The prevalence of neural tube defect (NTD)-affected pregnancies ranges between 0.4 and 2/1000 pregnancies in EU. NTDs result in severe malformations and sometimes miscarriages. Children born with NTD suffer for the rest of their life of disability and chronic healthcare issues, and many women therefore choose termination of pregnancy if NTD is diagnosed prenatally. Women planning for pregnancy are recommended to eat 400 μg folic acid/d, whereas average figures across Europe indicate intakes of ∼250 μg/d for women of fertile age, a gap that could be bridged by implementation of folic acid fortification. The results of mandatory folic acid fortifications introduced in USA and Canada are a decrease between 25 and 45% of NTD pregnancies.
Conclusion: Evidence-based NTD prophylaxis is now practised in more than 60 countries worldwide. EU countries worry over possible cancer risks, but ignore a wealth of studies reporting decreasing cancer risks with folate intakes at recommended levels. Currently, there are indications of a U-shaped relationship, that is, higher cancer risks at low folate intakes (<150 μg/day) and highly elevated folate intakes (>1 mg/day), respectively. However neither the global World Cancer Research review nor EU’s European Food Safety Authority report present data on increased cancer risk at physiological folate intake levels. Therefore, EU should act to implement folic acid fortification as NTD prophylaxis as soon as possible.
The preventive effect of a high folic acid intake (400 μg/day) against neural tube defects (NTD) is considered one of the most important nutritional discoveries during the last 50 years (1).
Currently, over 60 countries practise mandatory folic acid fortification of flour. As seen on the map in Figure 1 provided by the Flour Fortification Initiative (FFI) (2), the countries implementing mandatory folic acid fortification are all on the American continent and Australia (dark blue), but also include many countries in Africa and the Middle East. The map also indicates countries planning to introduce mandatory folic acid fortification (yellow) and those countries allowing voluntary folic acid fortification of flours (turquoise). To date, no European country has implemented mandatory folic acid fortification of foods. Ireland, UK and the Netherlands are currently considering doing so following recommendations by their national expert committees (3).
One risk aspect that has influenced Europe’s decision is whether folic acid fortification constitutes a cancer risk. Most studies, but not all, judged by the World Cancer Research Foundation reported a protective effect on cancer with increasing dietary folate intakes (4). However, during the last decade, many studies were published emphasizing possible cancer risks of long-lasting exposure of folic acid in high doses (>800 μg/day), that is, well above the physiological intakes ranging between 100 and 500 μg/day. These worries made several EU countries stop planned mandatory folic acid fortifications in awaiting investigations of possible cancer risks. EU asked European Food Safety Authority (EFSA) to investigate the state-of-the-art, and EFSA formed a working group, EFSA’s Scientific Cooperation Working Group (ESCO WG), which published their report: Analysis of Risks and Benefits of Fortification of Food with Folic acid in 2009 (3).
The purpose of this study is to briefly present the outcome of World Cancer Research Foundation (WCRF) evaluation of dietary folate intake and cancer from 2007 and the ESCO WG report on cancer risk from folic acid supplementation. First, some background of NTD prevalence and dietary folate intakes among EU countries is presented followed by discussions of folic acid and cancer. Finally, ethical considerations of mandatory folic acid fortification are discussed.
Neural Tube Defects Prevalence
Neural tube defects is the most common malformation of the central nervous system. The development and closure of the neural tube are normally completed within 28 days of conception. The clinical terms used to describe NTDs include spina bifida, anencephalus and encephaloceles. NTD pregnancies affect neonatal deaths, preterm abortions, stillbirths and live birth defects. Available data show the total prevalence of NTD-affected pregnancies range between 0.4 and 2 per 1000 in European countries (Table 1). The live birth rate of NTD is, however, much lower in countries practising ultrasound investigation around 16th to 18th week of gestation followed by induced abortion (3). There are also decreasing prevalence figures in Europe, not fully explained by termination of pregnancies following prenatal screening.
|European Country||NTD||Dietary folate intake||Folate recommendations–women of fertile age|
|Prevalence of NTD-affected pregnancies 1000−1||Median (range) or mean (SD) (μg/day)*||Daily intake (μg/day)||Pregnancy (μg/day)||Lactation (μg/day)|
|Denmark||1.2||295 (163–495) 478 (256–735)†||300–400||500||500|
|France||0.6–1.4||245 (SD; 90)||300||400||400|
|Germany||1.1–2.0||225 (188–273) 290 (220–431)†||400||600||600|
|Hungary||0.7||133 (SD; 47.5)||200||–||–|
|Italy||0.4–0.7||339 (SD; 121)||200||400||350|
|The Netherlands||1.0||173 (SD; 50)||300||400||400|
|Switzerland||1.1||284 (total pop)||400||600||600|
Neural tube defects born will suffer from a lifelong disability, for example, neurological motor deficits. About 50% are not ambulant. In addition, 85–90% are affected by hydrocephalus, with shunt problems and further on learning disabilities and most of all cognitive dysfunctions (dysexecutive syndrome). Some of the most common disorders are urinary tract dysfunction.
