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Homocysteine Lowering Trialists' Collaboration Correspondence to: Dr Robert
Clarke, Homocysteine Lowering Trialists' Collaboration, Clinical Trial
Service Unit, Radcliffe Infirmary, Oxford OX2 6HE
robert.clarke{at}ctsu.ox.ac.uk
Objective: To determine the size of reduction in
homocysteine concentrations produced by dietary supplementation with folic acid and with vitamins B-12 or B-6.
Epidemiological studies have consistently reported that patients
with occlusive vascular disease have higher blood homocysteine concentrations than control subjects, and that these differences precede the onset of disease and are independent of other risk factors.1-5 A meta-analysis of the observational studies
of blood homocysteine and vascular disease indicated that a prolonged
lowering of homocysteine concentration by 1 µmol/l was associated
with about a 10% reduction in risk throughout the range
10-15 µmol/l.1 Blood concentrations of homocysteine are
inversely related to blood concentrations of folate, vitamin B-12, and,
to a lesser extent, vitamin B-6.6 Dietary supplements of
these vitamins are used to reduce homocysteine concentrations in
subjects with homozygous homocystinuria, who have particularly high
blood concentrations of homocysteine.7 Several randomised
controlled trials of the effects of folic acid based supplements on
homocysteine concentrations have been conducted. Our study aimed, by a
meta-analysis of data from individual participants in these trials, to
determine more reliably the size of the reduction in blood homocysteine
achieved with different doses of folic acid and with the addition of
vitamin B-12 and vitamin B-6. This should help in the design of
randomised trials of the effects of lowering homocysteine
concentrations on vascular disease.
Studies included
Information collected
Table 1
Table 2
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Abstract
Top
Abstract
References
Design: Meta-analysis of randomised controlled trials
that assessed the effects of folic acid based supplements on blood
homocysteine concentrations. Multivariate regression analysis was used
to determine the effects on homocysteine concentrations of different
doses of folic acid and of the addition of vitamin B-12 or B-6.
Subjects: Individual data on 1114 people included in
12 trials.
Findings: The proportional and absolute reductions in
blood homocysteine produced by folic acid supplements were greater at
higher pretreatment blood homocysteine concentrations (P<0.001) and at
lower pretreatment blood folate concentrations (P<0.001). After
standardisation to pretreatment blood concentrations of homocysteine of
12 µmol/l and of folate of 12 nmol/l (approximate average
concentrations for Western populations), dietary folic acid reduced
blood homocysteine concentrations by 25% (95% confidence interval
23% to 28%; P<0.001), with similar effects in the range of 0.5-5 mg
folic acid daily. Vitamin B-12 (mean 0.5 mg daily) produced an
additional 7% (3% to 10%) reduction in blood homocysteine. Vitamin
B-6 (mean 16.5 mg daily) did not have a significant additional effect.
Conclusions: Typically in Western populations, daily
supplementation with both 0.5-5 mg folic acid and about 0.5 mg vitamin
B-12 would be expected to reduce blood homocysteine concentrations by
about a quarter to a third (for example, from about 12 µmol/l to 8-9 µmol/l). Large scale randomised trials of such regimens in high risk
populations are now needed to determine whether lowering blood
homocysteine concentrations reduces the risk of vascular disease.
Key messages
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Introduction
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Methods
We aimed to identify all published and unpublished randomised
trials that had assessed the effects on blood homocysteine
concentrations of folic acid supplements, with or without the addition
of vitamins B-12 or B-6. Studies were not eligible if they did not
include an untreated control group, assessed treatment after methionine loading, or treated patients for less than 3 weeks.8-15
Eligible studies were identified by Medline searches (using search
terms and widely used variants for folic acid, vitamin B-12, vitamin B-6, and homocysteine, and including the non-English language literature), scanning reference lists, and personal contact with relevant investigators. The 14 trials we identified that fulfilled the
eligibility criteria16-24 included two completed trials
(involving 50 and 144 subjects; V Howard, I Brouwer, personal
communications) from which data are not available for collaborative
analyses until their publication. The 12 available trials included 1114 subjects. Ten of these trials had a parallel group
design16-24 and two had a crossover design21
(for which, to avoid any carryover effects, we used only data from the
first period). The allocated treatment was blinded in all trials except
two that had untreated controls.
16 23
For each subject entered in these trials, we sought details of
age, sex, smoking habits, history of vascular disease or hypertension, and vitamin use before randomisation, and of their randomly allocated treatment regimen (daily dose of folic acid, vitamin B-12 or vitamin B-6, and scheduled duration) and blood concentrations of homocysteine, folate, vitamin B-12, and vitamin B-6 before treatment and at the end
of the scheduled treatment period.
