Context.— Epidemiological studies have established a relationship between cholesterol
and low-density lipoprotein cholesterol (LDL-C) concentrations and the risk
of ischemic heart disease (IHD), but up to half of patients with IHD may have
cholesterol levels in the normal range.
Objective.— To assess the ability to predict the risk of IHD using a cluster of
nontraditional metabolic risk factors that includes elevated fasting insulin
and apolipoprotein B levels as well as small, dense LDL particles.
Design.— Nested case-control study.
Setting.— Cases and controls were identified from the population-based cohort
of the Québec Cardiovascular Study, a prospective study conducted in
men free of IHD in 1985 and followed up for 5 years.
Participants.— Incident IHD cases were matched with controls selected from among the
sample of men who remained IHD free during follow-up. Matching variables were
age, smoking habits, body mass index, and alcohol consumption. The sample
included 85 complete pairs of nondiabetic IHD cases and controls.
Main Outcome Measures.— Ability of fasting insulin level, apolipoprotein B level, and LDL particle
diameter to predict IHD events, defined as angina, coronary insufficiency,
nonfatal myocardial infarction, and coronary death.
Results.— The risk of IHD was significantly increased in men who had elevated
fasting plasma insulin and apolipoprotein B levels and small, dense LDL particles,
compared with men who had normal levels for 2 of these 3 risk factors (odds
ratio [OR], 5.9; 95% confidence interval [CI], 2.3-15.4). Multivariate adjustment
for LDL-C, triglycerides, and high-density lipoprotein cholesterol (HDL-C)
did not attenuate the relationship between the cluster of nontraditional risk
factors and IHD (OR, 5.2; 95% CI, 1.7-15.7). On the other hand, the risk of
IHD in men having a combination of elevated LDL-C and triglyceride levels
and reduced HDL-C levels was no longer significant (OR, 1.4; 95% CI, 0.5-3.5)
after multivariate adjustment for fasting plasma insulin level, apolipoprotein
B level, and LDL particle size.
Conclusion.— Results from this prospective study suggest that the measurement of
fasting plasma insulin level, apolipoprotein B level, and LDL particle size
may provide further information on the risk of IHD compared with the information
provided by conventional lipid variables.
OVER THE LAST 30 years, several epidemiological studies have reported
a direct relationship between total plasma cholesterol and low-density lipoprotein
cholesterol (LDL-C) concentrations and the risk of coronary artery disease
(CAD),1,2 and elevated total plasma
cholesterol levels are considered by many to be the main cause of coronary
atherosclerosis. However, the ability to adequately identify individuals at
high risk for the development of CAD solely on the basis of total cholesterol
or LDL-C concentration has recently been challenged by evidence suggesting
that a considerable proportion of patients with CAD may have cholesterol levels
in the normal range (Genest et al3 reported
the proportion to be as high as 50%).4 There
are also data to suggest that a notable proportion of patients undergoing
cholesterol-lowering therapy and who achieve significant LDL-C reduction may
still develop CAD.5 These observations have
emphasized the need to find additional markers of risk that would allow a
more refined identification of individuals at high risk for CAD.
Clinical data have provided evidence that elevated plasma triglyceride
levels and reduced high-density lipoprotein cholesterol (HDL-C) concentrations
may be associated with a considerable increase in CAD risk. Although the independent
contribution of plasma triglycerides to CAD remains controversial,6 the clinical relevance of elevated triglyceride levels
should no longer be overlooked as hypertriglyceridemia may reflect additional
metabolic disturbances highly predictive of CAD risk.7
Results from the Helsinki Heart Study8 and
from the Prospective Cardiovascular Münster (PROCAM) study9
have suggested that hypertriglyceridemia should be considered an important
risk factor for CAD, particularly when combined with elevated LDL-C and reduced
HDL-C concentrations. This cluster of risk factors may represent the metabolic
condition most predictive of CAD risk.10
With the 5-year prospective data from the Québec Cardiovascular
Study, we have recently reported that elevated fasting plasma insulin levels,11 elevated apolipoprotein B concentrations,12,13 and the presence of small, dense
LDL particles14 were strongly associated with
the development of ischemic heart disease (IHD) in men, independent of established
risk factors. Plasma LDL-C, triglyceride, and HDL-C levels were also significant
correlates of IHD in the Québec Cardiovascular Study.7,12,13,15
In the current study, we investigate whether the ability to identify individuals
at high risk for the development of IHD could be improved by measuring 3 nontraditional
risk factors, namely fasting plasma insulin and apolipoprotein B levels and
LDL particle diameter, over and beyond what can be achieved using more traditional
lipid risk factors, triglyceride, LDL-C, and HDL-C levels.
