BMJ 2004;328:211-214 (24 January),
doi:10.1136/bmj.328.7433.211
Clinical
review
Science, medicine, and the future
Is folic acid the ultimate functional food component for disease prevention?
Mark Lucock,
lecturer1
1 Human Nutrition, School of Applied Sciences,
University of Newcastle, Brush Road, Ourimbah, NSW 2258, Australia Mark.Lucock@newcastle.edu.au
We are entering a
new era in preventive medicine, which focuses on diet as a means to
health. Folate has received much attention as a vitamin that can
protect against many diseases, but do we know enough about the long
term effects of supplementation?
 |
Introduction |
Mankind
has been relatively unsuccessful in the search for the ultimate
panacea for all ills; however, in the field of functional foods, few
nutritional components have so many fundamental and diverse
biological properties as folic acid and related B group vitamins.
Moreover, few nutrients can claim to modulate, if not overtly
benefit, such a wide array of clinical conditions.
Around 2500 years ago Hippocrates first espoused the "food as
medicine" philosophy, which fell into obscurity by the 19th
century. The first 50 years of the 20th century saw the discovery
of the essential elements and vitamins, particularly in the
context of deficiency diseases. Indeed, by 1912 Casimir Funk
had put forward the "vitamine theory," proposing four different
"vitamines" that would cure scurvy, pellagra, beri-beri, and
rickets. During the 1970s the shift in emphasis from undernutrition
to overnutrition and disease led to a flood of public health
guidelines on optimising nutritional parameters. By the 1990s,
with an ageing health conscious population, scientists from
academia and the commercial world coalesced their thinking to
create the trend we now know as functional foods.
Enrichment of flour as a US government programme to correct
problems with nutrient deficiency was probably the first modern
attempt to design a food for functional purposes related to
nutritional outcome. The first consequence of this was the
eradication of pellagra with niacin, and a current programme in the
United States aims to do the same for neural tube defects through
mandatory fortification of grain with folate at source.1
2
The ramifications of this mass use of folate as a functional food
are likely to have even wider effects. Folate status and common
variations in genes that code for folate dependent enzymes are linked
to many types of cancer, vascular disease, birth defects, and
complications of pregnancy.3
This arises because several molecular mechanisms that underpin the
genomic machinery are sensitive to B vitamin status and, in
particular, are responsive to the interaction between folate
nutrition and folate dependent enzyme polymorphisms (folate
nutrigenomics). Mechanisms that may be affected include the
maintenance of genomic CpG methylation patterns for regulated gene
expression and the proficient synthesis of nucleotides to prevent DNA
strand breakage.4
5
The same nutrigenomics also influence plasma homocysteine status and
thus risk for vascular disease.3
These relations are shown in figure
1.
Summary points
B vitamins, particularly folate, may give considerable
protection against serious diseases such as cancer, heart
disease, and birth defects
The method of protection is by lowering homocysteine
or through epigenetic mechanisms
Common single nucleotide polymorphisms of several
genes coding for folate dependent enzymes can modulate
risk for several serious clinical conditions
The level of risk or protection with some single
nucleotide polymorphisms is further influenced by a
person's nutritional folate status; this is the basis of
a new subdiscipline within human nutrition—"nutrigenomics"
Folate used in food fortification is not a natural
coenzyme; we do not know the long term biological effects
of exposure to unmodified synthetic folate
Future trends will emphasise dietary practices that
complement the genetic variation of an individual person
| |
 |
Worldwide interest in folate research
|
Clearly,
the use of folate fortification has immense potential benefit.
Interest in folate over the past decade has rocketed in comparison
with other nutrients, largely because scientists have recognised the
importance of this vitamin in treating a broad range of both
developmental and degenerative disorders that are sensitive to even
marginal deficiencies in B vitamins.6
Although Lucy Wills's 1931 description of yeast extract being
effective against the tropical macrocytic anaemia of late pregnancy
in India represents the first record of folate being used for
prevention of disease, folate as the critical factor involved
was not isolated, nor was its structure elucidated, until later.
Furthermore, it was not until more than half a century later
that the significance of folate in preventive medicine was once
again shown in a series of papers culminating in that by the
Medical Research Council Vitamin Study Group in 1991, documenting
how periconceptional folate prevents spina bifida.7
This discovery was followed by a meta-analysis published in 1995,
which presented data from 27 studies involving more than 4000
patients with occlusive vascular disease and a similar number of
controls.8
Data showed that homocysteine was an independent, graded risk
factor for atherosclerotic disease in the coronary, cerebral,
and peripheral vessels. This was of particular interest, as
dietary folate lowers homocysteine through de novo biosynthesis
of methionine,9
and it opened new avenues for intervention with vitamins to prevent
disease. Several single nucleotide polymorphisms that are related to
folate and other B vitamins were also discovered in 1995. These
affect the risk not only of birth defects and vascular disease but
also of several cancers. The two most important cancers in this
context are colorectal cancer and leukaemia.
