Plasma vitamin C concentrations predict the risk of incident stroke over 10 years in 20,649 participants of the EPIC-Norfolk prospective population study

Department of Public Health and Primary Care, University of Cambridge, United Kingdom.
American Journal of Clinical Nutrition (Impact Factor: 6.77). 02/2008; 87(1):64-9.
Source: PubMed
ABSTRACT
The relation between plasma vitamin C and risk of stroke remains unclear. Although clinical trials showed no significant benefit of vitamin C supplementation in reducing stroke risk, they were not able to examine the relation between plasma vitamin C concentrations and stroke risk in a general population.
The objective was to examine the relation between baseline plasma vitamin C concentrations and risk of incident stroke in a British population.
A population-based prospective study was conducted in 20,649 men and women aged 40-79 y without prevalent stroke at baseline and participating in the European Prospective Investigation into Cancer-Norfolk prospective population study. The participants completed a health questionnaire and attended a clinic during 1993-1997 and were followed up for incident strokes through March 2005.
Over 196,713 total person-years (average follow-up: 9.5 y), 448 incident strokes occurred. In a Cox proportional hazards model, persons in the top quartiles of baseline plasma vitamin C concentrations had a 42% lower risk (relative risk: 0.58; 95% CI: 0.43, 0.78) than did those in the bottom quartile, independently of age, sex, smoking, body mass index, systolic blood pressure, cholesterol, physical activity, prevalent diabetes and myocardial infarction, social class, alcohol consumption, and any supplement use. Similar results were obtained after exclusion of persons with illnesses, users of ascorbic acid-containing supplements, and persons with a history of early strokes during the initial 2 y of follow-up.
Plasma vitamin C concentrations may serve as a biological marker of lifestyle or other factors associated with reduced stroke risk and may be useful in identifying those at high risk of stroke.

Full-text

Available from: Phyo K Myint
See corresponding editorial on page 5.
Plasma vitamin C concentrations predict risk of incident stroke over
10 y in 20 649 participants of the European Prospective
Investigation into Cancer–Norfolk prospective population study
1–3
Phyo K Myint, Robert N Luben, Ailsa A Welch, Sheila A Bingham, Nicholas J Wareham, and Kay-Tee Khaw
ABSTRACT
Background: The relation between plasma vitamin C and risk of
stroke remains unclear. Although clinical trials showed no signifi-
cant benefit of vitamin C supplementation in reducing stroke risk,
they were not able to examine the relation between plasma vitamin
C concentrations and stroke risk in a general population.
Objective: The objective was to examine the relation between base-
line plasma vitamin C concentrations and risk of incident stroke in a
British population.
Design: A population-based prospective study was conducted in
20 649 men and women aged 40 –79 y without prevalent stroke at
baseline and participating in the European Prospective Investigation
into Cancer–Norfolk prospective population study. The participants
completed a health questionnaire and attended a clinic during 1993–
1997 and were followed up for incident strokes through March 2005.
Results: Over 196 713 total person-years (average follow-up:
9.5 y), 448 incident strokes occurred. In a Cox proportional hazards
model, persons in the top quartiles of baseline plasma vitamin C
concentrations had a 42% lower risk (relative risk: 0.58; 95% CI:
0.43, 0.78) than did those in the bottom quartile, independently of
age, sex, smoking, body mass index, systolic blood pressure, cho-
lesterol, physical activity, prevalent diabetes and myocardial infarc-
tion, social class, alcohol consumption, and any supplement use.
Similar results were obtained after exclusion of persons with ill-
nesses, users of ascorbic acid– containing supplements, and persons
with a history of early strokes during the initial2yoffollow-up.
Conclusions: Plasma vitamin C concentrations may serve as a bi-
ological marker of lifestyle or other factors associated with reduced
stroke risk and may be useful in identifying those at high risk of
stroke. Am J Clin Nutr 2008;87:64 –9.