Study Population and Follow-up
The Québec Cardiovascular Study cohort has been described in
detail previously.13,16 In 1973,
a random sample of 4637 men aged 35 to 64 years was recruited from 7 suburbs
of the Québec metropolitan area for an evaluation of cardiovascular
risk factors using the provincial electoral lists. Subsequent evaluations
were performed at regular intervals and data collected in 1985 were used as
the baseline characteristics for the present prospective analyses. In 1985,
2443 (61%) of the living cohort came to the lipid clinic in a fasting state
for their evaluation. Among the 1557 other potential living subjects, 150
(10%) could not be located, 302 (19%) came to the clinic in a nonfasting state,
and 1105 (71%) either refused to participate or were evaluated in a nonfasting
state at their home by project nurses. Analyses of data collected in 1973
revealed that the age distribution of the 2443 subjects in 1985 was representative
of the original cohort. At the end of follow-up (September 1, 1990), all subjects
were contacted by mail and invited to answer a short standardized questionnaire
on smoking habits, medication use, history of cardiovascular disease, and
diabetes mellitus. For those who reported such diseases and those who died,
hospital charts were reviewed. Telephone calls were made to subjects who did
not answer a second letter and if the call was unsuccessful, another call
was made to a close family member. Mortality and morbidity data were obtained
in 99% and 96%, respectively, of the subjects of the initial 1973 screening.
Evaluation of Risk Factors
Data on demographic and lifestyle variables as well as medical history
and medication were obtained in 1985 through a standardized questionnaire
administered to each subject by trained nurses and further reviewed by a physician.
Body weight and height were recorded. Resting blood pressure was measured
after a 5-minute rest in a sitting position. The mean of 2 blood pressure
measures taken 5 minutes apart was used in the analyses. Information on personal
and family history of IHD and diabetes mellitus, smoking habits, alcohol consumption,
and medication use was also obtained. Diabetes mellitus was considered in
men who self-reported the disease or who were treated with hypoglycemic agents.
Only 2% of men were using hypolipidemic drugs in 1985 (mainly clofibrate and
cholestyramine), whereas 8% and 4% of men were using β-blockers and diuretics,
respectively, on a regular basis at the 1985 screening. Data on drug use at
the time of follow-up were not available. Alcohol consumption was computed
from the type of beverage (beer, wine, or spirits) consumed in ounces per
week and then standardized as an absolute quantity (1 oz of absolute alcohol
was equivalent to 22.5 g of alcoholic beverage). Family history of IHD was
considered positive if at least 1 parent or 1 sibling had a history of IHD.
The diagnosis of a first IHD event included typical effort angina, coronary
insufficiency, nonfatal myocardial infarction, and coronary death. All myocardial
infarction cases met the criteria previously described,16
namely diagnostic electrocardiographic (ECG) changes alone or 2 of the following
criteria: typical chest pain of at least 20 minutes in duration, creatine
kinase enzyme level at least twice the upper limit of normal, or characteristic
ECG changes. Coronary insufficiency was considered if typical retrosternal
chest pain of at least 15 minutes in duration was associated with transient
ischemic ECG changes but without significant elevation in levels of creatine
kinase. Diagnoses of myocardial infarction and coronary insufficiency were
confirmed by hospital charts. All ECG tracings were read by the same cardiologist,
who was unaware of the subjects' risk profiles. The diagnosis of effort angina
was based on typical symptoms of retrosternal squeezing or pressure-type discomfort
occurring on exertion and relieved by rest and/or nitroglycerine. Criteria
for the diagnosis of coronary death included confirmation from death certificate
or autopsy report confirming the presence of coronary disease without evidence
for noncardiac disease that could explain death. Myocardial infarction was
considered fatal if death occurred within 4 weeks of the initial event or
if it was diagnosed at autopsy. Deaths related to IHD were confirmed from
the Provincial Death Registry. Informed consent was obtained to review relevant
hospital files. Autopsies were performed in about one third of deaths. The
total IHD event frequency during the 5-year follow-up period was similar in
men participating in the study (5.4%) and in nonparticipants (6.5%).
