For centuries the causation of neural tube defects remained unknown. Neural tube defects, of which anencephaly and spina bifida are the main types, occur throughout the world. In some places, as many as 1 in every 200 births is affected by a neural tube defect. While anencephaly is always fatal, spina bifida is usually viable, leaving the affected individual with severe disabilities, notably paralysis, hydrocephalus, and incontinence of urine and faeces.1 In the early part of the twentieth century surgery resulted in the management of hydrocephalus, mainly through the insertion of a valve (Spitz–Holter valve) that drained excess cerebro spinal fluid from the head into the circulation. This treated the severe cranial complications of spina bifida, without alleviating the other complications. The cause or causes of neural tube defects remained unknown until a few decades ago and, consequently, there was no known means of preventing the disorder from occurring. This review outlines the discovery of their main cause (folate deficiency), the struggle involved in carrying outsome of the research and the continuing struggle to adopt the public health measures necessary to prevent the disorder.
The first clue to a cause
It had long been suspected that genetic and environmental factors were involved in the causation of neural tube defects, but this, while true, was not particularly helpful in identifying a specific cause; most disorders are a combination of genetic and environmental factors. The challenge was to identify one or more environmental factors that could be modified to prevent the disorder. The most likely environmental factor was a dietary component. However, neural tube defects were common in many economically developed countries such as the UK, and many researchers felt that a nutritional deficiency was unlikely. Renwick suspected that the cause was a mycotoxin from blighted potatoes, but this hypothesis came to nothing.2,3 A clue emerged when Hibbard and Smithells in 19654 used a relatively new test, the formiminoglutamic acid (FIGLU) test, which, while not particularly specific, tends to identify a metabolic abnormality or deficiency relating to folate. The FIGLU test tended to be positive in women who had delivered babies with birth defects, including neural tube defects, and negative in women who had delivered unaffected babies and served as controls. Later, Smithells and his colleagues prescribed a multivitamin pill, Pregnavite forte F (Bencard, London), that contained eight vitamins [A (4000 U),D (400 U), B1 (15 mg), B2 (1.5 mg), B6 (1.0 mg), folic acid (0.36 mg), C (40 mg) and nicotinamide (1.5 mg);doses given are daily doses) and two minerals [iron(ferrous sulphate 120 mg) and calcium (dicalcium phosphate 240 mg)] to women who had previously had a pregnancy associated with a neural tube defect. While there were no controls, in the strict sense, the recurrence rate of neural tube defects in the treated women (1 out of 200) was much lower than in untreated women who had also had an affected pregnancy (13 out of 300), a statistically significant difference. The original series was published in 1981.5–7 There remained two scientific uncertainties. First, the women who took the multivitamin preparation may have been a selected group of women who were, in any case, at low risk of neural tube defects, for example by virtue of the fact that they were of higher socioeconomic status and may have benefited from a healthier diet generally and this selection may have explained the results. Second, even if the multivitamin supplement had prevented some cases of neural tube defects, it was not possible to say which vitamin conferred the benefit. Overall, there was a general scepticism because it was widely thought to be unlikely that women in Europe and North America suffered from a vitamin deficiency disorder. In 1981, a randomized trial of 4 mg folic acid was reported from south Wales,8 but it was too small to be informative; 2 out of 60 women allocated to the folic acid group had fetal neural tube defects compared with 4 out of 63 women allocated to the placebo group. So uncertainty remained.
