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Serum Folate and Risk of Fatal
Coronary Heart Disease
Howard I. Morrison, PhD;
Douglas Schaubel; Marie Desmeules, MSc;
Donald T.
Wigle, MD, PhD
Objective.\p=m-\Toassess the relationship between serum folate level and the risk
of fatal coronary heart disease (CHD) among men and women.
Design.\p=m-\Retrospectivecohort study with serum folate levels measured from
September 1970 to December 1972, with follow-up through 1985.
Setting.\p=m-\Participantsin the Nutrition Canada Survey.
Participants.\p=m-\Atotal of 5056 Canadian men and women aged 35 to 79 years
with no history of self-reported CHD.
Main Outcome Measure.\p=m-\Fifteen-yearCHD mortality.
Results.\p=m-\Atotal of 165 CHD deaths were observed. We found a statistically
significant association between serum folate level and risk of fatal CHD, with rate
ratios for individuals in the lowest serum folate level category (<6.8 nmol/L
[3 ng/mL]) compared with the highest category (>13.6 nmol/L [6 ng/mL]) of 1.69
(95% confidence interval, 1.10-2.61).
Conclusions.\p=m-\Thesedata indicate that low serum folate levels are associated
with an increased risk of fatal CHD.
(JAMA. 1996;275:1893-1896)
WELL-ESTABLISHED risk factors for
coronary heart disease (CHD) include
cigarette smoking, diabetes, hyperten¬
sion, and elevated
serum
cholesterol
levels.13
Although there is only limited
epidemiologie evidence that folate con¬
centration is associated with risk of
CHD,4 folate consumption has been
found to be a determinant of homocysteine-related carotid artery thickening.56
Moderate hyperhomocysteinemia has
been linked to both cerebrovascular and
CHD.78 Several mechanisms have been
proposed,6 including the toxic effects of
homocysteine on vascular endothelium,
and in proliferation of arterial wall
smooth muscle cells. Folate is required
for the conversion of homocysteine to
methionine.9 A strong inverse correlaFrom the Cancer
Bureau, Health Canada, Ottawa,
Ontario.
Reprints: Howard Morrison, PhD, Cancer Bureau,
Laboratory Centre for Disease Control Building, Tunney's Pasture, Ottawa, Ontario, Canada K1A 0L2.
tion has been noted between serum
folate and homocysteine levels among
subjects with elevated homocysteine
levels.10
The purpose of this article is to assess
the relationship between serum folate
level and the risk of fatal CHD among
both men and women in the Nutrition
Canada Survey (NCS) cohort.
METHODS
The NCS was conducted between Sep¬
tember 1970 and December 1972 using
a 3-stage stratified cluster design. A to¬
tal of 12 795 people responded to the
initial invitation to participate (46% re¬
sponse rate); 3295 unsolicited volunteers
also participated.11,12 Each survey cen¬
ter included a laboratory for the collec¬
tion and initial processing of blood and
urine samples. Blood samples were centrifuged to obtain serum, and a stabi¬
lizing solution was added to a portion of
the serum for later determination of vi¬
tamin C level. All samples were then
immediately frozen and shipped to a cen¬
tral laboratory for further biochemical
analyses.11
Participants were followed up retro¬
spectively between the ages of 35 and 79
years. Thus, those younger than 35 years
at interview began follow-up when they
reached the age of 35 years, while those
aged 80 years or older at interview were
excluded.
For editorial comment
see
1929.
The results of this article are based on
the mortality experience of both solic¬
ited and unsolicited respondents. Among
the 16090 NCS participants, individu¬
als who were interviewed as members
of specially targeted populations (preg¬
nant women, Native Indians, and Inuit)
were excluded (n=2828). A total of 3568
subjects were excluded because they did
not contribute follow-up to the 35- to
79-year-old age category. Of the 8792
remaining individuals, an additional 3736
were excluded, either because of selfreported heart disease (n=987) or be¬
cause of missing data (n=2749), result¬
ing in a final study population of 5056
subjects. Work to extend the follow-up
period to the end of 1993 is ongoing.
We examined the following survey
variables in our analysis: age; sex; re¬
spondent status (solicited or volunteer);
smoking habits (never, former, current);
diastolic blood pressure; history of dia¬
betes or presence of glucose in the urine
(as detected with a glucose oxidase re¬
agent strip); serum total cholesterol
level; serum vitamin C, serum vitamin
A, and serum vitamin E levels; dietary
folie acid consumption; use of folie acid
supplements; use of vitamin and/or min-
Downloaded from jama.ama-assn.org by guest on April 8, 2011
eral
supplements; serum iron level; and
folate level. Folate levels were
determined microbiologically using Lactobacillus casei. Serum homocysteine
levels were not measured.