Between 1985 and 1990, 114 of the 2103 men who had no clinical evidence
of IHD at baseline had a first IHD event: 50 had a myocardial infarction,
40 had effort angina, 9 had coronary insufficiency, and 15 died of IHD-related
causes. Each case subject was matched with a control subject selected from
among the remaining 1989 men without IHD during follow-up. Subjects were matched
on the basis of age, cigarette smoking, body mass index, and weekly alcohol
intake. The mean difference within pairs was 0.6 years, 0.2 kg/m2,
and 0.2 oz/wk for age, body mass index, and alcohol intake, respectively.
The mean difference within pairs for cigarette smoking was 0.3 cigarettes
per day. Subjects who had an IHD event and who were classified as nonsmokers
were systematically matched with nonsmoking control-group subjects.
Fasting lipoprotein lipid and apolipoprotein levels were measured in
plasma in 1985 when subjects came to the clinic for evaluation. Aliquots of
fasting plasma were frozen at the time of collection and were later used for
the assessment of LDL diameter and fasting insulin concentrations. Total cholesterol
and triglyceride levels were determined on a multianalyzer (Technicon RA-500,
Bayer Corp, Tarrytown, NY) as previously described.17
High-density lipoprotein cholesterol was measured in the supernatant fraction
after precipitation of apolipoprotein B–containing lipoproteins with
heparin–manganese chloride.18 Low-density
lipoprotein cholesterol levels were estimated by the equation of Friedewald
et al.19 Subjects with triglyceride levels
higher than 4.5 mmol/L (399 mg/dL) (n=52) were excluded from the analyses.13 Plasma apolipoprotein B levels were measured by the
rocket immunoelectrophoresis method of Laurell,20
as described previously.17 Serum standards
for the apolipoprotein assay were prepared in the laboratory and calibrated
against serum samples from the Centers for Disease Control and Prevention.
The standards were lyophilized and stored at−85°C until use. The
coefficients of variation for cholesterol, HDL-C, triglyceride, and apolipoprotein
B measurements were less than 3%.
Low-density lipoprotein particle diameter was assessed using nondenaturing
2% to 16% polyacrylamide gradient gel electrophoresis of whole plasma according
to Krauss et al21 and McNamara et al,22 as described previously.23
Plasma samples were applied on gels in a final concentration of 20% sucrose
and 0.25% bromophenol blue. Following a 15-minute pre-run, electrophoresis
was performed at 200 V for 12 to 16 hours and at 400 V for 2 to 4 hours. Gels
were stained with Sudan black B according to standardized procedures and stored
in a solution of 9% acetic acid and 20% methanol until analysis using an optical
densitometric image analyzer (BioImage Visage 1101DGEL, Genomic Solutions,
Ann Arbor, Mich) coupled with a computer (SPARC Station 2 Sun, Genomic Solutions).
Low-density lipoprotein diameter was estimated by comparing the migration
distance on the gel of the predominant LDL subspecies for each individual
with the migration distance of standards of known diameters. One assay was
performed for each subject. Analyses of pooled plasma standards revealed that
the assessment of LDL diameter using this method was highly reproducible with
a coefficient of variation of less than 3% (A.T., unpublished data, 1996).
Fasting plasma insulin concentrations were measured with a commercial
double-antibody radioimmunoassay (human insulin-specific radioimmunoassay
method; Linco Research, St Louis, Mo) according to the manufacturer protocol.
