The liver is an easily saturated system, and fortification could lead to significant unmetabolised folic acid entering the blood stream, with the potential to cause a number of health problems
Scientists at the Institute of Food Research have highlighted possible consequences of fortifying flour with folic acid due to new evidence of how it is absorbed by the body.
In May, the board of the UK Food Standards Agency agreed unanimously that 'mandatory fortification' with folic acid should be introduced to make sure the number of babies born with neural tube defects is reduced.
This means that it would be compulsory to add folic acid to either bread or flour.
Folic acid is a synthetic form of folate, a B vitamin found in a wide variety of foods including liver and green leafy vegetables.
Folates are metabolised in the gut, whereas in a paper published in the British Journal of Nutrition this month IFR scientists suggest that folic acid is metabolised in the liver.
"Fortifying UK flour with folic acid would reduce the incidence of neural tube defects", said Sian Astley of the Institute of Food Research.
"However, with doses of half the amount being proposed for fortification in the UK, the liver becomes saturated and unmetabolised folic acid floats around the bloodstream.
"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.
"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 exhibit Jekyll and Hyde characteristics, providing protection in some people while causing harm to others.
For example, studies have confirmed that unmetabolised folic acid accelerates cognitive decline in the elderly with low vitamin B12 status, while those with normal vitamin B12 status may be protected against cognitive impairment.
Most over 65s in the UK have low B12 status.
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.
Since the 1980s a consensus formed that that folic acid is metabolised in the small intestine in a similar way to naturally-occuring folates.
This consensus was used to assess the safety of folic acid fortification.
"We challenge the underlying scientific premise behind this consensus", said Astley.
"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".