In order to examine research trends in this area, I did a systematic
key word search of the PubMed National Library of Medicine for
the years 1992-2002. I noted the number of citations for folate
in association with a range of developmental and degenerative
conditions for each year. Figure
2 shows how interest in this vitamin has changed over the past 10
years. It is clear that all areas of research related to folate and
disease have expanded, but interest in folate in relation to cancer
has increased in particular (fig
2 (top)). However, when the data are expressed as a percentage of
all papers on folate, only research in relation to vascular disease
has expanded year on year in real terms (fig
2 (bottom)), presumably owing to worldwide interest in the
beneficial effects on vasculotoxic homocysteine. Although not shown,
numbers of publications on folate in general far exceed those
relating to other similar micronutrients. To provide a limited
illustration of how popular the field of folate-homocysteine
interrelations has become, figure
3 shows how folate research and vitamin C research had similar
outputs until the mid to late 1990s, when interest in folate began to
surge. However, publications on homocysteine were fewer than those on
either folate or vitamin C up to 1996, but at the time that
research on folate surged (around 1997) homocysteine research
substantially outstripped the number of publications dealing with
either of the other vitamins. The antioxidant nature of vitamin C
makes it a popular vitamin for biomedical studies, so it
provides a fair baseline for comparison.
 View larger
version (39K): [in
this window] [in a new window] |
Fig 2 Number of publications on
folate and specific diseases (top); publications on folate and
specific diseases as a percentage of all publications on folate
(bottom)
| |
 |
Folate nutrigenomics |
Much of
the current interest in folate stems from the discovery of several
single nucleotide polymorphisms that modulate risk for a range of
important diseases associated with considerable morbidity and
mortality.5
Box 1 gives some examples of these. Of even greater importance is the
fact that dietary folate can interact with the proteins that are
encoded by these variant genes and ameliorate risk to the extent that
an overt protection against the disease is conferred. Box 2 shows the
functional consequence of the C677T single nucleotide polymorphism of
5,10-methylenetetrahydrofolate reductase, in terms of dTMP nucleotide
biosynthesis, DNA methylation, and homocysteine metabolism, and
relates these to the pathology of cancer, developmental disorders,
and vascular disease.
 |
How much and what type of folate do we need in our
diet? |
Despite
the heightened interest and clear benefits of folate supplements,
caution is needed. The form of folate in supplements and in fortified
foods is pteroylmonoglutamate (PGA), a form that does not occur in
nature. It is both cheap and stable unlike most native forms of the
vitamin. The body metabolises PGA into methylfolate, the normal form
of the vitamin transported in plasma. However, research shows that
this absorption and biotransformation process is saturated at doses
in the region of 400 µg PGA or less.23
24
Thus at doses at or just below 400 µg PGA all this synthetic form of
folate is converted into biologically active methylfolate during
absorption. At higher doses synthetic PGA is also transported into
the blood in a manner that is directly proportional to dose. This
raises the possibility of a lifetime's exposure to unmetabolised PGA
where mandatory fortification is undertaken. Although such exposure
may present no health risk at all, we cannot know this for certain.
Also, what of the increasing tendency among clinicians to give 5 mg
doses of PGA (more than 10 times that needed to give maximal
methylfolate concentrations)? These doses are not uncommon in
patients with vascular problems or with an elevated homocysteine. In
one recent study we examined B vitamins and polymorphic markers in
thrombotic vascular disease.19
The serum folate measurements were usually in the range 3-30 ng/ml
but were interrupted by several exceedingly high values (100-200
ng/ml). These values were real, as they were mirrored by the
corresponding red cell folate values, which were also particularly
high (by high pressure liquid chromatography and routine
haematological assay). When we subsequently examined some of these
high serum folates by high pressure liquid chromatography, we found
that only around half of the serum folate measured by routine
haematological assay was methylfolate. The assumption was that the
remainder was unmodified PGA, as only this and other partially
oxidised folates do not fluoresce and thus are not directly
measurable by high pressure liquid chromatography.25
Box 1: Some important
single nucleotide polymorphisms of B vitamin genes
associated with various clinical conditions
- C677T variant of 5,10-methylenetetrahydrofolate
reductase gene:
- Colon cancer10
11
- Elevated homocysteine, a risk for vascular
disease12
13
- Spina bifida14
- Down's syndrome15
- Oral cleft16
- Adult acute lymphocytic leukaemia17
- Complications of pregnancy (pre-eclampsia,
recurrent early pregnancy loss, fetal growth
restriction)3
- A1298C variant of 5,10-methylenetetrahydrofolate
reductase gene:
- Spina bifida18
- Adult acute lymphocytic leukaemia17
- A2756G variant of methionine synthase gene:
- Thromboembolic vascular disease19
- Influences homocysteine response to diet
- A66G variant of methionine synthase reductase
gene:
- Spina bifida20
- Down's syndrome21
- C1420T variant of serinehydroxymethyl transferase
gene:
Adult acute lymphocytic leukaemia22
- 2R3R variant of thymidylate synthase gene:
Adult
acute lymphocytic leukaemia22
- C1561T variant of glutamate carboxypeptidase