This assay shows essentially no cross-reactivity with human proinsulin (<0.2%).
The coefficient of variation was below 5.5% for both low and high fasting
insulin concentrations.11
Fasting insulin levels and LDL diameter were measured in 106 and 103
case-control pairs, respectively,11,14
but data for both variables were available simultaneously in 100 controls
and 102 cases. Men who reported having diabetes mellitus or who were receiving
hypoglycemic therapy at the baseline evaluation were excluded (15 cases and
1 control). We therefore had data on 87 IHD cases and 99 controls. After excluding
all pairs for which 1 of the 2 subjects had missing data, the study sample
included 85 complete pairs of IHD cases and matched controls. Baseline characteristics
of subjects who developed IHD during the 5-year follow-up (IHD cases) were
compared with the characteristics of those who remained IHD free using paired t tests for means and χ2 tests for frequency
data. Variables with a skewed distribution were log-transformed. Correlation
analyses were performed using the Pearson and the Spearman coefficients of
correlation for parametric and nonparametric variables, respectively.
The median of the control group was used as the cutoff point to identify
men with elevated or low levels of each variable of interest (LDL-C, 3.7 mmol/L
[143 mg/dL]; triglycerides, 1.52 mmol/L [135 mg/dL]; apolipoprotein B, 1.1
g/L [110 mg/dL]; fasting insulin, 72 pmol/L [10 µU/mL]; HDL-C, 1.01
mmol/L [39 mg/dL]; LDL particle diameter, 25.82 nm). Thus, by definition,
each of these risk factors was found in 50% of the control subjects. The proportion
of cases classified as having 1 or more risk factor based on these arbitrary
cutoff points was compared with that of control subjects. The proportional
hazards regression (PHREG) procedure on SAS (SAS Institute, Cary, NC) for
conditional logistic regression analysis was used to estimate the odds ratio
(OR) for IHD associated with the presence of each risk factor, as an isolated
condition or combined with others. Odds ratios were adjusted for medication
use at baseline (β-blockers and/or diuretics), family history, and systolic
blood pressure. The potential confounding effects of using β-blockers
and diuretics were combined because they both yielded similar risk. Thus,
medication use (yes or no) and family history (presence or absence) were treated
as categoric variables whereas systolic blood pressure was treated as continuous.
Table 1 presents the clinical
characteristics of the 85 controls and IHD cases. A higher proportion of case
patients was using β-blockers and/or diuretics on a regular basis at
baseline (17.7% vs 4.7%, P=.007). However, there
was no difference between cases and controls in the use of hypolipidemic medication
at baseline. As a result of the matching procedure, the frequency of smokers
(41%) and the number of cigarettes smoked per day (25 cigarettes per day)
were essentially the same in both groups. Systolic blood pressure was also
the same in both groups. As expected, there were marked differences in several
plasma lipoprotein-lipid parameters as well as in fasting insulin levels at
baseline between IHD cases and controls. Triglycerides (18.2%), fasting insulin
(18.9%), and apolipoprotein B (15.9%) levels showed the largest case-control
differences. Mean plasma HDL-C concentrations and LDL diameter were also significantly
different between cases and controls (P=.03). It
is important to note that although being tightly matched with IHD cases on
the basis of age, body mass index, smoking, and alcohol consumption, the risk
profile of control subjects in the current study is very similar to that of
the total sample of men who remained free of IHD during follow-up13 and from which they were selected.
Prevalence of Lipoprotein and Insulin Abnormalities
Because there are currently no reference values for apolipoprotein B
and insulin levels and for LDL diameter, and in an attempt to compare the
contribution to IHD risk of variables having different scales, lipoprotein-lipid
and fasting insulin levels were dichotomized using the median (50th percentile)
of the control group.