KEY WORDS Plasma vitamin C, epidemiology, fruit and veg-
etables, stroke
INTRODUCTION
The main source of vitamin C (ascorbic acid) in humans is
from the consumption of fruit, vegetables, and plant foods be-
cause they cannot synthesize ascorbic acid in the body (1). Fur-
thermore, ascorbic acid in the food is easily destroyed (eg, via
cooking in water, roasting, or grilling) (1) and has a short half-life
(30 min) in the blood (2, 3). Therefore, plasma concentrations
of vitamin C in a random blood sample are most likely to be
related to an individual’s habitual dietary pattern and method of
food preparation. For example, fresh fruit and vegetables are a
richer source of ascorbic acid than are cooked or boiled fruit and
vegetables. As a result of antioxidant activity, higher levels of
ascorbic acid are thought to be associated with reduced cardio-
vascular disease risk (4). Although randomized trials indicate
that supplementation with antioxidant vitamins, including
-carotene, vitamin E, and vitamin C, do not reduce cardiovas-
cular disease risk (5, 6), prospective studies indicate that high
fruit and vegetable intakes, of which plasma vitamin C is a good
biomarker, are associated with lower stroke risk (7-9). However,
few prospective studies have examined the relation between
plasma vitamin C concentrations and stroke (8) (10), and such
studies have been limited in their ability to account for potential
confounding factors. We explored the relation between baseline
plasma vitamin C concentrations and future stroke risk in British
participants in the European Prospective Investigation into Can-
cer (EPIC)–Norfolk.
SUBJECTS AND METHODS
Participants
Participants were drawn from the EPIC-Norfolk prospective
population study, which recruited men and women aged 40 –79
y at the study baseline during 1993–1997 from residents in Nor-
folk, United Kingdom. The detailed recruitment method and
study protocol of EPIC-Norfolk were described previously (11).
Briefly, all eligible community-dwelling adults from 35 partic-
ipating general practices were invited to participate. A total of
30 445 (40% response) persons provided written consent to
participate in the study (99.6% white British persons). The Nor-
wich Local Research Ethics Committee approved the study.
1
From the Department of Public Health and Primary Care, University of
Cambridge, United Kingdom (PKM, RNL, AAW, and K-TK); the MRC
Centre for Nutrition and Cancer, Cambridge, United Kingdom (SAB); and
the MRC Epidemiology Unit, Elsie Widdowson Laboratories, Cambridge,
United Kingdom (NJW).
2
EPIC-Norfolk is supported by research program grant funding from
Cancer Research UK and the Medical Research Council with additional
support from the Stroke Association, British Heart Foundation, Research Into
Ageing, Academy of Medical Sciences, and Wellcome Trust.
3
Address reprint requests and correspondence to PK Myint, c/o Clinical
Gerontology Unit, Level 2, F&G Block, Box-251, Addenbrooke’s Univer-
sity Hospital, Hills Road, Cambridge CB2 0QQ, United Kingdom. E-mail:
pkyawmyint@aol.com.
Received March 28, 2007.
Accepted for publication July 10, 2007.
64 Am J Clin Nutr 2008;87:64 –9. Printed in USA. © 2008 American Society for Nutrition
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Measurements
At the time of the baseline survey, participants completed a
detailed health and lifestyle questionnaire. The participants were
asked about their medical history with the question, “Has a doctor
ever told you that you have any of the following?”, in reference
to the following conditions: stroke, heart attack, diabetes, and
cancer. Smoking history was obtained by asking the following
yes or no questions: “Have you ever smoked as much as one
cigarette a day for as long as a year?” and “Do you smoke
cigarettes now?”. Participants who took any supplements or sup-
plements containing vitamin C were identified from the question,
“Have you taken any vitamins, minerals, or other food supple-
ments regularly during the past year (such as vitamin C, vitamin
D, iron, calcium, fish oils, primrose oil,
-carotene)?”, which
was followed by “name/brand and dose.”
Trained nurses examined individuals at a baseline clinic visit.
Height and weight were measured, and body mass index (BMI)
was calculated as weight (kg) divided by height squared (m).
Blood pressure (BP) was measured with an Accutorr monitor
(Datascope, Huntingdon, United Kingdom) after the participant
had been seated for 5 min. We used the mean of 2 measurements
for analysis. Nonfasting venous blood samples were taken into
plain and citrate bottles. After overnight storage in a dark box at
4 –7 °C, the sample bottles were centrifuged at 2100 ҂ g for 15
min at 4 °C. About 1 y after the start of the study, when funding
became available, extra blood samples from participants were
taken for ascorbic acid assays. Plasma vitamin C was measured
from blood collected into citrate bottles, and plasma was stabi-
lized in a standardized volume of metaphosphoric acid stored at
Ҁ70 °C. We estimated plasma vitamin C concentrations with a
fluorometric assay within 1 wk of sampling (12). The CV was
5.6% at the lower end of the range (mean: 33.2
mol/L) and 4.6%
at the upper end (mean: 102.3
mol/L). Other blood samples for
assay were stored at 4 °C and assayed at the Department of
Clinical Biochemistry (University of Cambridge, Cambridge,
United Kingdom) within 1 wk after samples were collected. We
measured serum total cholesterol, HDL cholesterol, and triacyl-
glycerol with the RA 1000 (Bayer Diagnostics, Basingstoke,
United Kingdom) and calculated LDL-cholesterol concentra-
tions with the Friedewald formula (13).
