Fortifying flour with folic acid to cut birth defects may lead to a range of health problems, warn scientists.
Folic acid in bread could help cut cases of spina bifida
The move was approved earlier this year by the Food Standards Agency as a way to reduce defects such as spina bifida.
However, an Institute of Food Research team has shown the liver could easily become saturated by folic acid.
Writing in the British Journal of Nutrition, they warn this could lead to unmetabolised folic acid entering the blood, which could damage health.
The latest study follows a letter to the Food Standards Agency from Sir Liam Donaldson, the Chief Medical Officer of England, requesting further expert consideration of two recent studies linking folic acid to bowel cancer before the government gives the final go-ahead for mandatory fortification.
But the Food Standards Agency said fortification was safe.
Folic acid is a synthetic form of folate, a B vitamin found in a wide variety of foods including liver and green leafy vegetables.
But while folates are broken down in the gut, the latest research shows that folic acid is metabolised in the liver.
The researchers warn that the liver is an easily saturated system, and mandatory fortification could lead to significant unmetabolised folic acid entering the blood.
Researcher Dr Sian Astley said fortifying flour would undoubtedly reduce the number of neural tube defects among babies.
Mandatory fortification has already been introduced in the US, Canada and Chile, where it cut defect rates by up to half.
But she said: "With doses of half the amount being proposed for fortification in the UK, the liver becomes saturated and unmetabolised folic acid floats around the blood stream.
"This can cause problems for people being treated for leukaemia and arthritis, women being treated for ectopic pregnancies, men with a family history of bowel cancer, people with blocked arteries being treated with a stent and elderly people with poor vitamin B status.
"For women undergoing in-vitro fertilisation, it can also increase the likelihood of conceiving multiple embryos, with all the associated risks for the mother and babies."
Dr Astley warned it could take 20 years for any potential harmful effects of unmetabolised folic acid to become apparent.
It has already been shown that folic acid fortification can cause harm to some people.
For example, studies have confirmed that unmetabolised folic acid accelerates cognitive decline in the elderly with low levels of vitamin B12.
Similarly, dietary folates have a protective effect against cancer, but folic acid supplementation may increase the incidence of bowel cancer.
It may also increase the incidence of breast cancer in postmenopausal women.
However, since the 1980s a consensus formed that folic acid is metabolised in the small intestine in a similar way to naturally-occuring folates.
Dr Astley said: "We challenge the underlying scientific premise behind this consensus.
"This has important implications for the use of folic acid in fortification, because even at low doses it could lead to over consumption of folic acid with its inherent risks."
In a statement, the Food Standards Agency said its recommendation was made after an extensive and scientifically robust assessment.
"The FSA Board would not have recommended mandatory fortification if the scientific evidence suggested that there were unacceptable health risks for some groups.
"As part of the process, an expert committee of scientists considered the evidence regarding unmetabolised folic acid and they concluded that the data in humans was insufficient to assess any associated risks.
"They also examined in detail the potential cancer risks to some groups from high folic acid consumption."
The FSA added that it had recommended controls on voluntary fortification, and clear guidance on the use of folic acid supplements.
Both were designed to ensure most people did not exceed the recommended upper daily limit for folic acid.
Andrew Russell, of the Association for Spina Bifida & Hydrocephalus, said the risks of unmetabolised folic acid had been "thoroughly investigated and discounted" by top experts.
Professor Nicholas Wald, director of the Wolfson Institute of Preventive Medicine, said: "Fortification would prevent many cases of spina bifida and would also benefit the health of the country as a whole.
"Further delay in this public health measure will result in hundreds more babies being disabled by this serious disorder, or pregnancies being needlessly terminated due to a neural tube defect."