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The addition of folic acid to bread will not increase the risk of cancer, the head of the University of Otago's department of human nutrition, Prof Murray Skeaff, says.
He said last night he had formed the view after attending a conference in Prague two weeks ago where he heard the results of an as-yet unpublished British analysis of the effects of the vitamin.
Australia and New Zealand are due to begin "fortifying" bread with the B vitamin from September in a bid to reduce the number of children born with diseases such as spina bifida.
The move has sparked debate in New Zealand over whether increased consumption of folic acid might lead to more cases of cancer.
Prime Minister John Key said yesterday it was likely folic acid would be added to bread but that it might be stopped soon after.
Mr Key said a ministerial review in October was the best option to stop the implementation.
Prof Skeaff said the Prague conference was told of the "pooled" analysis done by the clinical trials service unit at the University of Oxford.
The unit had looked at all the "randomised control trials of folic acid to date", which included the results from about 35,000 individuals who took part in studies of high-dosage folic acid supplementation.
"The results showed that there was no significant increase or decrease in the risk of cancer among those who were taking B vitamins as opposed to those who were not.
"In other words, there was no change in their risk of cancer."
Prof Skeaff said the trials used high-dosage folic acid, ranging from 600mg to about 2500mg per day.
The fortification of bread is designed to deliver an extra 140mg per day to pregnant women.
The analysis was of trials done mainly in Europe and North America from the mid 1990s and compared countries with different levels of fortification, compared men with women and those with either high or low folate status at the beginning of trials.
"They found that folic acid and B vitamins had no effect on cancer risk."
It also examined the studies linking folic acid and prostate or colorectal cancer.
Prof Skeaff said these studies were "very small" and "very weak" and their primary purpose was not to look at the connection between folic acid and cancer.
By comparison, the pooled analysis was a much bigger study that looked at the "totality of the scientific evidence".
"It is the best evidence we have so far, and it shows that there is no increase in cancer risk with high-dose folic acid."
Earlier this year Britain postponed the introduction of fortification while it awaited the results of two major trials.
Prof Skeaff said the Oxford study was the one British health officials wanted to see before making a decision to proceed with fortification.