Social class was classified according to the Registrar Gener-
al’s occupation-based classification scheme (14). Social class I
consists of professionals, social class II includes managerial and
technical occupations, social class III is subdivided into non-
manual skilled workers and manual skilled workers, social class
IV consists of partly skilled workers, and social class V com-
prises unskilled manual workers (15). Social class was also re-
categorized as nonmanual (I, II, and III nonmanual) and manual
(III manual, IV, and V) social classes. Alcohol consumption was
derived from a food-frequency questionnaire (FFQ) collected at
baseline. For the “drinks” category, responses ranging from
never to 6 times/d were given for 4 types of alcoholic drinks.
An in-house computer program, CAFE, was developed for data
entry and analysis (16). A 4-level physical activity index was
derived from the validated EPIC short physical activity ques-
tionnaire designed to assess combined work and leisure activity.
Participants were categorized into inactive, moderately inactive,
moderately active, and active categories. The validity and re-
peatability of this scoring system was detailed elsewhere (17).
We further categorized these into inactive (inactive and moder-
ately inactive) and active (moderately active and active) catego-
ries.
Case ascertainment
Incident stroke cases were ascertained by using death certifi-
cate data and hospital record linkage. All participants are flagged
for death at the UK Office of National Statistics. Death certifi-
cates are coded by trained nosologists using International Clas-
sification of Disease (ICD), revisions 9 and 10. Participants are
also linked to National Health Service hospital information sys-
tems so that admissions anywhere in the United Kingdom are
reported to EPIC-Norfolk through routine annual record linkage.
Stroke death was defined as ICD-9 430 438 or ICD-10 60 69
anywhere on the death certificate. Incident stroke was defined as
stroke death or hospital discharge code ICD-9 430 438 or
ICD-10 60 69. The current study is based on follow-up through
March 2005.
Statistical analysis
We excluded participants with a history of stroke at the base-
line and who had any missing value for the variables included in
the analyses. Statistical analyses were performed by using SPSS
for WINDOWS (version 14.0; SPSS, Chicago, IL). We used Cox
proportional hazards model to determine the independent asso-
ciation between plasma vitamin C concentrations at baseline and
risk of stroke during the follow-up. To estimate the independent
contribution of vitamin C concentration to stroke risk, multivar-
iate Cox regression models were constructed for quartile of
ascorbic acid concentration with control for 1) age and sex; 2)
age, sex, and smoking; 3) age, sex, smoking, BMI, systolic BP
(every 10-mm Hg increase), cholesterol concentration, physical
activity, history of myocardial infarction, and diabetes mellitus;
4) the preceding factors plus social class and alcohol consump-
tion; and 5) the preceding factors plus control for any supplement
use. Analyses were repeated (as per final model) after additional
exclusion of 1) those with prevalent myocardial infarction and
cancer, 2) users of vitamin C– containing supplements, 3) those
with a history of incident stroke occurring during the initial2yof
follow-up, and 4) additionally adjusted for average fruit and
vegetable consumption.
We further examined the relative risk of stroke stratified by 1)
sex (men and women), 2) age (70 and 70 y), 3) smoking
status (never, former, and current smokers) 4) BMI (27 and
27), 5) systolic BP (150 and 150 mm Hg), 6) cholesterol
concentration (6 and 6 mmol/L), 7) social class (manual and
nonmanual), 8) physical activity (inactive and active), and 9)
supplement use (users and nonusers) for every 20-
mol/L in-
crease (1 SD) in plasma vitamin C concentration.
RESULTS
A total of 20 649 participants were included in the current
study after exclusion of those who reported a stroke at the base-
line survey (n ҃ 455) and those with missing data for any of the
variables included in the main model (n ҃ 9341). There were a
total of 448 incident strokes over 196 713 total person-years of
follow-up (average: 9.5 y). Of these, 147 (33%) were fatal (main
cause of death).