Table 2 presents
the prevalence of each of the metabolic abnormalities in IHD cases. Based
on these prevalences, ORs for developing IHD during the 5-year follow-up were
estimated using conditional logistic regression while taking into consideration
the potential confounding effects of systolic blood pressure, medication use,
and family history of IHD. Eighty-one percent of cases had elevated fasting
insulin concentrations based on these criteria, yielding a 5.5-fold increase
in the OR for IHD (95% confidence interval [CI], 2.3-13.6, P<.001) compared with men having insulin levels below the 50th percentile
of controls. Elevated plasma triglyceride levels were also associated with
a marked increase in the risk of IHD (OR, 3.5; 95% CI, 1.6-7.4; P=.002). Elevated apolipoprotein B and LDL-C levels and small, dense
LDL particles were observed in a similar proportion of cases (69.4%, 68.2%,
and 69.4%, respectively). These 3 abnormalities were associated with a significant
2.4-fold to 2.7-fold increase in the OR for IHD. Finally, 62.4% of IHD cases
had HDL-C levels below the 50th percentile of controls. There was a 60% increase
in the risk of IHD associated with reduced HDL-C levels (OR, 1.6), which was
not significant after adjustment for confounders (95% CI, 0.85-3.0). This
analysis did not take into consideration the fact that cases with 1 abnormality
may also have had additional metabolic abnormalities in combination. Nevertheless,
results presented in Table 2 suggest
that among all variables of interest, elevated fasting plasma insulin concentrations,
irrespective of the presence or absence of other lipoprotein abnormalities,
were associated with the greatest relative increase in the risk of IHD.
Prevalence of Isolated Abnormalities
The prevalence rates of elevated plasma fasting insulin and apolipoprotein
B levels as well as of small, dense LDL in their isolated form (ie, associated
with none of the other 2 abnormalities) were low in both IHD cases and control
subjects. Isolated hyperinsulinemia was observed in only 11 (12.9%) of both
IHD cases and controls. However, when considering only subjects with elevated
fasting insulin levels (42 controls and 69 cases), 11 (15.9%) of 69 hyperinsulinemic
IHD cases did not have elevated apolipoprotein B levels or small, dense LDL
in combination compared with 11 (26.6%) of 42 controls. Only 2 (2.4%) of 85
IHD cases had isolated elevations in apolipoprotein B levels compared with
9 (10.6%) of 85 controls. Finally, the small, dense LDL phenotype in its isolated
form was found in only 5 (5.9%) of 85 IHD cases. In comparison, twice as many
controls (11 [12.9%] of 85) had small, dense LDL in isolation. These results
suggest that hyperinsulinemia, elevated apolipoprotein B levels, and small,
dense LDL particles may be observed more frequently in combination with each
other rather than as isolated conditions, and that a smaller proportion of
IHD cases may display these abnormalities in their isolated form compared
with controls. We therefore tested whether the cluster of these metabolic
risk factors may further increase the risk of IHD.
Prevalence of Nontraditional Risk Factors
Figure 1 compares the prevalence
rates of the cumulative number of abnormalities in IHD cases and controls.
To simplify data presentation, fasting plasma insulin levels, apolipoprotein
B levels, and small, dense LDL particles are referred to as nontraditional risk factors, whereas LDL-C, triglyceride, and HDL-C
levels are referred to as traditional risk factors.
As shown in Figure 1 (top), only
2 IHD cases (2.4%) had none of the 3 nontraditional metabolic risk factors,
compared with 14 controls (16.5%). One of every 5 IHD cases (n = 18, 21.2%)
had 1 of the nontraditional risk factors in its isolated form, compared with
more than a third of controls (n = 31, 36.5%). The proportion of cases that
simultaneously had elevated fasting insulin levels, elevated apolipoprotein
B levels, and small, dense LDL particles (cumulative number of risk factors,
3) was 2.6-fold greater than that of controls (45.8% vs 17.7%). Consequently,
98% of IHD cases had at least 1 of the nontraditional risk factors compared
with 83% of controls. On the other hand, 82% of controls did not have elevated
fasting plasma insulin levels, elevated apolipoprotein B levels, and small,
dense LDL simultaneously, compared with 54% of IHD cases.
