A jury that convicted a couple of poisoning their foster son was not given the chance to consider if he died of natural causes, a court has heard.
Ian and Angela Gay have always denied poisoning Christian
Jurors were given two options - either Ian and Angela Gay murdered Christian Blewitt by blunt force to the head or they force-fed the three-year-old salt.
Lawyers asked Court of Appeal judges on Tuesday to hear new evidence that Christian had a rare medical condition.
The Gays, of Halesowen, West Midlands, were convicted of manslaughter in 2005.
Both were sentenced to five years in prison after a seven-week trial at Worcester Crown Court.
Michael Mansfield QC told the Court of Appeal medical evidence from no fewer than 19 experts was presented to jurors who were asked to decide not just "whodunit but also whatdunit" in the absence of any eyewitnesses.
It was alleged Christian had been force-fed up to six teaspoons of salt - the equivalent of a litre of sea water.
But Mr Mansfield said the new evidence of a medical condition could explain why Christian was retaining sodium in his system instead of excreting it through his healthy kidneys.
A new report by Dr Glyn Walters concluded there was nothing in the case that could be explained by salt poisoning that could not also be explained by the resetting of osmostats - mechanisms in the body which keep sodium at a safe level.
It showed experts in the study of this principle are still at the frontiers of knowledge and there were several features which could not be explained by poisoning but could be by the osmostat principle.
Mr Mansfield said had Dr Walters' evidence been available at the trial the verdict would have depended solely on a serious disagreement between reputable experts, there being virtually no other evidence.
Ian Gay, 39, and his 40-year-old wife were present for the first day of the appeal that is expected to last three days.
Christian died in hospital four days after being found unconscious at the couple's house in Bromsgrove, Worcestershire, in December 2002.
Former engineer Mr Gay and his wife, a £200,000-a-year insurance actuary, have always insisted they loved Christian and his younger brother and sister, who have since been successfully adopted elsewhere.
After the original trial Home Office pathologist Dr Peter Acland, a defence witness, suggested there was "significant doubt" about the guilty verdict and he was concerned there might have been a miscarriage of justice.
The case was adjourned until Wednesday.