PLASMA VITAMIN C CONCENTRATIONS AND RISK OF STROKE 65
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The distribution of sample characteristics by quartiles of
plasma vitamin C concentration by sex-combined and then sex-
specific analyses are shown in Table 1. Quartile 1 represents the
lowest quartile, whereas quartile 4 represents the highest plasma
vitamin C concentration. Men and women who were in the top
quartile category were younger and had a lower BMI, systolic
BP, and cholesterol concentration. Participants in the highest
quartile were less likely to be smokers, were more likely to be in
nonmanual occupations, were physically more active, were more
likely to take supplements, consumed more fruit and vegetables,
and had a lower prevalence of myocardial infarction and diabetes
than did those in the lower quartile categories. Of those variables
included in the analyses, vitamin C ҂ sex interaction terms were
significant only for BMI and vitamin C– containing supplement
use (P 0.0001 for both).
The relative risks (RRs) and corresponding 95% CIs for inci-
dent stroke are shown in Table 2. With increasing adjustments
for covariates, RR estimates were slightly attenuated. In model E,
participants who were in the top quartile had a 42% lower RR
(0.58; 95% CI: 0.43, 0.78) than did those who were in the bottom
quartile of plasma vitamin C, independently of age, sex, smok-
ing, BMI, systolic BP, cholesterol, physical activity, prevalent
diabetes, myocardial infarction, social class, alcohol consump-
tion, and any supplement use. The results were similar after the
exclusion of participants with prevalent myocardial infarction
and cancer (model F), those who used vitamin C– containing
supplements (model G), and after additional adjustment for fruit
and vegetable consumption (model H) and after exclusion of
incident strokes occurring within the first2yoffollow-up (model
I). There appeared to be an inverse dose-response linear relation
TABLE 1
Distribution of sex-combined and selected sex-specific sample characteristics by plasma vitamin C quartiles in 20 649 men and women of the European
Prospective Investigation into Cancer (EPIC)–Norfolk cohort at baseline (1993–1997)
1
Plasma vitamin C concentration quartile
P for trend1( 41
mol/L) 2 (41–53
mol/L) 3 (54 65
mol/L) 4 (66
mol/L)
No. of subjects 5298 5184 4824 5343
Men 3403 2767 1893 1386
Women 1895 2417 2931 3957
Age (y) 59.5 9.4
2
58.4 9.4 57.8 9.0 58.0 9.1 0.0001
Height (cm) 169.0 9.1 168.1 9.2 166.4 8.9 164.8 8.4 0.0001
BMI (kg/m
2
) 26.9 4.0 26.7 3.8 26.1 3.7 25.4 3.5 0.0001
Men 26.8 3.4 26.7 3.1 26.2 3.0 25.6 2.9 0.0001
Women 27.1 4.9 26.7 4.4 26.0 4.1 25.3 3.7 0.0001
Systolic blood pressure (mm Hg) 138 18 136 18 134 18 132 18 0.0001
Cholesterol concentration (mmol/L) 6.2 1.2 6.2 1.2 6.2 1.2 6.2 1.2 0.095
Daily alcohol intake (g) 8.9 14.4 9.0 13.1 8.3 11.9 8.6 12.1 0.037
Vitamin C concentration (
mol/L) 28.2 9.7 48.5 3.7 59.8 3.1 78.1 13.1 0.0001
Daily fruit and vegetable intake (mg) 354 203 444 221 492 251 527 276 0.0001
Smoking status [n (%)] 0.0001
3
Current smoker 1084 [20.5] 490 [9.5] 379 [7.9] 371 [6.9]
Former smoker 2379 [44.9] 2312 [44.6] 1955 [40.5] 2087 [39.1]
Never smoker 1835 [34.6] 2382 [45.9] 2490 [51.6] 2885 [54.0]
Occupational social class [n (%)] 0.0001
3
I 284 [5.4] 351 [6.8] 370 [7.7] 452 [8.5]
II 1658 [31.3] 1861 [35.9] 1890 [39.2] 2224 [41.6]
III, nonmanual 805 [15.2] 880 [17.0] 844 [17.5] 901 [16.9]
III, manual 1461 [27.6] 1256 [24.2] 991 [20.5] 1030 [19.3]
IV 838 [15.8] 672 [13.0] 583 [12.1] 614 [11.5]
V 252 [4.8] 164 [3.2] 146 [3.0] 122 [2.3]
Physical activity level [n (%)] 0.0001
3
Inactive 1803 [34] 1453 [28.0] 1198 [24.8] 1241 [23.2]
Moderately inactive 1423 [26.9] 1522 [29.4] 1479 [30.7] 1648 [30.8]
Moderately active 1142 [21.6] 1194 [23.0] 1158 [24.0] 1338 [25.0]
Active 930 [17.6] 1015 [19.6] 989 [20.5] 1116 [20.9]
Prevalent myocardial infarction [n (%)] 253 [4.8] 152 [2.9] 121 [2.5] 82 [1.5] 0.0001
Prevalent diabetes [n (%)] 177 [3.3] 129 [2.5] 80 [1.7] 51 [1.0] 0.0001
Any supplement user [n (%)] 1685 [31.8] 2136 [41.2] 2325 [48.2] 3222 [60.3] 0.0001
Vitamin C containing–supplement user
[n (%)]
101 [1.9] 216 [4.2] 262 [5.4] 559 [10.5] 0.0001
Men 49 [1.4] 104 [3.8] 99 [5.2] 161 [11.6] 0.0001
Women 52 [2.7] 112 [4.6] 163 [5.6] 398 [10.1] 0.0001
1
A general linear model was used for continuous variables, and a chi-square test was used for categorical variables. Sex-specific data are reported only
for those variables with a significant interaction between vitamin C and sex.
