Forensic pathologist Gert Saayman told the murder trial -- now in its second week -- that Pistorius's girlfriend ate at about 1:00 am, roughly two hours before her Valentine's Day death last year.
That contradicts the South African star sprinter's claim that the pair were in bed asleep in his upmarket Johannesburg home for five hours before Steenkamp was killed.
At around 3:00 am Pistorius shot the model and law graduate four times with a nine millimetre pistol in what prosecutors charge is premeditated murder but the double-amputee athlete says was an accident.
If found guilty of premeditated murder, Pistorius faces 25 years in South Africa's notoriously brutal jails and an abrupt end to his once glittering sporting career.
More From This Section
Saayman told the North Gauteng High Court that an autopsy revealed Steenkamp's stomach contained "vegetable material".
It is the latest in a chain of witness testimony that appears to call Pistorius's account of the fateful evening into question.
Neighbours have also testified that they heard a woman scream before the shots were fired, seemingly making it impossible Pistorius did not know where Steenkamp was when he fired the shots.
He asked whether she could have eaten a larger meal at an earlier time.
Saayman admitted that was not an expert on "gastric emptying", but academic studies would suggest the food had been ingested about two hours before, and pointed out he had performed between 10,000 and 15,000 post-mortems.
Saayman also testified that Steenkamp could have gone to the toilet anytime within an hour of her death, offering qualified support for Pistorius's account of events.
Yesterday, Pistorius retched loudly into a blue bucket as Saayman detailed the impact of Steenkamp's wounds.
Any of the head, arm or hip wounds could have caused Steenkamp's death, Saayman said.
The headshot "is in its own league," he said, "that will cause immediate incapacitation."
Pistorius used "Black Talon" hollow-point bullets which mushroom open like a jagged flower on contact to cause maximum tissue damage, according to the post-mortem report.
As the court adjourned for a break today, Pistorius laughed with his lawyers, a rare show of good spirits from the runner, who has cried often throughout the trial.