Putting folic acid in bread to prevent birth defects that can result in death or life-long disability has become an election issue.
Food Safety Minister Damien O'Connor aimed to introduce mandatory folic acid fortification of non-organic bread-making flour/bread this term, but failed after Covid-19 disruption and opposition from coalition partner NZ First.
National and Act are also opposed.
The Green Party supports the public health measure.
"I really wanted to get this progressed before the election but I wasn't able to secure Cabinet endorsement. I'm deeply disappointed but this remains a priority for me," O'Connor said in a statement.
The Ministry for Primary Industries last November called for feedback on putting folic acid in all non-organic bread or bread-making flour (a process called fortification), which it estimated could prevent about five to nine neural tube defect-affected pregnancies a year.
The Medical Association and College of Public Health Medicine say miscarriages earlier in pregnancy would also be prevented by the change. The college estimates as many as 200 a year could be prevented.
Getting enough folate, a natural B vitamin found in foods such as leafy green vegetables, before and in early pregnancy can considerably reduce the risk of neural tube defects (NTDs), which include spina bifida and often lead to severe disability, death or termination.
It's very difficult to get enough from diet alone, and women are advised to take folic acid tablets.
However, many don't, and more than half of pregnancies aren't planned - a proportion that rises for young mothers (83 per cent), Māori (75 per cent) and Pacific NZers (71 per cent). When those women realise they're pregnant it's often too late - the neural tube closes 15 to 28 days after conception.
Iconic Aussie chocolate-maker removes controversial ingredient
Peter Davis: Lessons from the Auckland Covid-19 outbreak
For this reason, NZ and Australia agreed to mandatory folic acid fortification from 2009, but the new National Government backed out after an opposition campaign by bakers and industry lobby group the Food & Grocery Council, who warned of unknown effects of "mass medication".
New Zealand currently has a voluntary system that aims to have up to half of packaged bread fortified with folic acid (about 38 per cent currently is).
• Read the full investigation into New Zealand's folic acid debate and legacy by clicking here
Countries that have made it compulsory for folic acid to be added to staple foods include the United States (in cereal grains since 1998), Canada (in flour and bread since 1998) and Australia (bread-making flour from 2009). Defect rates have fallen in those countries.
Last year, MPI called for feedback on three options: keep the status quo; boost the voluntary target to 80 per cent; or mandatory fortification of all non-organic bread, wheat flour for bread-making, or wheat flour.
In doing so, it cited a comprehensive 2018 report by the PM's chief science adviser and the Royal Society that concluded unequivocal benefits of mandatory fortification of packaged bread outweighed any potential adverse health effects.
"If mandatory fortification of bread had occurred in 2009, it is estimated that 120 to 171 pregnancies affected by a neural tube defect could have been prevented over the last 10 years," MPI estimated.
More than half of such pregnancies are terminated or result in stillbirths.
National was in power when the Ministry of Health commissioned then chief science adviser, Sir Peter Gluckman, to review folic acid fortification.
Despite his subsequent report's recommendation for mandatory fortification, the party remains opposed.
Its food safety spokesperson, Barbara Kuriger, said its past support for a voluntary system was to give consumers choice and "we continue to hold this view".
Act leader David Seymour said his party was opposed, but "not strongly".
"There is some sense to it, but it's a bad precedent. Is the answer to everything mass medication at the expense of personal responsibility?"
NZ First food safety spokesperson Mark Patterson said the party wanted increased education and labelling around the benefits.
"Some scientific studies suggest that excess folic acid could be a factor in prostate cancer and in principle we remain opposed to mandatory mass dosing of the public," Patterson said.
The Green Party now supports fortifying all non-organic bread-making flour - a big shift, after its past opposition.
Possible cancer links - raised by industry in past opposition - were carefully considered by the peer-reviewed report by the PM's chief science adviser and the Royal Society, which found "no evidence of harmful health effects of folic acid supplementation in adults, at least at low doses in the range suggested for fortification".
Most data suggests no cancer link, the June 2018 report found, but some limited evidence from genetic studies suggests, for some people, higher blood folate is weakly associated with increased risks of colorectal and prostate cancer, and lower risks of breast and total cancer.
"The associations seen in the genetic studies are not necessarily causal, and their public health significance remains uncertain especially when compared with the undoubted benefits of mandatory folate fortification," the scientists and panel of health experts agreed.
The Cancer Society supports fortification of bread-making flour, saying benefits outweigh any potential cancer effects, but wants ongoing research and monitoring.
Other groups in favour include the Ministry of Health, Auckland Regional Public Health Service, DHBs including Southern, Canterbury, Capital & Coast and Nelson Marlborough, Plunket, the Medical Association, the College of Obstetricians and Gynaecologists, the Paediatric Society, College of Physicians, Nurses Organisation, College of Midwives, Dieticians NZ, Consumer NZ, scientists, health academics and workers, including doctors and midwives.
The Food & Grocery Council wants the voluntary system to continue in the interim while more consideration is given to what it says is emerging evidence of possible health risks, including over-consumption of folic acid including by children.