2
x SD (all such values).
3
Overall P value.
66 MYINT ET AL
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between baseline plasma vitamin C concentration and stroke risk
across the sample population.
With a Cox-proportional hazards model, RRs for every 20-
mol/L increase in plasma vitamin C concentration were esti-
mated in stratified analyses categorized by 1) sex (male and.
female), 2) age (70 y and 70 y), 3) smoking status (never,
former, and current smokers), 4) BMI (27 and 27), 5) systolic
BP (150 and 150 mm Hg), 6) cholesterol concentration (6
and 6 mmol/L), 7) occupational social class (manual and non-
manual), 8) physical activity (active and inactive), and 9) sup-
plement use (user and nonuser). The covariates adjusted in these
models (except in the model in which the variable is of interest)
were age, sex, smoking status, BMI, 10-mm Hg increase in sys-
tolic BP, cholesterol concentration, physical activity, social
class, diabetes, myocardial infarction, alcohol consumption, and
any supplement use (data not shown). An increasing plasma
vitamin C concentration was associated with a reduced stroke
risk in all stratified analyses. The effects appeared to be some-
what attenuated in men, within manual social class, and among
participants with a cholesterol concentration 6 mmol/L. We
reported the main effect only because the interaction terms for
each of the subgroups were not significant. There was an overall
17% reduction in incident stroke (RR: 0.83; 95% CI: 0.75, 0.92)
for every 20-
mol/L increase in plasma vitamin C concentra-
tion.
DISCUSSION
Consistent with previous population-based studies (8–10, 18,
19), we found a significant inverse relation between plasma vi-
tamin C concentrations and subsequent stroke risk in a general
population who had no history of stroke at baseline. This result
was independent of known biological, social, and lifestyle risk
factors for stroke: age, sex, smoking, BMI, systolic BP, choles-
terol concentration, physical activity, prevalent diabetes and
myocardial infarction, social class, alcohol consumption, and
any supplement use.
Nearly 10 y ago, Simon et al (20) showed a cross-sectional
relation between serum ascorbic acid concentration and preva-
lence of stroke in 6624 US men and women enrolled in the second
National Health and Nutrition Examination Survey. Although
previous population-based prospective studies (8 –10) con-
firmed a relation between plasma vitamin C concentration and
incident stroke risk, these studies have generally been based on
small sample sizes with a lower number of incident strokes
(250) and often were not able to examine many covariates. The
current study is a large prospective population-based study
(20 000 participants) that was able to take into account biolog-
ical (age, BMI, BP, and cholesterol), social (occupational social
class), and lifestyle behavior (smoking, alcohol consumption,
physical activity level, and supplement use) factors that are as-
sociated with risk of stroke and had the additional ability to take
into account prevalent illnesses at baseline, including myocardial
infarction, diabetes, and cancer.
In an observational study, confounding and reverse causality
issues require attention. We addressed these issues in this study
in several ways. First, we adjusted for possible confounders that
potentially relate to both ascorbic acid concentration and known
stroke risk factors (21). Second, we examined the multivariate-
adjusted relation after excluding those with prevalent illness,
those who took supplements, and those with a history of stroke
within the first2yoffollow-up. Third, we performed analyses
stratifying by age, sex, BMI, systolic BP, cholesterol concentra-
tion, occupational social class, physical activity level, and sup-
plement use; the results did not differ significantly between the
various subgroups.