Age was categorized as 35 to 54,55 to
59, 60 to 64, 65 to 69, 70 to 74, and 75 to
79 years. The serum folate levels were
categorized into approximate quartiles,
ranging from <6.8 nmol/L (3.0 ng/mL)
to >13.6 nmol/L (6.0 ng/mL). We as¬
sessed food consumption using a 24-hour
food recall questionnaire. United States
Department of Agriculture13 and Cana¬
dian Health Protection Branch tables14
were used to convert the information on
diet to nutrient intake.
Diastolic blood pressure was recorded
in the survey only for subjects with a
reading of 100 mm Hg or greater; only
subjects who had this level or reported
receiving treatment for hypertension
from a physician were considered hy¬
pertensive. Fifth-phase diastolic blood
pressure was measured while subjects
serum
were
sitting.
The mortality history of the cohort
was obtained by linking the survey file
to Statistics Canada's National Mortal¬
ity Database for the years 1970-1985
using the generalized iterative record
Table 1.—Characteristics of
Nutrition Canada Survey Cohort
Study Subjects,
Men,
No.
Characteristic
(%)
Women,
No.
(%)
Serum cholesterol,
mmol/L (mg/dL)
<4.14(<160)
213(9.7) 279(9.8)
4.14-<5.17(160-<20 ) 788 (35.7) 941 (33.0)
5.17-<6.21 (200-<240) 838 (38.0) 978 (34.3)
==6.21 (==240)
368(16.7) 651(22.9)
Diabetes status
2131
Nondiabetic
Diabetic
(96.6) 2742(96.2)
76(3.4) 107(3.8)
Cigarette smoking
Never
Former
633(28.7) 1617(58.8)
526(23.8) 218(7.7)
1048(47.5) 1014(35.6)
Current
Hypertension
No
Yes
1839(83.3) 2222(78.0)
368(16.7) 627(22.0)
Serum folate,
nmol/L (ng/mL)
(22.7)
(21.9)
676 (30.6)
<6.8
501
6.8-0.1
484
(<3.0)
(3.0-<4.0)
9.1-<13.6(4.0-<6.0)
==13.6 (==6.0)
546
(24.7)
Table 2.—Risk Factors for Fatal Coronary Heart
Disease, Nutrition Canada Survey Cohort*
763
654
748
684
(26.8)
(23.0)
(26.3)
(24.0)
Rate Ratio
(95% CI)
1.00
1.17
1.72
(0.73-1.85)
(1.19-2.47)
Cigarette smoking
_
Never
Former
Current
Hypertension
No
Yes
Diabetes status
Nondiabetic
1.00
2.37
(1.73-3.24)
1.00
2.26
(1.38-3.72)
<4.14(<160)
4.14-<5.17(160-<200)
5.17-<6.21 (200-<240)
==6.21 (==240)
1.00
1.71
1.96
2.91
(0.61-4.78)
(0.71-5.40)
(1.05-8.09)
9.1-<13.6(4.0-<6.0)
6.8-0.1 (3.0-<4.0)
<6.8(<3.0)
1.20
1.69
Diabetic
Serum cholesterol,
mmol/L (mg/dL)
Serum folate,
nmol/L (ng/mL)
==13.6 (==6.0)
1.00
1.11
(0.71-1.74)
(0.75-1.92)
(1.10-2.61)
*Model included terms for age, sex, serum choles¬
terol level, smoking status, diabetes status, and diastolic blood pressure. CI indicates confidence Interval.
3.5
3.0
2.5
linkage computer system.15 Coronary
heart disease (8th revision of the Inter¬
national Classification ofDiseases,16 ru¬
brics 410 to 414) was selected when listed
as the underlying cause of death.
We created Poisson regression mod¬
els that included terms for age, sex, se¬
rum folate level, cigarette smoking, hy¬
pertension, serum cholesterol level, and
diabetes status. Volunteer status and
serum levels of vitamins A, C, and E did
not confound the folate-CHD associa¬
tion, and were therefore not included in
the final models. Sex-specific and agespecific models were also created, as
were models examining serum folate in¬
teraction by serum cholesterol levels.
RESULTS
Table 1 displays the baseline charac¬
teristics of study subjects. Approxi¬
mately 23% of the men and 27% of the
women had serum folate levels less than
6.8 nmol/L (3.0 ng/mL). There was rela¬
tively poor correlation between serum
folate and dietary folate (as determined
by a 24-hour food recall, data not shown).