(folate deconjugase) gene:
May affect cardiovascular
disease
(In many of the above examples, protection against as
well as susceptibility to disease is conferred)
| |
Box 2: Functional
consequence of C677T variant of 5,10-methylenetetrahydrofolate
reductase gene3
5
Direct biochemical effect
Folate stabilises the polymorphic enzyme encoded by
the C677T variant gene by preventing it from
relinquishing its flavin cofactor. As
5,10-methylenetetrahydrofolate reductase is a flavin
protein, several authors have suggested that people with
the TT recessive genotype may respond more rapidly to
riboflavin (vitamin B2) supplements as well as
folate to lower homocysteine
Nucleotide biosynthesis
dTMP used for DNA is synthesised by thymidylate
synthase from dUMP and requires the one-carbon unit of
5,10-methylenetetrahydrofolate. If folate levels are low,
uracil misincorporation occurs, leading to breaks in the
DNA strand, which predispose to cancer. The polymorphic
enzyme encoded by the C677T variant gene can enhance
synthesis of dTMP nucleotide if folate status is good
(bottleneck in one-carbon flux into methionine favours
diversion to nucleotides), and this is thought to afford
protection against cancer of the colon and leukaemia. If
folate status is poor, the single nucleotide polymorphism
may confer risk rather than protection
Biological methylations
Dietary methionine cannot provide all methyl groups
needed for cellular methylation reactions, leading to a
dependence on de novo synthesis of methionine through the
folate one-carbon pool. S-adenosylmethionine (AdoMet)
levels regulate protein, lipid, biogenic amine, and DNA
methylation. AdoMet dependent DNA methylation of specific
CpG sites regulates gene expression and plays a critical
role in the developmental process. Methylation of
clusters of CpG sites associated with promoter regions tends
to silence gene expression. A deficiency of methyl groups
may therefore alter the normal control of proto-oncogene
expression. The polymorphic enzyme encoded by the C677T
variant gene may reduce availability of de novo methyl
groups for these important reactions
Homocysteine metabolism
Polymorphic 5,10-methylenetetrahydrofolate reductase
reduces one-carbon flux to methylfolate, the donor
molecule for conversion of homocysteine into methionine.
This single nucleotide polymorphism may thus elevate
homocysteine, which is an independent risk factor in
heart disease. Homocysteine is atherogenic and undergoes
redox cycling in the presence of transition metal ions,
forming radicals that cause oxidative damage to low
density lipoprotein. Homocysteine may also react with
cysteine SH groups and modify apolipoproteins. It is also
a hypertensive compound, reacting with endothelium-derived
relaxation factor to form S-nitrosohomocysteine and
superoxide. This causes a loss of vasodilation action. It
also inhibits or downregulates anticoagulants, including
prostacyclin synthesis, activation of protein C,
thrombomodulin expression, heparin sulphate expression,
and fibrinolysis. In addition, it activates procoagulants
such as factor V and tissue clotting factor. Some other
effects include proliferation of vascular smooth muscle
and increased platelet coagulability. Its final effect is
to chelate copper and inhibit lysyl oxidase, which
impairs cross linking of collagen and elastin and leads
to connective tissue abnormalities
| |
Nobody yet knows what, if any, the long term biological effect of
mandatory fortification of grain products would be at a national
level. In a recent report on the situation in the United States,
more people than expected were subjected to the established
tolerable upper limit of exposure to PGA.26
Folate metabolism is extraordinarily complex, underpinning several
pathways crucial for life. Notwithstanding this fact, unmetabolised
non-native PGA should be assessed for its in vivo effects on folate
dependent enzymes. It is metabolised by dihydrofolate reductase, for
which it has a higher Km (lower affinity) than has dihydrofolate
itself—could this have an antifolate effect through competitive
interaction? Could similar phenomena occur at other enzymes,
particularly allosteric enzymes where structurally similar
dihydrofolate plays a regulatory role?27
In vitro studies do show that PGA derivatives act to inhibit certain
enzymes,28
including those associated with nucleotide biosynthesis.29
Although such in vitro interactions remain hypothetical to the in
vivo situation, research to test for their existence in humans seems
only prudent.
With some people receiving more than 600 µg/day of folate, largely
as PGA, and clinicians regularly prescribing 5 mg/day of PGA, we need
to know what the consequences of long term exposure to PGA are (no
matter how subtle). Perhaps we should also be assessing
supplementation with isomer specific native folates that represent
the natural food forms of the vitamin, as well as simple ways to
preserve native folate activity during the food preparation
process.
One thing is for sure—folate is of great interest and of great
clinical value, a veritable panacea among functional foods. Given the
fundamental importance of B vitamin nutrigenomics, and the pace of
development in molecular diagnostics, it is not hard to envisage a
new era in preventive medicine that has even greater emphasis on diet
as a means to a long and healthy life—indeed, a return to
Hippocrates' famous "Let food be thy medicine and medicine be thy
food" philosophy. As they say, "what goes round comes round," the
only difference today being the remarkable use of polymerase chain
reaction technology.
Competing interests: None declared.
Funding: None.
 |
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(Accepted 18 November 2003)
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