Neural Tube Defects Prophylaxis by Mandatory Folic Acid Fortification has Now been Practised for 15 Years
Randomized controlled trials have conclusively shown that folic acid supplementation can prevent up to two-thirds of NTDs (5,6). Therefore, to minimize the risk of NTDs, fertile women planning to be pregnant are recommended to eat 400 μg dietary folate or folic acid per day. As NTDs appear very early in pregnancy, as early as during the second to third week after conception, women are recommended to start folic acid supplementation at least 1 month before. The fact that women need to enhance their folate intake as early as 1 month before being pregnant and many pregnancies are unplanned are the main reasons why fortification and not just supplementation is the only realistic option. Information campaigns to women of fertile age to start supplementation in time have not been successful.
In 1998, USA and Canada decided to implement mandatory folic acid fortification of wheat flour and corn. Follow-up studies later showed a decrease in NTD of about 30–50% in USA and Canada, respectively. The prevention is not 100% as there are also other risk factors, for example, female adipositas, genetics, antiepileptics and milieu factors.
Chile followed in 2000, and from 2004 to 2007, the number of countries who introduced national regulations for mandatory wheat flour fortification increased from 33 to 54 (7). Some of these countries fortify folic acid in combination with iron and other B vitamins and some only add iron (UK, Venezuela, Trinidad & Tobago, Philippines and Nigeria; see Fig. 1).
Recently, EU introduced new rules to regulate voluntary food fortification. These are set in Regulation (EC) No 1925/2006 on the addition of vitamins and minerals and of certain other substances to foods. An overview of the voluntary folic acid food fortification practices in European countries can be found in the ESCO WG report (3). All countries except Sweden voluntarily fortify a wide range of foods with folic acid including flour/bread, breakfast cereals, dairy products, fruit juices and fat spreads. Some countries, such as Belgium, Denmark, the Netherlands and UK, have restrictions in place on the level of folic acid that can be added to food. In Norway and Denmark, approval is required before a folic acid-fortified food can be marketed.
The Gap Between Folate Intakes and Folate Recommendations in European Countries
It is clear from Table 1 that there is a significant gap between dietary folate intake and recommended intakes among European women. In most of the 12 countries reported, the approximate daily folate intake varied around 250 μg, without supplementation. For most of these countries, the recommendations of folate intake for women of fertile age and recommendations set for pregnancy and lactation periods are similar, that is, 400 μg ± 100 μg folate/d. A decision of mandatory folic acid fortification of flours is aimed to increase the folate intake by around 100 to 150 μg per day, which could bridge the gap between actual intake and recommendations.
Only three countries, Denmark, Germany and UK, have provided figures on daily folate intake among women of fertile age including supplementation. These figures amount to 478 μg/day vs. 295 (Denmark), 290 μg/day vs. 226 μg/day (Germany) and 292 μg/day vs. 251 μg/day with and without supplementation (UK) (3).
What are the Reasons Behind the Hesitation of European Countries to Introduce Mandatory Folic Acid Fortification?
‘During the last decade, a possible relationship regarding folic acid and cancer risk has emerged in the scientific literature. For instance, time trends in colorectal cancer incidence in the USA and Canada between 1986 and 2002 indicated an abrupt reversal in the downward trend in colorectal cancer incidence between 1996 and 1998 at around the time of the introduction of folic acid fortification. The downward trend later resumed with the incidence curve shifted upwards because of temporary increase. Mason et al. (8) hypothesized that folic acid fortification may have been responsible for the significant deviation from the pre-1996 trend resulting in an excess of about 4–6 additional cases of colorectal cancer cases per 100 000 individuals. This type of ecological evidence cannot exclude the possibility that the observed fluctuations in colorectal cancer were due to improved screening programmes for colorectal cancer. While there was an increase in colorectal cancer incidence at around the time of the introduction of folic acid fortification, there was no corresponding increase in cancer mortality, which is consistent with the fluctuations being due to improved screening rather than increased incidence of cancer. However, cancer mortality may not be a useful endpoint in this context because an ecological study cannot take account of the effects on cancer mortality of new cancer treatments that became available in the 1990’ (3).