Statistical analysis
The proportional reductions in blood homocysteine in the treated
groups compared with the control groups were determined by extending an
analysis of covariance25 that estimated the differences in
post-treatment, log transformed homocysteine values after adjustment for baseline values of homocysteine. The simple model was extended to
allow the extent of this adjustment to vary between studies and to take
account of factors such as folic acid dose, concomitant vitamin B-12 or
vitamin B-6, age, sex, and duration of treatment. More complex models
that allowed the effect of folic acid supplementation to differ in
individual studies were used to investigate sources of heterogeneity.
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Results |
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Characteristics of individual trials
Among the 1114 subjects in the trials, the mean age was 52 years
(range of trial means 23 to 75 years) and the mean duration of
treatment was 6 weeks (range 3 to 12 weeks) (table 1). The median
pretreatment blood concentration of homocysteine was 11.8 µmol/l and
of folate was 11.6 nmol/l, but there were substantial differences
between the trials. All of the trials compared folic acid alone versus
control or folic acid plus vitamin B-6 or B-12, or both, versus
control, although two trials
17 18
also involved within-trial comparisons of folic acid alone versus combination therapy
(table 2). A correlation coefficient of 0.87 for homocysteine in
pretreatment blood samples collected from 664 of these patients on two
separate occasions shows that there was relatively little variation
within subjects and that the homocysteine measurements were reliable.
Compliance with the study protocols was good, with blood homocysteine
measurements at the end of study treatment available from 98% of those
randomised.
Exploration of heterogeneity between the results of different
trials
The effect of folic acid on blood homocysteine concentrations
seemed to differ among the trials. This heterogeneity of the homocysteine lowering effect was not explained by differences in age,
sex, or duration of treatment (although the longest duration studied
was only 12 weeks). The proportional and absolute reductions in blood
homocysteine concentrations seemed, however, to be influenced by the
pretreatment blood concentrations of homocysteine and folate, but not
of vitamin B-12. Even after adjustment for differences in the folic
acid regimen, the homocysteine lowering effect of folic acid ranged
from a proportional reduction of 16% (11% to 20%) among subjects in
the bottom fifth of pretreatment blood homocysteine concentrations to a
39% (36% to 43%) reduction among those in the top fifth (fig 1; P
for trend <0.001). Conversely, the blood homocysteine lowering effect
of folic acid was greater at lower pretreatment blood concentrations of
folate (P for trend <0.001). These associations of the homocysteine
lowering effect with pretreatment concentrations of blood homocysteine
and blood folate remained significant (P<0.001) when both pretreatment
measurements were included simultaneously in the model. The model
provided no strong evidence that the variation in the homocysteine
lowering effect with baseline homocysteine depended on baseline folate or vice versa. Figure 2 shows that the proportional reductions in blood
homocysteine concentrations achieved by folic acid supplementation according to pretreatment blood levels of homocysteine and folate under
this assumption. (Exclusion of the two trials
18 19
in subjects with very high pretreatment blood homocysteine concentrations did not materially alter these findings, and nor did inclusion of the
two small completed but unpublished trials not yet formally available
for these collaborative analyses: data not
shown.)
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Effects of different folic acid doses on blood homocysteine
After pretreatment blood concentrations of homocysteine and folate
were adjusted for, there was no longer much evidence of heterogeneity
between the separate blood homocysteine lowering effects in the
different trials of daily folic acid doses of <1 mg (mean dose 0.5 mg; P value for heterogeneity=0.15), of 1-3 mg (mean dose 1.2 mg;
P=0.05), or of >3 mg folic acid (mean dose 5.7 mg; P=0.69). Nor was
there any evidence of differences between the blood homocysteine
lowering effects of these different folic acid doses. For individuals
with pretreatment blood concentrations of homocysteine of 12 µmol/l
and of folate of 12 nmol/l (approximate average concentrations for
Western populations), folic acid doses of <1 mg, 1-3 mg, and >3 mg
daily were each associated with reductions in blood homocysteine of
about one quarter (fig 3).
Effects of adding vitamin B-12 or vitamin B-6 to folic acid
The addition of vitamin B-12 (0.02-1 mg daily; mean 0.5 mg) to
folic acid further reduced blood homocysteine concentrations by about
7% (3% to 10%). Hence, among people with pretreatment blood
concentrations of homocysteine of 12 µmol/l and of folate of
12 nmol/l, adding vitamin B-12 to folic acid changed the reduction in
homocysteine from 25% (23% to 28%) to 31% (27% to 35%). Adding vitamin B-6 (2-50 mg daily; mean 16.5 mg) to folic acid did not lower
blood homocysteine any further.
