The fittest people could be wrongly diagnosed as having heart disease because their hearts have become enlarged through frequent exercise, a study suggests.
Scientists knew that the hearts of athletes could grow because of excessive training but did not believe the same effect could happen in amateurs.
Now Imperial College has proved that even people who exercise for as little as an hour three hours a week could be increasing their heart muscle.
While a larger stronger heart is beneficial for keeping up fitness levels it may be confusing doctors into thinking that patients are suffering from heart problems, because a diseased organ also appears swollen and enlarged on scans.
"It's well known that the hearts of endurance athletes adapt in response to exercise, a phenomenon called 'athlete's heart'," said Declan O'Regan, of the Medical Research Council Clinical Sciences Centre, at Imperial.
"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."