VITAMIN SUPPLEMENTATION IN THE PREVENTION OF NEURAL TUBE DEFECTS

Introduction

One of the first parts of the body to develop is the central nervous system. The neural tube, from which the spinal cord and brain develop, is formed within the first 25 days of pregnancy. Spina bifida is caused by the failure of the neural tube to develop properly, hence the term 'neural tube defects'. Related defects are anencephaly (the absence of a brain) and encephalocele (a malformation of the brain and skull).

Why does the neural tube develop incorrectly?

The causes are not yet known and are thought to be connected with both genetic and environmental factors. If a couple have had one affected child, the risk of recurrence in another pregnancy is calculated at about 1 in 35.

What has research shown?

Research on the addition of folic acid (vitamin B9) to the diet has shown conclusively that women who have had a pregnancy affected by neural tube defect can reduce their chances of having a second affected pregnancy - by a dramatic 72%.

In April 1983, the Medical Research Council began a clinical trial of folic acid and other vitamin supplements in the prevention of neural tube defects.

The trial was conducted at 33 centres in seven countries and the results were announced in July 1991. The dose given to women taking folic acid by itself was 4mg a day.

The women recruited to the trial and each child born during the trial were checked for possible adverse effects. The infants continued to be monitored until their third birthday. As yet, no ill-effects caused by folic acid supplements have been reported in either the mothers or their babies.

What does this mean in practice?

These results show that, by taking a daily folic acid supplement, a woman can reduce the risk of having another pregnancy affected by neural tube defect. Women considered to be at higher risk of having a neural tube defect pregnancy include those who have had a previously affected pregnancy, those who have an NTD themselves and those who have a family history of NTD or whose partner has a family history of NTD.

Women in these higher risk groups should take a daily 5mg tablet of folic acid for at least one month before conception and then throughout the first 12 weeks of pregnancy. These 5mg tablets have to be obtained on prescription from a doctor.

At present, the only tablets available on prescription in the United Kingdom contain 5mg of folic acid. Some doubts remain that this dosage may be larger than necessary for preventative effect and it is 1mg larger than the amount used in the Medical Research Council trial. The Chief Medical Officer says 5mg will be replaced by a 4mg tablet as soon as this dosage is available.

Women who have not been identified as being in a high-risk group may also benefit from taking folic acid tablets. These are available in lower dosage 0.4mg tablets, found among the vitamin displays in pharmacies and health food shops. Again, these should be taken for at least one month before conception and then until the end of the first 12 weeks of pregnancy.

Women in families where there has been spina bifida in other relatives may wish to receive genetic counselling. In these cases, they should ask their doctors to refer them to a genetic counsellor. ASBAH has an information sheet on this topic which is available free of charge.

A healthy diet can help

Many studies have shown that diet is a contributory factor to the well-being of the foetus and it would seem prudent for women who are planning to become pregnant to ensure that their diet is reinforced with items in which folic acid naturally occurs. These include barley, baked beans, brewer's yeast, endive, chick peas, green leafy vegetables, lentils, orange juice, oranges, peas, rice, soya beans, split peas, sprouts, wheat and wheat germ. Vegetables should be lightly cooked as over-boiling destroys their vitamin content. Advice to eat liver should not be heeded: liver contains concentrations of vitamin A which, when added to vitamin A intake from other foods, can damage the unborn baby.

Some substances may affect the absorption of folic acid so, if you are taking any medication, do check this out with your doctor.

If you are on anti-convulsant medication, you should consult your neurologist before taking folic acid, as folic acid and some anti-convulsants can be antagonists (ie work against each other).

Folic acid supplements taken before conception can reduce the risk of neural tube defects recurring. They will not however, guarantee the elimination of neural tube defects, such as spina bifida.