In 2008, ESCO WG on ‘Analysis of risks and benefits of fortification of food with folic acid’ was asked to: i) review current practice in member states regarding the level of voluntary fortification of foods with folic acid and ii) consider new evidence regarding the risk of high intakes of folic acid and the need to review current guidance on safe upper levels of folic acid for all population groups (3).
ESCO WG convened a 2-day scientific meeting in January 2009 to consider the evidence on the possible relationship regarding folic acid and cancer. Over 60 scientific experts from the EU, Switzerland, USA and Canada attended the meeting. The ESCO WG group focused on cancer risks from folic acid supplementation by pills (pharmacological doses) from two categories of trials: recurrence studies of colorectal adenomas and B vitamin reducing effects on coronary vascular disease (CVD). The ESCO WG group summarized the following statements of folic acid and cancer risk: ‘Evidence on folic acid and cancer risk is available from a number of randomized control trial. Some of these trials were specifically designed to test the effect of folic acid on recurrence of colorectal adenomas. These have produced different results: four studies with 3-year interventions reported no adverse effects, whereas one longer-term study reported adverse effects on adenomas in the intervention group. Results on cancer risk have also been brought together in a meta-analysis from other randomized control trials designated to test the hypothesis that folic acid and other B vitamins would reduce CVD risk. The evidence from these trials does not suggest that folic acid intakes are associated with increased cancer risk; however, interpretation of these data is limited by a number of issues including duration of the trials and power of the meta-analysis.’
A more recent systematic review and meta-analysis of cancer risk with folic acid supplements (9) concludes: ‘A meta-analysis of 10 randomized controlled clinical studies showed no benefit but a borderline significant increase in frequency of overall cancer in the folic acid group compared with controls. When analysing site-specific cancers, prostate cancer was the only cancer type where an increase in the risk was shown for folic acid supplements’. The supplementation of folic acid ranged between 800 μg and 40 mg/d and included also simultaneous supplementation of vitamin B12 and sometimes also vitamin B6. Yet, the conclusions were claimed to be a result of folic acid, only.
World Cancer Research Foundation Reports Some Evidence of Cancer-Protective Effects with Increasing Dietary Folate Intakes
Although the ESCO WG group according to their report title should also consider the benefits of folic acid fortification, this aspect was given very little attention. This is indeed surprising as most epidemiological studies have reported significant protective effects of dietary folate intakes vs. cancer risks. The best source of the state-of-the-art of this area is the evaluations presented by WCRF (4). WCRF is an organization of high recognition and integrity for evaluations of the relationship between diet and cancer. Its experts only evaluate epidemiological studies meeting clearly defined quality criteria. Further, WCRF emphasizes that only studies of dietary intakes are included. This means studies where diet might include items mandatory or voluntarily fortified with folic acid, but not studies where folic acid is supplemented by pills. In their last evaluation, published in 2007 (4), the strongest significance between folate intake and relative risk (quotient between highest quartile/quintile vs. the lowest quartile/quintile) was found for pancreatic cancer, where WCRF stated that: ‘There is evidence suggesting that foods containing folate probably protect against pancreatic cancer.’ For cancer in oesophagus, WCRF judged the evidence to be limited (suggestive decreased risk). For colorectal cancer, WCRF states: ‘The evidence from cohort studies is plentiful, with a dose–response relationship, but there is unexplained inconsistency. Residual confounding from dietary fibre is possible. There is limited evidence suggesting that foods containing folate protect against cancer’. Other cancer sites were not evaluated. WCRF does not report any study where the quotient between the highest and lowest quartiles of dietary folate intake was significantly higher than 1, that is, increased cancer risk with increasing folate intake. They did, however, refer to an animal study reporting increased cancer risks in experimentally produced colon cancer following huge supplementation of folic acid by the feed. The dose of folic acid given was pharmacological rather than physiological.
Folic Acid and Cancer – Possible Mechanisms
Folate is a generic term for a naturally occurring family of B-group vitamins (essential nutrients) (10–14). It is found naturally in a variety of foods including green leafy vegetables, roots and legumes, seeds, citrus fruits, strawberries, liver, egg and yeast. Folic acid is the synthetic oxidized form of folate, which is widely used in supplements and for food fortification. Folic acid is more stable in foods and is better absorbed than natural folates.