Naturally, our study had limitations. Because of the require-
ment of subjects to provide detailed health and lifestyle infor-
mation and to be able to undergo health checks, the initial re-
sponse rate was modest (40%). This may have introduced a
healthy responder bias. Nevertheless, the baseline characteristics
of the study population were similar to those of other UK pop-
ulation samples, except for a slightly lower prevalence of smok-
ers (11). Moreover, the truncation of distribution because of
TABLE 2
Relative risks (and 95% CIs) for risk of stroke by quartile of plasma vitamin C concentration at baseline in the European Prospective Investigation into
Cancer (EPIC)–Norfolk population (1993–1977 to 2005)
1
No. of
events
Plasma vitamin C quartile
P1( 41
mol/L) 2 (41–53
mol/L) 3 (54 65
mol/L) 4 ( 66
mol/L)
Model A 448 1.00 0.76 (0.61, 0.96) 0.57 (0.43, 0.75) 0.49 (0.37, 0.64) 0.0001
Model B 448 1.00 0.80 (0.63, 1.01) 0.60 (0.45, 0.79) 0.51 (0.39, 0.68) 0.0001
Model C 448 1.00 0.83 (0.66, 1.05) 0.63 (0.48, 0.83) 0.57 (0.43, 0.76) 0.0001
Model D 448 1.00 0.84 (0.67, 1.07) 0.64 (0.48, 0.84) 0.58 (0.44, 0.78) 0.001
Model E 448 1.00 0.84 (0.66, 1.07) 0.64 (0.48, 0.84) 0.58 (0.43, 0.78) 0.001
Model F 381 1.00 0.91 (0.70, 1.17) 0.67 (0.50, 0.91) 0.61 (0.45, 0.84) 0.006
Model G 428 1.00 0.80 (0.63, 1.02) 0.62 (0.46, 0.82) 0.58 (0.43, 0.78) 0.001
Model H 448 1.00 0.83 (0.65, 1.05) 0.67 (0.47, 0.83) 0.57 (0.42, 0.76) 0.0001
Model I 427 1.00 0.91 (0.72, 1.16) 0.69 (0.52, 0.92) 0.60 (0.44, 0.81) 0.003
1
A Cox-proportional hazards model was used. Model A was adjusted for age and sex. Model B was adjusted for age, sex, and smoking status. Model C
was adjusted for age, sex, smoking status, BMI, systolic blood pressure (by 10-mm Hg increase), cholesterol, physical activity, and prevalent myocardial
infarction and diabetes. Model D was adjusted as in model C and for social class and alcohol consumption. Model E was adjusted as in model D and for any
supplement use. Model F was adjusted as in model E after exclusion of prevalent myocardial infarction and cancer. Model G was adjusted as in model E after
exclusion of vitamin C supplement users. Model H was adjusted as in model E and for fruit and vegetable consumption. Model I was adjusted as in model E
after exclusion of strokes occurring within2yoffollow-up.
PLASMA VITAMIN C CONCENTRATIONS AND RISK OF STROKE 67
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healthy responders likely attenuated the associations, but this
should not have affected the relation between vitamin C and
stroke observed within the study participants; if anything, trun-
cation of the distribution is likely to reduce the power of any
associations. We used death certification and a hospital record
linkage system using ICD coding to identify stroke cases. Al-
though follow-up with the use of these methods is virtually com-
plete, this approach may underestimate incident nonfatal stroke
cases that are not admitted to the hospital. The use of self-
reported stroke to exclude prevalent cases may have missed some
prevalent strokes. We were not able to separately examine stroke
subtypes. Nevertheless, the primary focus of the study was to
assess the risk prediction of clinical stroke event severe enough
to lead to hospitalization or death regardless of stroke subtype. In
any case, the misclassification of strokes was likely to only at-
tenuate any associations.
Only single measurements of plasma vitamin C and other
covariates, such as cholesterol and BP, were made at baseline.
These measures as well as lifestyle behaviors, which may affect
vitamin C concentration, may have changed over the follow-up
period. Moreover, the blood sample taken was a nonfasting sam-
ple and was therefore less standardized for some of the variables
(eg, cholesterol concentration) than was a fasting blood sample.
Nevertheless, random measurement error was likely only to at-
tenuate any relations observed between plasma vitamin C and
stroke.
Plasma vitamin C concentration was a good biomarker of plant
food, namely fruit and vegetable intakes, in our cohort. The
20-
mol or 1-SD increase in plasma vitamin C concentration is
associated with approximately one additional serving of fruit and
vegetables daily (22). This agrees with more recently published
literature on the relation between higher dietary fruit and vege-
table intake and reduced stroke risk (23, 24). However, observa-
tional studies are not all consistent (25, 26), and antioxidant
supplementation including vitamin C did not produce a substan-
tial benefit in clinical trial settings in high risk individuals (6, 27).
