Supermarket suppliers and bakers want the Government to protect them from lawsuits as questions grow about the health impact of a food additive about to be introduced to all bread.

In four months, bakers will be forced to begin putting a synthetic form of folic acid into almost every loaf made in New Zealand.

This is despite a market research survey carried out by the Government that shows 87 per cent of New Zealanders oppose the move.

The plan aims to reduce the number of brain-damaged babies, although the fall may be a few as four a year.

But new research shows folic acid may cause an increase in colon cancer cases. And another study suggests it may cause colon cancer to grow faster.

The Bakers' Association has labelled the compulsory introduction "mass medication" of the population, and warned that bread containing folic acid will be less safe than it is now.

While about 50 countries already have voluntary schemes, the September introduction would put New Zealand among select few that make it mandatory, or are planning to do so.

The United Kingdom is among them, but it has put plans on hold while it awaits new research.

Ireland canned plans to make adding folic acid mandatory after a voluntary scheme was shown to raise women's folate levels.

New Zealand Food Safety Minister Kate Wilkinson said the Government was concerned about the scheme, which it had "inherited" from Labour.

"We need to make sure the health benefits outweigh the risk," she said. "We have to make sure the evidence is science-based and not emotion-based. We are concerned and we are looking at it."

Wilkinson said Food Safety Authority officials were preparing advice on the latest research. She expected to take it to Cabinet before the end of the month, when a "course of action" would be decided.

"We have to take into account 87 per cent of New Zealanders didn't want mandatory fortification of bread with folic acid," she said.

The scheme was a favourite of former Health Minister Annette King but never went before Parliament. It was passed under special rules which do not allow the same level of public scrutiny.

The mandatory scheme was developed after it was decided the current scheme - in which specific brands are fortified with folic acid - was unsuccessful.

Neither the Food Safety Authority nor the Ministry of Health could say if any money had been spent on a public education campaign to promote the voluntary scheme.

Papers released under the Official Information Act show the authority told Wilkinson that "research on the health effects of folic acid, both positive and negative, is a rapidly developing area".

It said any "public health risk" would be "acted upon immediately" but New Zealand might not be able to pull out of the scheme without causing a diplomatic rift with Australia.

Both countries are subject to the transtasman Food Standards Australia New Zealand agency, which led compulsory introduction in both countries.

Wilkinson is the sole New Zealand minister on the group that runs the agency.

Briefing papers supplied to her before a meeting with supermarket and bakers' representatives last month warn that pulling out of the scheme "would be likely to have an undesirable effect on the Australia and New Zealand relationship".

Wilkinson said she had consulted the Ministry of Foreign Affairs and Trade and had also raised her concerns at the latest meeting of the transtasman council.

Bakers' Association head Laurie Powell said it was difficult to address the issue because the industry did not want to put consumers off bread. "Our products are safe but probably not as safe with folic acid."

He confirmed concerns about the scheme had led the association to ask the Government for legal indemnity.

"If it is found in 15 years' time this stuff is bad and it causes health problems, we would be sued," he said.

Powell was also concerned the industry could not regulate the amount of folic acid going into each individual loaf.

"It is a mass medication experiment that won't work," he said. "A trip to your baker should not be a trip to the pharmacy."

Food and Grocery Council chief executive Katherine Rich said there was no good reason to medicate an entire nation without clear benefits and known risks.

"They are embarking on a medical experiment of grand proportions," she said. "If there are long-term effects and the Government is keen on adding folic acid, they should indemnify."

Authority officials confirmed pregnant women would not get enough folic acid from fortified bread and would still need to take supplements.

They said it was not possible to eliminate risk from any food product, but thorough monitoring would pick up any health problems.

* Bread facts
What's happening to bread?

From September, all producers will be forced to put folic acid into bread. The only exceptions will be loaves made from scratch by a small number of bakers, organic bread and unleavened bread.

Why is it being done?

Higher levels of folic acid in pregnant women reduce the number of babies born with neural tube defects, which result in serious brain damage. Compulsory fortification is expected to reduce numbers by 4-14 cases a year. It has other health benefits, with some studies suggesting it prevents premature births.

Is it safe?

Probably, although there are concerns over new research which shows folic acid at higher levels can accelerate the growth of cancer cells in the colon and prostate. Research has also shown that it causes fundamental changes in genes in the colon and liver.

What happens next?

Food Safety Minister Kate Wilkinson has asked her officials for a briefing on the latest research and plans taking the results to Cabinet. She may call on colleagues to can the scheme, although has been warned that doing so could harm diplomatic relations with Australia.