Prevalence of Traditional Risk Factors
A similar analysis was performed using the traditional risk factors
(LDL-C, triglycerides, and HDL-C levels) as discriminating variables for the
determination of IHD risk (Figure 1,
bottom). Although differences in the proportion of cumulative number of traditional
risk factors between IHD cases and controls were slightly attenuated compared
with differences in the proportion of nontraditional risk factors, a similar
pattern of distribution was observed. There was a greater proportion of controls
that had relatively low LDL-C and triglyceride levels and high HDL-C levels
(number of risk factors, 0) compared with IHD cases (18.8% vs 7.1%), whereas
the proportion of IHD cases that had elevated LDL-C and triglyceride levels
and low HDL-C concentrations simultaneously (cumulative number of risk factors,
3) was 1.9-fold greater than that of controls (41.2% vs 21.2%).
Risk of Developing IHD During Follow-up
Based on the prevalence of the cumulative number of risk factors presented
in Figure 1, the crude OR for developing
IHD during the 5-year follow-up was increased 18.2-fold in subjects who had
all 3 nontraditional risk factors simultaneously compared with those who had
none of the 3 risk factors (results not shown). By comparison, the OR for
IHD in subjects with the 3 traditional risk factors simultaneously was 5.2
(not shown). Multivariate conditional logistic regression analysis was performed
to compare the ability to predict IHD using traditional and nontraditional
risk factors. The prevalence of IHD cases in subjects with no risk factor
(2 and 6 IHD cases for nontraditional and traditional risk factors, respectively)
was too small to accurately assess the risk of IHD using this group as a reference.
We have therefore performed the multivariate logistic regression analysis
by combining subjects with 0 and 1 risk factor only, and by using this group
as a reference (OR, 1). As shown in Table
3, subjects that had elevated LDL-C and triglyceride levels and
reduced HDL-C concentrations simultaneously (cumulative number of traditional
risk factors, 3) showed a 3-fold increase in the risk of IHD (model 1: OR,
3.0; 95% CI, 1.4-6.4; P=.005) compared with men having
none or only 1 of these risk factors. This increased risk was no longer significant
after multivariate adjustment for fasting insulin and apolipoprotein B levels
and LDL particle diameter (model 2: OR, 1.4; 95% CI, 0.5-3.5; P=.50).
The impact of having elevated fasting insulin and apolipoprotein B levels
and small, dense LDL particles in combination with each other on the odds
of developing IHD was more prominent. The risk of developing IHD was increased
almost 6-fold when subjects simultaneously had elevated fasting insulin and
apolipoprotein B levels and small, dense LDL particles (model 3: OR, 5.9;
95% CI, 2.3-15.4; P<.001). This increase in risk
was essentially unmodified when LDL-C, triglyceride, and HDL-C levels were
included as confounders in the multivariate logistic regression model (model
4: OR, 5.2; 95% CI, 1.7-15.7; P=.003).
An analysis was carried out to test the 2-way and 3-way interaction
terms as predictors of IHD risk. It was found that none of the 2-way or 3-way
interaction terms for continuous variables were significant. However, because
of the small sample size, the possibility of a significant interaction among
the 3 nontraditional or the 3 traditional risk factors cannot be excluded.
Univariate associations between the traditional and nontraditional risk
factors and the variables that were used to match IHD cases to controls were
investigated. Plasma triglyceride levels (r=0.15, P=.05) and HDL-C levels (r=−0.17, P=.02) showed significant associations with body mass index.
Plasma triglyceride levels also showed a significant but inverse correlation
with age (r=−0.23, P=.003)
whereas HDL-C levels were positively associated with weekly alcohol consumption
(r=0.26, P<.001). Low-density
lipoprotein particle size was also a significant correlate of age (r=0.19, P=.01) but the most significant correlation
between risk factors and matching variables was observed between plasma fasting
insulin concentrations and body mass index (r=0.40, P<.001).
Results of the present prospective study emphasize the potential of
plasma fasting insulin and apolipoprotein B levels as well as of small, dense
LDL particles as clinically relevant markers of the risk of developing IHD.
