Researchers fear that unless food manufacturers implement a government recommendation to add the vitamin to staple foods such as bread and cereals, many women - particularly the majority who have unplanned pregnancies - may not benefit from this information.
Spina bifida results when the embryo's neural tube, which develops into the spinal cord, fails to close properly at about 28 days after conception. The effects include paralysis, incontinence and anencephaly, or absence of the brain, which results in death.
In 1964 in England and Wales there were 1,335 cases of anencephaly and 1,395 of spina bifida, dropping to 34 and 135 respectively in 1989. Much of the drop is due to the availability of antenatal screening and abortion - a traumatic experience that the latest dietary recommendations may help to avoid.
A study by the Medical Research Council in July 1991 found that a daily tablet of 4 milligrams of the B-group vitamin folic acid, taken by a woman who has already had one affected child, could reduce by 75 per cent the risk of having a second.
The Department of Health set up a working group to examine the implications for women's diets. The group submitted its report last spring, and it was published by the department in December.
The report, Folic Acid and the Prevention of Neural Tube Defects, recommends supplements of folic acid for women planning pregnancies, to be continued until the 12th week. Those who conceive unexpectedly are advised to begin taking supplements as soon as they discover they are pregnant.
Women planning to become pregnant are recommended to eat plenty of foods rich in folates (natural chemicals related to folic acid), including broccoli, Brussels sprouts and spinach. The report also recommends that manufacturers should add folic acid to bread and cereals.
Studies have shown that the average daily intake of folates is about 0.2mg, and Christopher Schorah, senior lecturer in chemical pathology at the University of Leeds, says it would be difficult for women to bring their intake up to effective levels by diet alone.
Dr Schorah is one of the researchers who first worked on the link between spina bifida and diet. He says: 'Those women most at risk - young mothers, single mothers and those who are socio- economically disadvantaged - are the ones least likely to eat the right sort of diets, no matter how much education is channelled into them. Nor are they likely to take folate supplements during the preconceptual period, because many of their pregnancies are unplanned.'
So what should a woman do if she wants to reduce her risk of bearing a child with a neural tube defect?
The report recommends that any woman who has already had an affected pregnancy, and is therefore at high risk of having another, should take daily folic acid supplements of 5mg while trying to conceive. Folic acid tablets of this strength are available only on prescription from a doctor. The original MRC study used a dose of 4mg of folic acid and as soon as a tablet of this size becomes available the dose should be dropped to this level, the report says.
But about 95 per cent of cases of neural tube defects occur in women who have never had affected pregnancies. For prevention in this group, the report recommends that women take a much smaller dose of 0.4mg (sometimes written as 400 micrograms and abbreviated on labels to 400mcg or 400hg).
This dose, available without prescription, is high enough to be effective but low enough to reduce the risk of side-effects. Although folic acid is very safe, it can be harmful in high doses for people who are deficient in vitamin B12, if this is not treated. Women taking anticonvulsants for epilepsy should ask the advice of their doctors before starting to take folic acid.
Women who cannot find folic acid tablets should 'choose a supplement of B group vitamins which includes folic acid at around a level of 0.4 mg per day,' says the department.
Folic acid alone is more likely to be available in small independent health-food stores than in high- street pharmacies. Two examples are Solgar 0.4mg folic acid tablets, costing pounds 2.07 for 100, and Health- Aid 0.4mg folic acid tablets, pounds 3.19 for 90 tablets.
The report was submitted to the Department of Health last spring. Asked why the department had not done more to ensure that suitable preparations were widely available, a spokesman said it was encouraging the production of a 0.4mg folic-acid-only supplement for retail sale.
This recommended dose is, however, a leap in the dark. No conclusive trial has been done to show that 0.4mg of folic acid protects against a first occurrence of neural tube defect.
Nicholas Wald, professor of environmental and preventive medicine at St Bartholemew's Hospital Medical College, London, and a member of the expert group that produced the report, says: 'A woman who is particularly concerned over the dose of folic acid and who thinks that she might be better off on the higher dose, may well be correct.'
But he warns women not to try to achieve the recommended dose of folic acid by taking multiple doses of multivitamin pills containing small amounts of the vitamin. They would risk harmful effects from overdosing on other vitamins, he says.
Professor Wald regrets that the report does not recommend legislation to compel food manufacturers to add the vitamin. 'In the absence of legislation to compel fortification, I hope that consumer pressure will act to encourage manufacturers to fortify bread with folic acid,' he says.
Some bakeries already produce supplemented breads. Mighty White, made by Allied Bakeries, provides 0.105mg per slice and Tesco's Healthy Eating Bread delivers 0.15mg per 125 grams of bread. The Federation of Bakers, whose members produce three- quarters of all the bread sold in Britain, will discuss the report on 21 January.
Tony Casdagli, director of the federation, says: 'If this is related only to pregnant women, we need to discuss whether we need to fortify bread across the board.'
'Folic Acid and the Prevention of Neural Tube Defects' is available free from the Health Publications Unit, Heywood Stores, No 2 Site, Manchester Road, Heywood, Lancashire OL10 2PZ.
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