The level of folic acid in the body can be monitored by measuring its concentration in red blood cells and plasma. With insufficient stores of folic acid we suffer a type of anaemia called megaloblastic anaemia, and a quarter of pregnant women are found to suffer from this condition. Without enough folic acid soon after conception the foetus may suffer a neural tube defect, of which spina bifida is the most common type. The Department of Health has recently issued the report of a committee chaired by Professor Dame June Lloyd, entitled Folic Acid and Prevention of Neural Tube Defects. Its key recommendation is that women who wish to become pregnant should take folic acid supplements, and the Government's leaflet Pregnancy, Folic Acid and You advises them to take one 400 microgram (mcg) tablet - bought from chemists' or health food shops - a day.
By the time women discover that they are pregnant, it may be too late for them to start taking the folic acid supplements, because the baby's neural tube forms about four weeks after conception. Women of childbearing age can get the 400mcg per day they need from their food, provided that they know what to eat. Our liver stores folic acid and generally there will be several weeks' supply in hand.
Folic acid has the chemical formula C19 H19 N7O6 and can be extracted from natural sources such as spinach, yeast, wheat bran or liver. It can be made synthetically from its molecular components, glutamic acid, p-aminobenzoic acid and pteridine. Pure folic acid will grow as deep yellow crystals from warm water, but it decomposes in boiling water and it is also destroyed by light. For these reasons much of the vitamin is lost in cooking because those vegetables with a lot are the ones we tend to cook the longest, such as brussels sprouts, cauliflower and spinach. Ideally they should be eaten raw or only lightly boiled or steamed.
Folic acid is found in plants, animals and micro-organisms such as fungi and yeasts. It is present in grass, butterflies' wings and fish scales. Foods with the highest levels of folic acid are liver (250 parts per million); brussels sprouts (100ppm); spinach (90ppm); broccoli (65ppm) and oranges (50ppm). Some cornflakes and bran flakes are now fortified with folic acid up to 250ppm, but the richest sources are Bovril and Marmite with more than 1,000ppm.
Folic acid has several effects on the human metabolism, some so far unexplained, such as an increased tolerance of pain. It was once called vitamin M - although it is now recognised as one of the B vitamins - and is an essential part of everyone's diet, not only that of pregnant women. Mostly it is needed to carry out chemical reactions that build other molecules, and its speciality is in supplying a single carbon unit. Folic acid is very good at picking up such carbons from other sources and then handing them over when needed to cell components such as DNA, and amino acids such as methionine.
The role of folic acid in preventing neural tube defects was highlighted by Emeritus Professor Richard Smithells of the University of Leeds in 1983, and as a result an extensive study was undertaken by the Medical Research Council. This involved 1,800 women who had previously given birth to a spina bifida baby. They were given various vitamin supplements, of which half received folic acid. A report published in 1991 showed that of the 1,200 women who subsequently gave birth, 27 had babies with neural tube defects. Of these, 21 were born to women not taking extra folic acid, and only six were born to those who were taking it.
But folic acid alone may not prevent all cases of neural tube defects. In the latest issue of the Quarterly Journal of Medicine, a group of scientists from Trinity College, Dublin, report on a study of 56,000 pregnant women. According to one member of the group, Dr Ann Molloy, 'Our research shows that lack of both folic acid and vitamin B12 are risk factors, and that if folic acid is to be used to fortify food there may also be a case for including B12.'
The Dublin team even suggests that current allowances of both vitamins are still too low. The World Health Organisation recommends a daily intake of 200mcg, while the Department of Health says it should rise to 300mcg for pregnant women. The trials with pregnant women used 500mcg and the new guidelines suggest at least 400mcg.
Dr John Emsley is science writer in residence in the department of chemistry, Imperial College, London.