Dietary folates are reduced and usually linked to a polyglutamic chain. As their metabolic role is as carriers of C1 groups, the folate molecule is substituted by one of five different exocylic reduced C1 groups, that is, methyl-, formyl-, methylene-, methylene- and formimino-, attached to position N5 and/or N10. The C1 groups are supplied from the catabolism of certain amino acids and further converted enzymatically.
Intracellularly, the reduced folates occur in polyglutamyl forms carrying reduced one-carbon groups. These folates are directly involved in the nucleotide biosynthesis (DNA and RNA) by supplying two carbon atoms to each purine molecule. Another key step where C1 groups from reduced folates participate directly is methylation of uridine to thymidine. Low cellular folate levels result in limited supply of C1 groups, that is, an ineffective DNA and RNA synthesis, leading to inhibition of tumour growth and suppression. On the contrary, high folate intakes are believed to enhance growth and replication of all cells including cancer cells.
Additionally, low cellular folate levels retard methylation of uracil to thymidine, leading to thymidine depletion and elevated uracil concentrations. As uracil and thymidine differ only by a single methyl group, uracil is incorporated into DNA in place of thymidine, but is quickly removed by DNA repair enzymes. The low cellular folate levels induce DNA strand breaks, chromosomal and genomic instability, uracil misincorporation and impaired DNA repair, which increases mutations ultimately leading to cancer (10–13). The genome instability caused by low folate levels can be corrected by higher folate intakes and that is the supposed mechanism behind decreased cancer risks at dietary folate intakes in recommended ranges.
In addition, folate is one of the participants of the methylation cycle particularly the methylation of homocysteine to methionine by a methylcobalamin (vitamin B12)-dependent route. Folate/vitamin B12 is one pathway to restore homocysteine to methionine. Supplementation with these two vitamins can normalize elevated homocysteine levels seen at low folate and/or vitamin B12 status; elevated homocysteine levels have been intensively investigated as a risk factor for cardiovascular diseases. Another methyl donor restoring methionine from homocysteine is betaine formed from phospholipids, for example, choline. Methionine is the precursor of SAM (S-adenosylmethionine), the key methyl donor in all cellular methylation reactions. Methionine is also supplied from dietary protein intake and protein catabolism in cells. Betaine and its precursors (phospholipids) can be synthesized by the cells and also be supplied by dietary intakes.
Altered DNA methylation has been reported to enhance the cancer risk. Global and gene-specific DNA hypomethylation and site-specific hypermethylation are common features in tumorigenesis (14). As a consequence, many scientists point to the role of folate in creating hyper- and hypomethylation of DNA either in subnormal or pharmacological levels. However, linking only folate intakes/status to DNA methylation is a considerable simplification, as folate is just one of several actors supplying C1 groups to the SAM pool.
Whereas the beneficial effect of folic acid on NTDs only required intervention studies lasting for <1 year, beneficial and adverse effects on cancer risks need much longer observational times, up to 30 years. Hitherto, the majority of epidemiological studies regarding folate and cancer are based on simple biomarkers such as folate status and/or folate intakes. Instead, a multitude of biomarkers need to be considered, for example, DNA methylation, DNA instability, intakes and status of some more B vitamins and other relevant methyl donors, mentioned previously. This means that it will take several decades to reach scientifically evidence-based knowledge on the relationship between folate and cancer, if at all possible, considering the complexity of the issue.
Meanwhile, we know that the beneficial effect of mandatory folic acid fortification of flours is scientifically evidence-based, although the protective mechanisms of folic acid supplementation towards NTD are not fully understood. Other beneficial effects, for instance on CVD, cancer, cognitive functions and other birth defects, have not yet reached consistent scientific evidence. For cancer risks, there are indications of a U-shaped relationship, that is, higher cancer risks at low folate intake and elevated folic acid intakes, respectively, with least risks at optimal folate intakes around 300–400 μg folate/d. Mandatory folic acid fortification should enhance folic acid intakes by 100–150 μg/d, which will bring the current average intake typical of European populations into a minimum cancer risk level.
Apart from taking advantage of the benefits on NTD prevalence for women of fertile age planning for pregnancies, the 10% of the population with the lowest folate intake this might be protected from enhanced cancer risks. In the long-term perspective might prevent cancer also for that part, approximately 1% of the population, that statistically can reach intakes of folate close to or exceeding the safe upper level, 1 mg/day.
Today, 15 years has passed since the first countries, USA and Canada, decided to introduce mandatory folic acid fortification of wheat and corn flour to prevent NTD. No scientific evidence of any harmful effects of the folic acid fortification for health has emerged forcing these pioneer countries to change their mandatory fortification decision.