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Discussion |
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Among the vitamins studied in these trials, folic acid had the dominant blood homocysteine lowering effect, and this effect was greater among subjects with higher blood homocysteine concentrations or lower blood folate concentrations before treatment. After standardisation for differences in pretreatment blood homocysteine and folate concentrations, the effect of folic acid was similar for daily doses ranging from 0.5 to 5 mg daily, and vitamin B-12 produced a small additional effect. Supplementation with vitamin B-6 did not seem to have any material effect on blood homocysteine concentrations, but these trials did not assess effects on blood homocysteine after methionine loading, which may be determined to a greater extent by the transulphuration pathway in which vitamin B-6 is a cofactor.8
Our results suggest that a daily dose of at least 0.5 mg of folic
acid, along with a similar amount of vitamin B-12, would produce a
proportional reduction in blood homocysteine of about a quarter to a
third. The addition of about 1 mg daily of oral vitamin B-12 to folic
acid would also be expected to avoid the theoretical risk of neuropathy
due to unopposed folic acid therapy in patients deficient in vitamin
B-12, even those with intrinsic factor deficiency or malabsorption
states.26-28 Studies in the United States and Britain
indicate that the average concentration of blood homocysteine in a
typical Western population is about 12 µmol/l,
4 6
and
so a reduction of about a quarter to a third would correspond to an
absolute reduction of about 3-4 µmol/l. A previous meta-analysis of
the observational studies suggests that a prolonged lower blood
homocysteine concentration of 3-4 µmol/l would correspond to 30-40%
less vascular disease.1 Consequently, even if as much as
half of the epidemiologically predicted benefit is achieved within a
few years of lowering blood homocysteine (as seems to be the case with
cholesterol lowering29-31), trials of folic acid
supplements may well need to be large, and to include people at high
risk, to be able to detect the sort of reductions
15% to 20%
in
cardiovascular risk that might realistically be anticipated.
Supplementation with folic acid is a cheap and effective method of
lowering blood homocysteine concentrations. If large scale trials in
high risk populations do show reliably that blood homocysteine reductions with such supplements can be sustained over time and that
this strategy reduces the risk of vascular events (and is safe), this
could have important public health implications. Higher dose
supplements could be used in people at high risk, and population mean
concentrations of blood homocysteine could be reduced by fortifying
flour with folic acid.
1 32
Introducing fortified flour
for the prevention of neural tube defects before trials of folic acid
on vascular disease are conducted could, however, complicate the
overall assessment of any benefits
or risks
of lowering homocysteine
concentrations in this way.
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Acknowledgments |
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The following investigators were members of the Homocysteine Lowering Trialists' Collaboration. Abbreviated trial names are listed alphabetically, along with the institutions and names of the principal investigators. Brattström (University of Lund: L Brattström, F Landgren, B Israelsson, A Lindgren, B Hultberg, A Andersson); Cuskelly (University of Ulster: G Cuskelly, H McNulty, SS Strain; Trinity College, Dublin: J McPartlin, DG Weir, JM Scott); den Heijer (Leyenburg Hospital, the Hague, and University of Nijmegen: M den Heijer, IA Brouwer, HJ Blom, GMJ Bos, A Spaans, FR Rosendaal, CMG Thomas, HL Haak, PW Wijermans, WBJ Gerrits); Naurath (University of Leuven and Witten-Herdecke: HJ Naurath, E Joosten, R Riezler, SP Stabler, RH Allen, J Lindenbaum); Pietrzik (University of Bonn: K Pietrzik, R Prinz-Langenohl, J Dierkes); Saltzman (USDA-HNRC at Tufts University: E Saltzman, JB Mason, P Jacques, J Selhub, D Salem, E Schaefer, IH Rosenberg); Ubbink (University of Pretoria: J Ubbink, A van der Mere, WJH Vermack, R Delport, PJ Becker, HC Potgieter); Woodside (Queen's University of Belfast: JV Woodside, JWG Yarnell, D McMaster, IS Young, EE McCrum, SS Patterson, KF Gey, AE Evans).
Secretariat: Clinical Trial Service Unit, University of Oxford (R Clarke, P Appleby, P Harding, P Sherliker, R Collins) and Medical Statistics Unit, London School of Hygiene and Tropical Medicine (C Frost, V Leroy).
Writing committee and guarantors: R Clarke, C Frost, V Leroy, R Collins.
This paper is dedicated to the late Dr John Lindenbaum.
Funding: British Heart Foundation and Medical Research Council.
Conflict of interest: None.
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References |
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an innocuous means of reducing plasma homocysteine.
Scand J Clin Lab Invest
1988;
48:
215-221[Medline].(Accepted 27 November 1997)
T polymorphism and coronary heart disease: does totality of evidence support causal role for homocysteine and preventive potential of folate?
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