Resolution of the uncertainty
The uncertainty needed to be resolved, and if a component in the multivitamin preparation prevented women from having neural tube defect pregnancies, it needed to be specifically identified. Only a large trial in which women were allocated at random to a folic acid supplement or a placebo would resolve the issue. The trial needed to include the non-folic acid vitamins used by Smithells to assess their effect. It also would preferably use a 4-mg daily dose of folic acid as used in the south Wales study instead of the 0.36 mg as used by Smithells and his colleagues. In this way a negative trial result could confidently exclude an effect if one existed. The case for the trial was published in 1984.9 This paper helped gain support for the trial, but not without further debate.10,11 Producing a trial protocol and enlisting a team of investigators was a significant undertaking, but the major challenge was coping with the polarization of opinion. It is often the case when there is scientific uncertainty that people tend to take strong views on one or other side of the issue– some regarding the idea that neural tube defects were a folate deficiency disorder as preposterous and not worth investigating, whereas others regarded the evidence as sufficient and of no importance whether the protective agent was folic acid or another vitamin in the supplement mixture used by Smithells and his colleagues. It took nearly 3 years, from 1980 to1983, to bring this trial to fruition. During this period there was a fierce debate. Some said that the trial was unlikely to succeed.12 Others questioned whether or not it was necessary,13,14 and questions to this effect were asked in the House of Commons. The Minister of Health at the time, Kenneth Clarke, fielded anti-trial sentiment. In a letter to Nature, published on 11 November 1982, Arthur Wynne accused me and the Medical Research Council (MRC), that had, in principle, agreed to pay for the trial, of contravening the Helsinki Code of Medical Ethics by withholding folic acid from some women, based on the belief that folic acid was already known to be effective.15 The trial was about to start and I responded to his allegations in a letter that was to be published in Nature on 25 November 1982. After submitting the letter and discussing it with the head of the MRC, Sir James Gowans, I felt that such philosophical battles were unproductive and my letter would serve nouseful purpose, so I withdrew it. What was needed was the results from the trial that the MRC had agreed to fund, subject to its Council confirming its approval, a decision expected on 7 December 1982. What was published in Nature was a blank space on page 310 of the 25 November 1982 issue that was to have contained my letter, and instead an editorial by John Maddox, the editor of Nature at the time, on page 302 entitled ‘Biting off others' tongues’16 stating that I (‘the researcher’) had been pressured into withdrawing the letter, which was not the case. Maddox stated in his editorial that ‘The blank space on page 310 is intended to be spectacular’. Nonetheless, there was broad support for the trial. The Department of Health and the Association for Spina Bifida and Hydrocephalus17 and The Lancet18 backed the trial, as did the British Medical Association.19 What was a scientific debate had become a political spat and, not surprisingly, the press picked up on this.20 The lesson from the episode is the crucial link between science and ethics. The ethical considerations relating to a proposed study must follow the scientific assessment of what is known at the time. Once scientific uncertainty is recognized it is ethical to randomize people to receive either the treatment to be studied or a placebo. If there were no uncertainty and a proposed remedy was either useful or useless, it would be unethical to conduct a further study on the proposed intervention. The ethics of a human intervention study stem principally from an honest appraisal of the scientific position. The two cannot be considered separately. As stated in our paper on the need for the trial, ‘If an important medical question has been reliably answered then it is both unethical and scientifically unnecessary to mount a randomized trial, while if it has not, it is likely to be both ethically and scientifically appropriate to do so’.9
Once approved, the MRC vitamin study was launched in 1983 and recruitment started in July that year. The study was a randomized double-blind placebo-controlled trial with a factorial design conducted at 33 centres in seven countries. The trial was stopped early because of a clear-cut result that showed the efficacy of folic acid in the prevention of neural tube defects. The trial women were randomized into one of four groups: (i) folic acid (4 mg daily), (ii) folic acid and the other vitamins in Pregnavite Forte F used in the previous study by Smithells at the same daily dose, (iii) the other vitamins alone, or (iv) placebo (the two minerals were used as the placebo). The results shown in Figure 1 are taken from the report of the study in 199121 and show the preventive effect in the women who took folate compared with those who did not. To avoid selection bias in such trials, the primary statistical analysis needs to be performed on the women allocated to the different regimes (intention-to-treat analysis), and on this basis there was a 71% preventive effect (the original report incorrectly indicated 72%) with a 95% confidence interval of 29% to 88%. Figure 2 shows the results of a secondary analysis limited to women who were not already pregnant at randomization and who said that they had taken their pills – an 83% preventive effect (95% confidence interval 41% to 95%). There was no evidence that the other vitamins had a preventive effect (see Figure 3).
Thus, the results of the MRC vitamin study provided firm evidence that 4 mg of folic acid taken everyday immediately before pregnancy and in the early stages of pregnancy could prevent as many as four out of five cases of neural tube defect. It is rare for a clinical trial to show such clear-cut results with such a large beneficial effect. In 1992, Czeizel and Dudas22 reported the results of a randomized trial using a multivitamin supplement containing 0.8 mg folic acid; there were no neural tube defect pregnancies in the 2104 women in the vitamin group, but there were six in the 2052 in the placebo group. During the 1980s, observational epidemiological studies produced further support, although these studies lacked the scientific rigour of a randomized trial.
Assessing the evidence as a whole
Table 1 summarizes the results of the studies that used vitamin supplements in intervention studies, both randomized and non-randomized. Table 2 summarizes observation studies of dietary folate and neural tube defects. Table 3 summarizes observational studies of the use of periconceptional vitamins containing folic acid and neural tube defects in the general population. All of the studies summarized in the three tables show a remarkably coherent set of evidence showing the preventive efficacy of folic acid and natural folate in different populations and in women with and without a history of having had a neural tube defect pregnancy. Nonetheless, three issues needed resolution: (i) explaining the results of the blood folate studies, which appeared negative, (ii) quantifying the effect of different doses of folic acid, and (iii) why people were deficient in folate.