As seen in Table 2, an increased risk
of fatal CHD was associated with ciga¬
rette smoking, hypertension, elevated
serum cholesterol level, diabetes, and
decreased serum folate levels (lowest
quartile compared with highest quartile
rate ratio =1.69, 95% confidence inter¬
val [CI] =1.10-2.61). Risk increased in a
step-wise fashion as the serum folate
levels decreased (Figure). Sex-specific
0.5
0
4.5(2)
9.1(4)
Serum Folate,
13.6(6)
18.1
(8)
22.7(10)
nmol/L(ng/mL)
Serum folate level and fatal coronary heart disease, Nutrition Canada Survey Cohort, 1970-1985. The rate
ratio (95% confidence interval) was adjusted for age, sex, smoking, diabetes status, and serum cholesterol
level. Rate ratios are plotted against median serum folate levels for each category.
estimated relative risks (RRs) for folate
are displayed in Tables 3 and 4 for men
and women, respectively. Estimated
RRs for the lowest folate category com¬
pared with the highest were greater for
women (RR, 2.83; 95% CI, 1.30-6.18) than
for men (RR, 1.36; 95% CI, 0.80-2.31);
however, there was no statistically sig¬
nificant interaction by sex.
Although there was no statistically
significant interaction by age, the doseresponse relationship appears to be
slightly stronger for individuals younger
than 65 years compared with those 65
years and older (Table 5).
To test whether the serum choles¬
terol level modifies the relationship be¬
tween serum folate level and fatal CHD
risk, models were fitted with serum cho¬
lesterol levels less than 6.2 mmol/L (240
mg/dL) and 6.2 mmol/L (240 mg/dL) or
higher. Coronary heart disease rate ra¬
tios according to serum folate level did
not differ significantlyby cholesterol sta¬
tus
(data
not
shown).
There were too few exposed to folate
supplements to assess risk (<1% of re¬
spondents). We found no important as¬
sociations between either use of vitamin
and/or mineral supplements or dietary
folie acid consumption and risk of fatal
CHD.
COMMENT
Using computerized record linkage, a
total of 165 CHD deaths were identified
in the Canadian Mortality Database
among the 5056 subjects included in the
analysis. The potential for deaths hav¬
ing been missed is expected to have been
small given the quality of the personal
identifiers, the completeness of the Ca¬
nadian Mortality Database, and the ac¬
curacy of the record linkage process used
by Statistics Canada.17
This study observed a statistically sig¬
nificant inverse relationship between se¬
rum folate level and risk of fatal CHD.
A previous case-control study of early
onset coronary artery disease noted a
reduction in risk with increased serum
folate levels.4 Folate consumption is a
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Coronary Heart Disease,
Table 3.—Serum Folate Level and Risk of Fatal
1970 Through 1985, Men*
Serum Folate,
nmol/L (ng/mL)
213.6 (==6.0)
9.1-<13.6(4.0-<6.0)
6.8-0.1 (3.0-<4.0)
<6.8(<3.0)
Survey Cohort,
Nutrition Canada
Deaths, No.
Person-Years
RRt
30
7577
1.08
30
5511
1.36
95% CI
0.80-2.31
*RR indicates estimated relative risk, and CI, confidence interval. Test for trend, P=.02.
tAdjusted for age, serum cholesterol level, smoking status, diabetes status, and diastolic blood pressure.
Coronary Heart Disease, Nutrition Canada Survey Cohort,
Table 4.—Serum Folate Level and Risk of Fatal
1970 Through 1985, Women*
Serum Folate,
nmol/L
Deaths, No.
(ng/mL)
Person-Years
95% CI
RRt
(==6.0)_S)_8252_
_
9.1-<13.6(4.0-<6.0)_13_8995_L24_0.53-2.91
6.8-<9.1 (3.0-<4.0)_9_7614_1^32_0.52-3.27
==13.6
22
<6.8(<3.0)
8663
2.83
1.30-6.18
*RR indicates estimated relative risk, and CI, confidence interval. Test for trend, P=.04.
tAdjusted for age, serum cholesterol level, smoking status, diabetes status, and diastolic blood pressure.
Coronary Heart Disease Estimated Relative Risks by Age,
Table 5.—Serum Folate Level and Risk of Fatal
Nutrition Canada Survey Cohort*
Age,
Serum Folate,
nmol/L (ng/mL)
=
Deaths,
No.
