Dr Michael Hunter reveals his conclusion in a new episode of the TV documentary series Autopsy: The Last Hours Of… which airs tonight in the US and later this year in the UK.
In a painstaking examination of Cash’s medical records from childhood to his death in September 2003, aged 71, Dr Hunter identifies “reckless” drug abuse, out-of-control diabetes and a smoking habit since the age of 10 as contributory factors.
He says the gravel-voiced star’s history of heart problems, including a double bypass operation, and regular bouts of pneumonia, possibly caused by the air he inhaled growing up on a Dust Bowl farm in Arkansas, all played a part in his downward health spiral.
However, Dr Hunter is convinced an extreme case of a far more common ailment – acid reflux or heartburn – was the underlying reason behind his death almost 15 years ago.
He reached the conclusion after discovering Cash was still taking an astonishing cocktail of 30 prescription drugs a day, right up until he died.
Friends confirmed Cash was suffering so intensely from heartburn he could only sleep sitting upright in bed.
Dr Hunter declared: “This was an extreme case of polypharmacy, where so many drugs were being taken there was a risk of dangerous interaction between them. Too many medications can be poisonous and toxic.”
At the same time, nerve damage caused by Cash’s diabetes was severely affecting his stomach, according to the doctor, “which was unable to regulate the amount of gastric acid being produced.
“Over time, acid built up in the stomach until it was ejected into his oesophagus and into his trachea. At this point, the acid would seep into his throat and lungs, causing enormous damage”.
X-rays showed Cash’s lungs to be virtually black as a result of six decades of chain-smoking.
Dr Hunter added: “What little healthy lung tissue that was left was compromised by the acid, leading to respiratory failure and his death.”
He declared: “Johnny lived up to his tough, outlaw image.
“He made his own rules and lived the life he wanted, despite putting his body through everything he did.
It shows what an incredible fighter he was and just how resilient the human body can be.”
Cash sold 90 million records worldwide, including the hits Ring Of Fire and I Walk The Line.
However, Dr Hunter dismisses the poignant notion that he died of a broken heart just four months after the death of June, his beloved wife of 35 years.
“The pain of her being gone is more than I can stand,” Cash had declared.
But Dr Hunter said: “Although it sounds improbable, there is a condition called stress-induced cardiomyopathy, which is also known as broken heart syndrome.
“It’s commonly described as the physical pain in the heart or chest area resulting from a surge in stress hormones.
“This can be triggered by an emotionally stressful event, such as the death of a loved one.”
He added: “Broken heart syndrome mimics symptoms of a heart attack and, even though Johnny had heart problems, there is no evidence he suffered a heart attack when he died, so I can rule this out as a cause of death.”