|Study||Number of neural tube defects||Relative risk||95% confidence interval|
|MRC vitamin study (1991)21||27||0.29||0.10 to 0.74|
|Laurence et al.(1981)8||6||0.42||0.04 to 2.97|
|Czeizel and Dudas (1992)22||6||0.00||0.00 to 0.85|
|Combined||39||0.25||0.12 to 0.55|
|Smithells et al. (1983)7||27||0.14||0.03 to 0.47|
|Vergel et al. (1990)23||4||0.00||0.00 to 2.13|
|Combined||31||0.12||0.04 to 0.41|
|Study||Number of neural tube defects||Relative risk (high- vs. Low- folate diet)||95% confidence interval|
|Laurence et al. (1980)25||8||0||0.00 to 0.33|
|Yates et al. (1987)26||20||0.33||0.09 to 1.27|
|Bower and Stanley (1989)27||77||0.31||0.10 to 0.97|
|0.16||0.06 to 0.49|
|Milunsky et al. (1989)28||39||0.42||0.16 to 1.15|
|Study||Neural tube defect pregnancy||Unaffected pregnancy||Relative risk(95% confidence interval)|
|Mulinare et al. (1988)29||24||154||404||1066||0.41 (0.26 to 0.66)|
|Mills et al. (1989)30||100||465||77||461||0.87 (0.73 to 1.02)|
|103||464||0.94 (0.80 to 1.10)|
|Milunsky et al. (1989)28||10||39||10 703||11 905||0.29 (0.15 to 0.55)|
|Werler and Mitchell (1993)31||36||253||339||1253||0.6 (0.4 to 0.9)|
Explaining the blood folate studies
These studies showed little difference (generally 1 ng/ml or less) between serum folate levels in women who had pregnancies with neural tube defects and in those women who had unaffected pregnancies. This appeared to conflict with a lack of folate as a cause of neural tube defects. The red cell folate studies were less in conflict. The explanation presented in 199324 was that the distribution of serum folate levels in the population is narrow, say between 4 and 6 ng/ml. There is little scope for serum folate levels to differ between women with affected and unaffected pregnancies. They must be similar, even if a lack of folate were a major cause of neural tube defects, as we now know is the case. Smoking and lung cancer may offer an explanation analogy; if everyone smoked 20 cigarettes a day, there would be no difference in the number of cigarettes smoked in people with and without lung cancer, but that would not mean smoking is not a cause of lung cancer. Epidemiologically, there needs to be sufficient variation in exposure (cigarette use) or in blood levels (serum folate) for an association to be observable. In practice, there is usually some variation in exposure to the causal factor but this may be small – there is a ‘narrow-window effect’24 that limits the ability to detect an association and can lead to a false-negative inference on whether or not a factor causes a disease. In the MRC vitamin trial, the median folate level in women in the non-folic acid groups was about 5 ng/ml compared with about 50 ng/ml in those in the folic acid groups – a wide difference capable of showing the folic acid effect.
Quantifying the effect of different doses of folic acid
Randomized prevention trials are usually not able to assess dose–response relationship because trials using multiple doses of a preventive agent would have to be very large indeed. The cost of using several doses with sufficient statistical power to show a dose effect is usually prohibitive. Observational studies and short-term supplementation studies can, however, provide the missing information.32 Short-term randomized folic acid supplementation studies using different doses show that serum folate concentration increases by about 1 ng/ml for every 0.1 mg/day increase in folic acid intake in young adults. Observation studies in a setting with a reasonable range of serum folate levels showed that every doubling of serum folate approximately halves the risk of a neural tube defect. These two pieces of information used together predict that, with a background serum folate of 5 ng/ml and a 0.2 mg/day increase in folic acid intake, the risk of a neural tube defect would fall by about 20%; an increase of 0.4 mg/day would reduce risk by nearly 40%, 1 mg by 60% and 4 mg by about 80%. These predictions fit what has been observed in the US33 and Chile34 fortification experiences, the case–control studies based on supplements containing about 0.4 mg folic acid and the direct evidence from the MRC vitamin study, which used 4mg/day. The practical implication is that folic acid supplements for the prevention of neural tube defects should be about 4 mg daily, not 0.4 mg, as is widely recommended, and 4 mg (or 5 mg) folic acid supplements should be available without prescription, as is the case in some countries such as Australia, but not others such as the UK or the USA.
Dietary folate and food fortification
Eating more fruit and vegetables and less calorie-dense foods is sensible advice, but dietary change alone is not enough. It would require too large a change in dietary habits to be practical. There are several reasons to explain why dietary modification alone is inadequate. Folate is present in some fruit and vegetables, but large amounts must be eaten to achieve the desired intake, for example eight portions of brussel sprouts or ten of spinach is necessary to achieve an intake of 0.4 mg folate.35 Liver is a source of folate but many people do not eat liver or do so only rarely. The problem is made worse because the cooking of food tends to destroy folate and white flour loses much of the folate in the wheat. In this respect the parent molecule, folic acid, which is not found naturally, is stable and preserved in cooking, an important advantage in using folic acid itself in fortification. Another relevant issue is that over the twentieth century many communities have become more sedentary so their total calorie requirements have fallen, leading to eating less food. While this is necessary from the point of view of macronutrients, such as sugars and carbohydrates, it means that micronutrients, such as folic acid, are consumed to a smaller extent than in the past. One of the nutritional problems of a population becoming wealthier and more sedentary is excess macronutrient intake because the reduction in calorie intake is not sufficient to avoid being overweight but enough to cause or exacerbate micronutrient deficiency.