13.6(26.0)
9.1-<13.6(4.0-<6.0)
(3.0-<4.0)
<6.8 (<3.0)
6.8-0.1
10
<65 y
Age,
Person-Years
RRf
10 539
1.0
12156
1.49
0.58-3.80
9747
1.58
0.60-4.17
1.81
0.69-4.71
95% CI
Deaths,
No.
265 y
95% CI
28
Person-Years RRf
3888
1.0
4416
1.02
0.61-1.70
25
1.10
0.64-1.90
1.67
1.03-2.71
3560
*RR indicates estimated relative risk, and CI, confidence interval.
tAdjusted for age, sex, serum cholesterol level, smoking status, diabetes status, and diastolic blood pressure.
determinant of homocysteine-related
carotid artery thickening.5 Arterial
thickening has been associated with both
stroke18·19 and CHD.20·21
This study is one of the first to note an
association between serum folate level
and fatal CHD. It has 2 significant ad¬
vantages over previous work. The NCS
cohort was derived from a national nu¬
tritional survey. Previous studies of fo¬
late examined white men4 and the elder¬
ly.5 The current study noted significant
associations between serum folate level
and fatal CHD in both men and women.
It did not rely on indirect measures of an
intermediate health effect, such as ca¬
rotid artery thickening.
Increased risks were not restricted to
individuals with extremely low serum fo¬
late levels, but were observed for indi¬
viduals with normal levels as well, sug¬
gesting that current definitions of
appropriate serum folate levels be reas¬
sessed. Although a significant exposureresponse relationship was observed, it is
possible that some of the observed re¬
duction in risk associated with serum fo¬
late level reflects unmeasured dietary fac¬
tors for which folate is a good marker.
However, serum levels of vitamins A, C,
and E, also markers of a good diet, did not
confound the association.
Although neither interaction was sta¬
tistically significant, the magnitude of
the protective effect of folie acid ap¬
peared to be greater for women than for
men and slightly greater for those
younger than 65 years compared with
those aged 65 years and older. Relative
risks for most CHD risk factors decline
with age.1 It is not known ifthe increased
RR among women as compared with
men was a chance finding, or whether it
reflects true differences in risk between
the sexes.
We found no clear relationship be¬
tween dietary folie acid consumption es¬
timated by 24-hour food recall and fatal
CHD. Our crude measure of dietary fo¬
lie acid consumption did not include the
contribution from vitamin supplements.
Although a 24-hour food recall question¬
naire is a valid means of determining
population folate levels,22 within-indi¬
vidual variation in diet severely limits
its usefulness in classifying individuals.
In addition, determination of dietary fo¬
late consumption was particularly prob¬
lematic in the NCS,14 and there was only
a weak correlation between estimated
dietary folate consumption and serum
folate levels. There were too few ex¬
posed people to adequately assess folie
acid supplements. No effect was noted
for individuals reporting use of vitamin
and/or mineral supplements. However,
it is not known how many of these were
actually taking supplements that con¬
tained folie acid.
The failure to note a dietary effect
should not be taken as evidence that
dietary intervention is not to be encour¬
aged. Dietary folate was inversely as¬
sociated with carotid artery stenosis in
the Framingham Study.5 Dietary folie
acid supplementation has also been as¬
sociated with a reduced risk of neural
tube defects.23 Low serum folate levels
have been reported to enhance the ef¬
fect of risk factors for cervical dysplasia,24 and low folate intake has been as¬
sociated with an increased risk of rectal
cancer.25
Folie acid fortification of staple foods
has been recommended for consideration
as a means to reduce the risk of cancer26
and neural tube defects.23 Folie acid for¬
tification of grain to prevent neural tube
defects alone has been estimated to be
cost-effective.27 Although folie acid is
generally considered safe even at high
levels of intake, high intake can mask
the clinical signs of pernicious anemia,28
and can interfere with the effectiveness
of anticonvulsants.29 Roughly one half of
US adults on a given day consume less
than the newly lowered recommended
dietary allowance for folate,30,31 and an
estimated 88%32 consume less than the
levels needed to produce low, stable ho¬
mocysteine levels.6
Efforts should be made to increase
folate consumption by a modification of
the diet to include more vegetables and
legumes. However, in spite of health
promotion efforts, not all people will in¬
crease their dietary consumption of fo¬
late and the bioavailability of folate from
food is significantly less than that from
folie acid supplements.32 The recent de¬
cision of the Food and Drug Adminis¬
tration to require folie acid fortification
of food products to prevent spina bifida
may have the additional benefit of re¬
ducing CHD risk.33
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