Our results suggest that this cluster of metabolic abnormalities may even
provide more information on IHD risk than the more traditional lipid risk
factors, LDL-C, triglycerides, and HDL-C. Indeed, almost 1 (45.8%) of every
2 IHD cases had elevated insulin and apolipoprotein B levels as well as small,
dense LDL particles, and this combination of metabolic risk factors resulted
in a remarkable 18-fold increase in the risk of IHD. Adjustment for the more
traditional cluster of risk factors through multivariate logistic regression
did not attenuate this relationship. These observations have consequential
clinical implications, particularly in terms of primary prevention of IHD.
They imply that identification of individuals at risk could be substantially
improved by measuring fasting plasma insulin and apolipoprotein B levels and
LDL particle diameter. It should be kept in mind that these findings do not
in any way lessen the clinical importance of assessing LDL-C, triglyceride,
and HDL-C concentrations. The current study should not be considered an attempt
to discredit the well-described and accepted relationship between the so-called
lipid triad and the risk of IHD.8-10
It was apparent that an important proportion of IHD cases was characterized
by this dyslipidemia compared with controls.
It may be argued that the paired nature of the study population may
have had the adverse effect of overmatching for the traditional risk factors,
thereby understating their true impact on a randomly selected population.
As expected, there were significant correlations between risk factors and
some of the variables used to match IHD cases and controls. Although significant,
these correlations were of very low magnitude (with shared variances lower
than 7%), with the exception of the relationship between plasma fasting insulin
levels and body mass index (with a shared variance of 16%). The paired nature
of the study is therefore very unlikely to have biased the estimation of the
contribution of the traditional risk factors to IHD risk compared with that
of the nontraditional risk factors.
We reported that a very small proportion of IHD cases had no risk factor
and that abnormalities in insulin and apolipoprotein B levels and in LDL particle
diameter were more frequently observed in combination and not in isolation
compared with controls. It is therefore apparent that the risk of developing
IHD is largely dependent on the presence of risk factors that, in most cases,
emerge as a cluster of metabolic abnormalities. In this context, arguments
have been proposed for why plasma insulin and apolipoprotein B levels and
LDL particle size may represent better markers of IHD risk than LDL-C, triglyceride,
and HDL-C levels.
Small, Dense LDL and the Risk of IHD
Plasma LDL-C levels are merely measurements of the cholesterol content
of a lipoprotein particle that has been described as being very heterogeneous
in terms of composition, size, and density. Although the cholesterol content
of LDL certainly contributes to its heterogeneity, we have failed to find
a significant association between LDL density or size and LDL-C levels.14,23 Recognition of the atherogenic potential
of small, dense LDL largely came from cross-sectional case-control studies
that reported a higher prevalence of small, dense LDL in patients with IHD
compared with healthy controls.24-26
Observations from 3 recent prospective reports provided further support for
a critical role of small, dense LDL particles in the etiology of atherosclerosis.14,27,28 The greater susceptibility
of these particles to oxidation29 and their
reduced affinity for the hepatic LDL receptor30
have been proposed as potential mechanisms for the increased atherogenic potential
of small, dense LDL.
Apolipoprotein B and the Risk of IHD
Apolipoprotein B is the protein moiety of LDL. The clinical interest
of this protein lies in the fact that it provides a relatively accurate estimate
of circulating LDL particle numbers. Total plasma apolipoprotein B concentration,
as opposed to LDL apolipoprotein B, also accounts for the number of triglyceride-rich
lipoproteins (very low-density lipoprotein and intermediate-density lipoproteins),
and recent data suggest that these 2 lipoprotein subfractions may also play
an important role in the etiology of IHD.31,32
Plasma apolipoprotein B concentration can therefore be considered a crude
marker of the number of atherogenic particles in plasma.33
Results from the Québec Cardiovascular Study suggest that plasma apolipoprotein
B concentration is a strong predictor of IHD risk, independent of traditional
risk factors.12,13 It is therefore
suggested that apolipoprotein B, as a measure of the number of atherogenic
particles in plasma, may yet provide more information than the amount of cholesterol
transported by these particles.