For these reasons, food fortification with folic acid is needed, not as a substitute for periconceptional folic acid supplement use, but in addition to such supplementation. Fortification is a population-wide ‘safety net’ conferring a level of protection but not maximal protection, since there is likely to be a reluctance to fortify at levels necessary to achieve an extra intake above about 0.4 mg per person per day. The MRC vitamin study used 4 mg folic acid, and this, or the more commonly available 5 mg dose, is the dose of choice for a supplement to be used by all women who may become pregnant, not just those who have had a neural tube defect pregnancy.
There is no evidence that folic acid fortification or periconceptional folic acid supplementation is harmful at the levels and doses indicated in this review. Several possible hazards have been suggested, such as an increase in the risk of cancer or miscarriage. A link with multiple pregnancy has been noted. None of these possibilities have any proper scientific basis [see for example Bayston et al. (2008),36 Bayston et al. (2007)37 on colon cancer, and Gindler etal. (2001)38 on miscarriage, and Mathews et al.(1999)39 on multiple pregnancies].
Public health action
One could perhaps be forgiven for believing that, with the evidence available, governments throughout the world would have taken action to advise all women planning a pregnancy to take a 4 mg or 5 mg folic acid supplement if they might become pregnant and to fortify a staple food ingredient such as flour with folic acid, so that the population as a whole would have a degree of protection without completely relying on women having to take supplements. Many pregnancies are unplanned and would therefore be unprotected without fortification. Sadly, in many parts of the world this did not happen. The USA, under the prudent guidance of Dr Godfrey Oakley at the Center for Disease Control, was the first to issue a national recommendation that women should take folic acid before pregnancy in 1991, shortly after the publication of the MRC study report, and later again, in 1996, under his guidance the USA was the first country to fortify grains with folic acid. Over 70 countries have followed the US fortification policy, but no European country has done so, including the UK, which funded and led the critical research.
There is no sound reason for not introducing folic acid fortification and every reason to do so. The failure to implement this simple measure illustrates a general problem in public health policy-making in many countries; unless there is a visible disaster such as the fire at King’s Cross station in London in 1987 that led to the prohibition of smoking on the London underground, there is not the political motivation to take necessary public health action. Perhaps politicians fear that altering the composition of foodwill lose them votes. The fact that all manufactured food necessarily involves some alteration to food seems to escape attention. While industry has wide discretion in formulating the composition of food and drink, some governments are reluctant to fortify a staple food with a vitamin that would prevent serious birth defects because of a fear of political repercussions, that is probably groundless. In countries that have introduced fortification, there have been no negative effects. It has been accepted, even welcomed, by the public. The level of fortification in countries that have introduced fortification has, however, generally been too low. The country that has probably fortified flour closest to the optimal level is Chile, equivalent to about 0.4 mg/day increased intake, resulting in the approximate 40% reduction in neural tube defects referred to above. In practice, most fortification policies add about 0.2 mg of folic acid to the daily diet, and this could be doubled and would result in a useful increase in the preventive effect.
Lessons to be learnt
Oakley states that ‘one of the most remarkable successes of epidemiology was the demonstration in the late twentieth century that spina bifida and anencephaly – two of the most common and severe birth defects – are caused by folate deficiency’.40The same article states that only about 10% of cases of spina bifida and anencephaly that can be prevented by folate are prevented. It is more than an opportunity lost – it is a case of collective public health neglect. A sense of urgency is needed to prevent the estimated 200 000 or more cases of folic acid preventable neural tube defects each year throughout the world.41 The two steps to be taken in all countries are:
Mandatory fortification of flour with folic acid at a level providing about 0.4 mg/day.
Recommend that all women who might become pregnant take a 4 mg or 5 mg folic acid supplement daily and continue during the first trimester of pregnancy.
There are also two general lessons to be learned from the folic acid and neural tube defects story. First, when a study is needed to resolve an important medical question, the decision-making process must be more effective than is often the case. Such studies should not have to rely on researchers struggling for years as champions for the study. Second, the translation of research into public health practice needs to be streamlined, with public health officers having the authority to make decisions. The decisions should come from a public health agency that can not only implement preventive measures, but can also monitor the action taken. Public health decisions such as whether or not to implement folic acid fortification should not be political decisions. They should fall outside the political arena and be delegated to public health officials, as is the case with the control of infectious diseases.