The recent Women’s Health Initiative also reported no reduction
in cardiovascular disease in the group allocated to a low-fat and
higher fruit and vegetable target diet (28).
There are some plausible explanations why the discrepancy
exists between cohort studies and trials. Many of the supplemen-
tation trials, such as the Heart Protection Study and the Finnish
Alpha-Tocopherol Beta-Carotene Cancer Prevention Study,
were conducted in high-risk or highly selected rather than general
populations (27, 29, 30). Additionally, combinations of antioxi-
dants, some of them in pharmacologic doses, such as vitamin E,
may have unpredicted biological effects.
Moreover, the lack of benefit of vitamin C in clinical trials
could be explained by the relation between vitamin C dose and
plasma concentration. At doses 100 mg/d, there is a large
change in plasma concentration for small changes in dose. Above
100 mg/d, there is little change in plasma concentration despite
large changes in dose. If the control group (ie, lowest quartile)
consumed 100 mg/d, then further increases in dose would be
predicted to cause little change in concentration. The outcome,
therefore, may not be affected at higher doses. This problem was
first pointed out by Levine et al in 1999 (31).
The association appeared to be independent of the most plau-
sible confounders (32), such as smoking, physical activity, and
social class, as we previously highlighted (22, 33). The mean
height of men and women by quartile of vitamin C concentrations
did not show material differences (Table 2); hence, life-course
factors are unlikely to be significant confounders. An intriguing
possibility is that the plasma vitamin C concentration is a good
marker of a wider range of health behaviors, such as fruit and
vegetable consumption, that may be protective against stroke.
Even then, it appears that the relation was independent of fruit
and vegetable consumption. It is also possible that the relation
could reflect measurement error related to the dietary instru-
ments. It is also plausible that vitamin C may biochemically
affect stroke risk. Given the current evidence, it unlikely that
long-term randomized controlled trials of isolated vitamin C
supplementation and cardiovascular disease endpoints will be
conducted. Nevertheless, the magnitude of the association be-
tween plasma vitamin C and subsequent stroke is substantial and
independent of known major risk factors for stroke.
We believe that these findings are of interest for several rea-
sons. First, the strong inverse association between plasma vita-
min C and stroke suggests that plasma vitamin C is likely to be a
good biomarker of whatever causal factors affect stroke risk,
most plausibly the dietary intake of plant foods. However, iden-
tification of the relevant factors may lead to better stroke pre-
vention. Second, irrespective of any causal associations, plasma
vitamin C appears to be a good predictive risk indicator of stroke,
independent of known risk factors such as age, BP, smoking,
lipids, diabetes, and BMI. Given that about half of the risk of
stroke is unexplained by conventional cardiovascular disease
risk factors (34) and that the predictive validity of traditional
cardiovascular disease risk factors appears to diminish with age
(35, 36), risk markers that may help to identify those persons at
greatest risk of stroke for targeted preventive interventions with
established therapies, such as BP reduction, may be of interest.
We thank the participants and general practitioners who took part in the
study, the staff of EPIC-Norfolk, and our funders.
The authors’ responsibilities were as follows—K-TK, SAB, and NJW:
principal investigators in the EPIC-Norfolk population study; RNL: respon-
sible for data management, computing, and data linkages; PKM: conducted
the analysis; and AAW: involved in the study design and in the writing of the
manuscript. All coauthors contributed to the writing of this article. KTK was
the guarantor. No conflicts of interest were reported by any of the authors.
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PLASMA VITAMIN C CONCENTRATIONS AND RISK OF STROKE 69
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  • Source
    • "study, serum AA was inversely related with the prevelance of coronary heart disease and stroke [48]. In several studies, including the Epic-Norfolk prospective population study, low plasma concentration of vitamin C was associated with increased stroke risk [49]. In cross-sectional studies, blood pressure in middle aged and elderly [50] as well as in young adults [51] was found to be inversely associated with blood AA. "
    [Show abstract] [Hide abstract] ABSTRACT: The role of vitamin C at the physiological and cellular levels is indisputable. In line with this, blood level of vitamin C is inversely related to disease parameters such as risk of cancer, cardiovascular disease and mortality in prospective cohort and correlational studies. At the same time, adequately powered clinical intervention studies consistently provide no evidence for a beneficial effect of supplementing vitamin C. Here we provide a framework to resolve this apparent conflict. Besides providing an overview of the widely-known facts regarding vitamin C, we review evidence that are of potential relevance but are seldomly mentioned in the context of vitamin C. We invoke the glucose-ascorbate antagonism (GAA) theory which predicts that as a consequence of their molecular similarity glucose hinders the entry of vitamin C into cells. Integrating data coming from research at the cellular level, those from clinical, anthropological and dietary studies, in the present hypothesis paper we propose an evolutionary framework which may synthesize currently available data in the relation of vitamin C and disease. We put forward that instead of taking vitamin C as a supplement, an evolutionary adapted human diet based on meat, fat and offal would provide enough vitamin C to cover physiological needs and to ward off diseases associated with vitamin C deficiency.
