocardiogram (ECG) measures, to identify people who may need treatment.
"The events in Rio will undoubtedly inspire many of us to put on our running shoes and get active," says Dr. Noel Faherty, Research Advisor at the British Heart Foundation, which helped fund the research.
"And this interesting research shows that even moderate physical activity is associated with changes in the heart's size and shape, which are visible on a cardiac MRI," he adds.
"Detectable changes to the heart on an MRI scan are common in elite endurance athletes but some heart conditions, like cardiomyopathy, can be diagnosed by detecting similar changes. This study demonstrates the importance of documenting the MRI appearance of healthy, active people's hearts, so normal adaptive changes are recognized by doctors and not mistaken for disease."
Dr. Noel Faherty
O'Regan and colleagues analyzed the genes of participants to rule out individuals with a family history and predisposition to heart disease to ensure all involved participants had a healthy heart, and that, therefore, any changes in heart structure were the result of exercise.