A senior doctor yesterday spoke of his conviction that three-year-old Christian Blewitt's would-be adoptive parents never gave him a massive overdose of salt.
Dr Glyn Walters said that, after reviewing the evidence, he had suspected from early on that the high level of sodium in Christian's bloodstream was due to "a failure of his regulatory systems" and not to sudden ingestion of a huge "load" of salt.
Responding to prosecution claims that Ian and Angela Gay caused Christian's "acute poisoning" by forcing a dose of between 30 to 40 grammes of salt into his mouth, he remarked: "That would be an enormous dose for a child of three-and-a-half".
The couple, from Halesowen, were each jailed for five years at Worcester Crown Court in January last year after being cleared of murder but convicted of Christian's manslaughter.
It was the prosecution's case that they had force-fed the boy with salt, when they became angry with him.
The couple have appealed against their convictions.
Christian was placed with the Gays at their then home in Bromsgrove, Worcestershire, in November 2002, but was admitted to hospital five weeks later. He died on December 12 2002, after life support machines were switched off.
Dr Walters, a chemical pathologist, said the boy died as a result of a condition known as "osmostat resetting" - said to be "at the frontiers of knowledge" - that meant the levels of salt in his body were wrongly regulated.
He told the court yesterday that, in cases of salt poisoning, it was normally "a piece of cake" to bring down sodium levels by simply giving a patient enough water to encourage excretion of the excess salt and dilute its concentration in the body.
Remarking that Christian's sodium levels "should have come down" during his 48 hours-plus in hospital, he said he had suspected from an early stage that a rare phenomenon called "resetting of the osmostat" had played a crucial part in the youngster's death.
Although it was "quite unknown" how the condition is triggered, Dr Walters said it was documented that, in some cases, patients can maintain body fluids as normal while afflicted by "grossly abnormal sodium concentrations" in their blood.
He said although "the mechanisms are unknown", Christian's failure to excrete enough salt to bring concentrations down to a safe level "leads me to suspect a failure of the regulatory systems".
Dr Walters said that, during his first 12 hours in hospital, Christian had excreted salt at a high rate and he would have expected that to continue for at least 48 hours.
However, once 12 hours had passed, the rate of excretion plateaued and the amounts of salt in Christian's blood never reduced to normal levels, the court heard.
Crown expert, Professor George Haycock, had suggested that the level of retention of sodium in Christian's blood stream could have been a result of heart failure.
However, Dr Walters said that would only happen in cases of acute heart failure and examination of Christian before his death showed his heart function was only "mildly impaired".
The hearing continues.