He drew applause at the recent New Zealand Medical Association's annual South General Practice Conference and Medical exhibition in Dunedin when he said if such a trial proved its efficacy he and "everyone in this room would change their opinion and wouldn't be ashamed of it".
In the meantime, he was not "too interested" in the experience of a few doctors who had used the treatment, he said.
He was responding to Mike Cushman, an Auckland pharmacist, who criticised aspects of Prof Holt's presentation, saying he had seen a lot of people helped by the treatment over a number of years.
Dr Monika Clark-Grill, who lectures on complementary medicine at the University of Otago, said she was troubled by factual errors in his presentation relating to the case involving swine flu and leukaemia patient Allan Smith, who was given intravenous vitamin C.
During his treatment, it was withdrawn and begun again. After it was withdrawn, his condition worsened, and then improved with the reinstatement of the vitamin C.
Prof Holt did not accept there were errors. He said he had spoken to the doctor involved in that case at great length.
Mr Smith, who had made a remarkable recovery, was having dozens of treatments at the time. It could not be shown whether it was the vitamin C or the other treatments which had led to the recovery.
He did not think it was vitamin C because the evidence from medical literature did not support that, he said.
Prof Holt, whose publications include a book Natural Remedies That Really Work, said there had been no major trials of the efficacy of large doses of vitamin C since the 1970s and "still vitamin C lingers on".
It was understandable that cancer patients reacted to a media report of research suggesting it could help fight cancer, but that report was based on a laboratory study which did not prove anything.
He spoke of the danger of some harmful complementary therapies, the risk of using ineffective unproven medicines over established ones, the psychological harm of false hope, and the financial harm patients could suffer.
He was critical of conspiracy theories about the quashing of therapies such as vitamin C by drug companies.
Pharmaceutical companies were not "snow white" but it was not reasonable and "pretty insulting" to accuse oncologists of denying patients effective treatments,.
The four-day conference, was attended by about 550 people.