Insulin and the Risk of IHD
The concept of insulin resistance as a central component of a potentially
atherogenic dyslipidemic state was first introduced in 1988 when it was suggested
that a large proportion of individuals resistant to the action of insulin
was also characterized by metabolic disturbances highly predictive of an increased
IHD risk.34 Using fasting or postglucose insulin
levels as crude indices of insulin resistance, univariate analyses of large
cohorts of nondiabetic populations have shown that hyperinsulinemia in the
fasting state or following a glucose load was associated with an increased
risk of IHD.35-37
Results from multivariate analyses have, however, yielded discordant conclusions.
We11 and others38
have recently reported that elevated plasma insulin levels measured with an
antibody showing essentially no cross-reactivity with proinsulin were associated
with an increased risk of developing IHD, independent of other risk factors
such as triglyceride, HDL-C, and LDL-C levels. Nevertheless, whether plasma
insulin should or should not be considered an independent risk factor for
the development of IHD remains a matter of considerable debate. It is well
accepted, however, that elevated plasma insulin concentrations are most frequently
associated with deteriorations in other cardiovascular risk factors.39 Hyperinsulinemia and insulin resistance also appear
to have direct effects on the arterial wall and contribute to a reduced fibrinolytic
potential.40 Plasma insulin levels may therefore
provide a crude but global description of a number of additional metabolic
abnormalities that may, in turn, be associated with an increased risk of IHD,
but that may not be adequately assessed by the traditional triad of lipid
risk factors. It is important to emphasize that results of the present study
apply to nondiabetic men, particularly because patients with type 2 diabetes
mellitus were excluded from the analyses. Although inclusion of men with type
2 diabetes mellitus in the study sample essentially had no impact on the results,
whether results of the present study can be applied to other populations such
as persons with type 2 diabetes mellitus, women, or the elderly population
will have to be established more specifically in future studies.
Beyond the mechanisms underlying the atherogenicity of hyperinsulinemia,
hyperapobetalipoproteinemia, and small, dense LDL, and irrespective of whether
these mechanisms share common paths, results of the present study suggest
that the risk of IHD is increased substantially when these metabolic abnormalities
cluster. The synergistic contribution of the nontraditional cluster of risk
factors to IHD risk and the fact that almost 1 of every 2 IHD cases had these
abnormalities simultaneously reflect the multifactorial etiology of IHD. It
also emphasizes the importance of defining the risk of IHD based on more than
1 risk factor.
There are a number of critical issues that have to be considered before
any decision can be made toward the measurement of these nontraditional risk
factors on a routine basis. Among others, results of this prospective case-control
study will have to be confirmed through larger population-based studies, as
the relatively low number of IHD cases allowed only a gross assessment of
risk. The relatively large CIs associated with the estimated risk in some
of the subgroups reflect this phenomenon. Population reference values such
as those used for LDL-C, triglycerides, and HDL-C also will be needed before
critical levels of fasting insulin, apolipoprotein B levels, and LDL particle
size or density at which a person becomes at greater risk for IHD are identified.
Means to achieve effective treatment of the nontraditional risk factors is
also a critical issue that deserves a great deal of scrutiny before decisions
can be made toward use of these variables in the risk management of IHD. There
are data to suggest that LDL particle size can be modulated by changes in
plasma triglyceride levels.41 Studies have
shown that triglyceride-lowering therapy with fibric acid derivatives can
lead to a significant increase in LDL particle size.42,43
There is also a large body of evidence demonstrating that LDL particle size,
apolipoprotein B level, and insulin resistance and/or hyperinsulinemia can
be effectively altered by diet and exercise-induced weight loss.44,45
Thus, the ability to favorably modify the nontraditional risk factors by diet,
exercise, and appropriate pharmacotherapy provides further support for the
use of these risk factors in the management of IHD risk. Finally, the cost-effectiveness
of implementing and using new risk factors as a basis for screening and treatment
in primary and secondary prevention of IHD should be established. Irrespective
of these important considerations, we hope that these results will help stimulate
research aimed at identifying means that could substantially improve the early
diagnosis and treatment of individuals at risk for IHD.
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