"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."
Scientists have not previously known the extent to which the hearts of healthy people adapt to the demands of moderate exercise.
More than 1000 people took part in the study and around one third of participants reported doing three to five hours of exercise a week.
The scientists found that one in five of those people had developed an enlarged heart as result.
Similar adaptations were seen in almost half of those who reported doing more than five hours of exercise.The adaptations allow the heart to pump more blood, which helps to supply muscles with the oxygen and nutrients they need when exercising.
"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," added O'Regan.
The study suggests that a person's exercise level should be noted alongside a heart scan and electrocardiogram, or ECG, to rule out those who do not need treatment.
Dr Noel Faherty, research adviser at the British Heart Foundation, which part-funded the study, said: "This demonstrates the importance of documenting the MRI appearance of healthy, active people's hearts so normal adaptive changes are recognised by doctors."