    Full-text · Article · Mar 2016
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    • "In a recent study however, it was found that 4 weeks of pre-treatment of stroke-prone spontaneously hypertensive rats with vitamins C and E lowered levels of lipid peroxidation and significantly lowered infarct volume following MCAO [175]. In large human observational studies, it was determined that an increase in vitamin C plasma levels correlated with a lowered incidence of stroke176177178. However, in randomised control trials, patients receiving antioxidant vitamin supplementation displayed no difference in incidence of stroke compared to those receiving placebo179180181. "
    [Show abstract] [Hide abstract] ABSTRACT: Transient or permanent interruption of cerebral blood flow by occlusion of a cerebral artery gives rise to an ischaemic stroke leading to irreversible damage or dysfunction to the cells within the affected tissue along with permanent or reversible neurological deficit. Extensive research has identified excitotoxicity, oxidative stress, inflammation and cell death as key contributory pathways underlying lesion progression. The cornerstone of treatment for acute ischaemic stroke remains reperfusion therapy with recombinant tissue plasminogen activator (rt-PA). The downstream sequelae of events resulting from spontaneous or pharmacological reperfusion lead to an imbalance in the production of harmful reactive oxygen species (ROS) over endogenous anti-oxidant protection strategies. As such, anti-oxidant therapy has long been investigated as a means to reduce the extent of injury resulting from ischaemic stroke with varying degrees of success. Here we discuss the production and source of these ROS and the various strategies employed to modulate levels. These strategies broadly attempt to inhibit ROS production or increase scavenging or degradation of ROS. While early clinical studies have failed to translate success from bench to bedside, the combination of anti-oxidants with existing thrombolytics or novel neuroprotectants may represent an avenue worthy of clinical investigation. Clearly, there is a pressing need to identify new therapeutic alternatives for the vast majority of patients who are not eligible to receive rt-PA for this debilitating and devastating disease.
    Preview · Article · Jan 2016
  • Source
    • "30% lower risk of death from stroke compared to subjects with lower vitamin C levels. Myint et al., 2008 20,649 men and women 78.1 µmol/L "
    [Show abstract] [Hide abstract] ABSTRACT: The recommended dietary allowance (RDA) of vitamin C has traditionally been based on the prevention of the vitamin C deficiency disease, scurvy. While higher intakes of vitamin C may exert additional health benefits, the limited Phase III randomized placebo-controlled trials (RCTs) of vitamin C supplementation have not found consistent benefit with respect to chronic disease prevention. To date, this has precluded upward adjustments of the current RDA. Here we argue that Phase III RCTs-designed principally to test the safety and efficacy of pharmaceutical drugs-are ill suited to assess the health benefits of essential nutrients; and the currently available scientific evidence is sufficient to determine the optimum intake of vitamin C in humans. This evidence establishes biological plausibility and mechanisms of action for vitamin C in the primary prevention of coronary heart disease, stroke, and cancer; and is buttressed by consistent data from prospective cohort studies based on blood analysis or dietary intake and well-designed Phase II RCTs. These RCTs show that vitamin C supplementation lowers hypertension, endothelial dysfunction, chronic inflammation, and Helicobacter pylori infection, which are independent risk factors of cardiovascular diseases and certain cancers. Furthermore, vitamin C acts as a biological antioxidant that can lower elevated levels of oxidative stress, which also may contribute to chronic disease prevention. Based on the combined evidence from human metabolic, pharmacokinetic, and observational studies and Phase II RCTs, we conclude that 200 mg per day is the optimum dietary intake of vitamin C for the majority of the adult population to maximize the vitamin's potential health benefits with the least risk of inadequacy or adverse health effects.
    Full-text · Article · Sep 2012 · Critical reviews in food science and nutrition
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