A jogger in his 50s was found lying on the ground along the MacRitchie Nature Trail near the Singapore Island Country Club (SICC) last Saturday morning.
The Straits Times understands that he suffered a cardiac arrest and died later in hospital.
A Singapore Civil Defence Force spokesman said it received a call for help at 240 Sime Road - SICC's address - at around 11.05am. It sent a fire bike and an ambulance to the scene.
The man was taken to Tan Tock Seng Hospital.
A fellow jogger, Mr Patrick Yap, told The Straits Times yesterday that he was about to complete his run when he saw the man, who wore a blue singlet and black shorts, lying on the ground in the distance.
"I just happened to pass SICC and saw from afar that there was a man lying on a patch of grass just beside the running path," said Mr Yap, 37, who is self-employed in the recruitment industry.
Together with a female jogger who was approaching from the opposite direction, they went over to attend to him but found him completely unresponsive.
Mr Yap called for an ambulance, before a family stopped to help.
"The wife was a former nurse, so she put him in the recovery position," Mr Yap recounted, referring to a position in which the person is lying down and turned to the side with the head facing sideways.
"That was when he could breathe for a bit, gasping for air.
"We did not do cardiopulmonary resuscitation (CPR) as there was a faint heartbeat," he added.
"But about two or three minutes later, I noticed he had stopped breathing, so that was when we started compressions.
"An SICC buggy happened to be driving by, and we shouted at the staff to get the automated external defibrillator."
Mr Yap said the man, who had only a set of car keys on him, was unconscious when taken into the ambulance.
When a person is exercising, his blood pressure and heart rate go up, and this could cause cholesterol deposits in the arteries to get dislodged, causing a cut in an artery, said Dr Kenneth Ng, a cardiologist at Novena Heart Centre.
"This would cause a blood clot to form in the artery, leading to a heart attack," he added.
While cholesterol deposits can form in men and women of all ages, the accumulation of such deposits in an older person could make the patient more susceptible to suffering a heart attack while exercising, he said.
If a person encounters a patient suffering a heart attack, he should call for an ambulance immediately.
CPR should also be administered if the patient is unconscious.
If the patient is conscious, putting him in a recovery position so that he can breathe without choking is also a good step.
Administering crushed aspirin, if available, could also help to thin the blood, said Dr Ng.
•Additional reporting by Audrey Tan