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Department of Internal Medicine, National Taiwan University Hospital, 7 Chung Shan South Rd., Taipei, Taiwan 10016.
a Author for correspondence. Fax 886-2-2395-9911; e-mail ytlee{at}ha.mc.ntu.edu.tw
| Abstract |
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Methods: We studied 2165 subjects, ages >35, from a community cohort.
Results: The distributions of fasting insulin were skewed to the right, with higher concentrations in women than in men. As age increased, insulin increased in women, but decreased in men. As fasting insulin concentrations increased, postloading insulin, glucose, blood pressure, body mass index, waist-to-hip ratio, total cholesterol, triglycerides, LDL-cholesterol, apoprotein B, plasminogen activator inhibitor 1, tissue plasminogen activator, and fibrinogen increased, but lipoprotein(a), HDL-cholesterol, and apoprotein A1 decreased. Multiple logistic regression showed that obesity, high LDL-cholesterol, and low HDL-cholesterol were significant predictors of hyperinsulinemic status.
Conclusion: The study subjects with insulin resistance syndrome and related risk factors may be at risk for atherosclerosis, thrombosis, and other coronary heart diseases.© 1999 American Association for Clinical Chemistry
| Introduction |
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Some studies have addressed racial differences in the clustering of insulin resistance syndrome, including hypertension, obesity, dyslipidemia, and hyperinsulinemia (8). Environmental studies showed marked differences in CHD mortality rates among countries and relative impacts on atherosclerosis among populations. Racial discrepancies have also been reported for diabetes mellitus and insulin concentrations (9). Pima Americans, aboriginal Australians, and South Asians have relatively higher prevalences of diabetes than Caucasians (10)(11)(12). Common to most of these groups are a recent trend toward urbanization, a decrease in physical activity, and the development of obesity.
Populations with high incidence rates for CHD are also at high risk for diabetes (11), which may be related to insulin resistance. One population-based study has demonstrated that fasting insulin concentrations were lower in the Japanese than in Caucasians(9). The distribution patterns of atherosclerotic risk factors between these two ethnic groups were also different.
For the Chinese population, the distribution of fasting insulin in the general population has not yet been addressed. Moreover, the relationships between insulin concentration, blood pressure, lipid concentrations, and coagulation factors in Chinese remain to be explored. A recent study in Mauritius found that both fasting and 2-h postload insulin concentrations in Chinese subjects were lower than that in Hindu and Muslim Indian subjects (13).
Community-based studies use many indicators of insulin resistance syndrome. Observations based on quartiles or tertiles of fasting insulin concentrations(2)(3)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25), insulin resistance indexed by calculation of fasting and postloading glucose and insulin concentrations (18)(26), or insulin sensitivity index profiles (18)(26) have revealed underlying pathophysiological mechanisms and have wide applications in population studies. Associations between these indicators of insulin resistance and atherosclerotic risk factors can be viewed as different aspects of insulin resistance syndrome. The Chin-Shan Community Cardiovascular Cohort (CCCC) study is a prospective community-based investigation of cardiovascular disease risk factors in Chinese adults in Taiwan. Here, we focus on the distribution of fasting insulin concentrations and the linkage between insulin concentration and various atherosclerotic risk factors.
| Materials and Methods |
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data collection and clinic exams
A clinic was set up at the Chin-Shan Community Health Center by
the study team, which consisted of 20 senior medical students, 2
assistant nurses, and 10 cardiologists and local practitioners. Trained
medical students canvassed door-to-door with the assistance of
community leaders to extend invitations for the baseline survey.
Information collected included sociodemographic characteristics,
lifestyle, personal and family histories of diseases, and the history
of hospitalization. With the consent of participants, a team of
physicians and medical students conducted physical examinations and
laboratory tests on those participants invited to the clinic. Blood
pressures were measured with subjects in the sitting position after
resting for 10 min. Body weight was measured using a calibrated
balance. Body mass index (BMI) was calculated as weight (kilograms)
divided by height (meters) squared. The circumferences of the smallest
part of waist and the thickest part of the hip in the standing position
were measured, and the waist-to-hip ratio (WHR) was calculated.
Electrocardiograms (12-lead) were recorded concurrently. Specimens for
blood analysis were also collected in the morning, before 1200.
blood sampling and analytic methods
All subjects with a minimum fasting period of 12 h underwent
an oral glucose tolerance test with 75 g of glucose loading in
accordance with the World Health Organization standard. Tests were
performed in the morning, before 1000. Immediately before the
glucose loading, serum samples for determinations of blood lipids,
plasma glucose, and serum insulin were obtained. A second serum sample
was taken 2 h later. The serum samples were refrigerated
immediately and transported to the National Taiwan University Hospital
within 6 h. Serum samples were then stored at -70 °C for batch
assays of total cholesterol, triglycerides, LDL-cholesterol (LDL-C),
HDL-cholesterol (HDL-C), apoprotein A1 (apo A1), apoprotein B
(apo B), and lipoprotein(a) [Lp(a)], as described previously(28). Standard enzymatic methods were used to determine
serum cholesterol and triglycerides (methods 14354 and 14366,
respectively; Merck). HDL-C was measured in the supernatant
after precipitation with magnesium chloride-phosphotungstate reagents
(method 14993; Merck). The LDL-C content was measured as "total
cholesterol minus cholesterol in the supernatant" by the
precipitation method (29) because the HDL-C was precipitated
by the use of heparin/citrate reagent (method 14992; Merck). apo A1 and
apo B concentrations were measured by turbidimetric immunoassay using
commercial kits (Sigma). Lp(a) was determined by enzyme-linked
immunosorbent assay (Organon) regardless of isoform.
Blood samples for glucose analysis were drawn into glass test tubes,
each containing 80 mol/L fluoride/oxalate reagent. After centrifugation
at 1500g for 10 min at 4 °C, glucose
concentrations were measured on supernatants by enzymatic assay
(commercial kit 3389; Merck) in a Eppendorf 5060 automated analyzer.
The plasma insulin concentration was determined using the ELISA method
in which a reagent kit supplied by the Dako was used. The plate
antibody binds
chains somewhere near the intrachain disulfide. The
conjugate antibody binds very close to the cleavage site in proinsulin,
and its epitope is partially composed of a lysine residue at position
30 on the ß chain. Thus, the assay will not measure intact proinsulin
and provides specificity for insulin (30). The lower limit
of detection for insulin was 0.158 pmol/L, with a CV of
5.0%.
The measurements of coagulation profiles were made in the following manner: Tissue plasminogen activator (tPA) was analyzed using an enzyme immunoassay (Asserachrom tPA; Diagnostica Stago), and plasminogen activator inhibitor (PAI-1) and fibrinogen were measured using commercial kits. PAI-1 was measured using an enzyme immunoassay (Asserachrom PAI-1; Diagnostica Stago), whereas fibrinogen was measured using a clotting assay (STA-Fibrinogen; Diagnostica Stago).
diagnostic criteria
Subjects were defined as hypertensive according to the Fifth Joint
National Committee criteria (31): a systolic blood pressure
of 140 mmHg and higher and/or diastolic blood pressure of 90 mmHg and
higher, or receiving regular antihypertensive therapy. The presence of
CHD was defined on the basis of a finding of abnormal Q or QS patterns
on an electrocardiogram or a clinical history of myocardial infarction
or angina pectoris with admission documents. A history of stroke was
defined on the basis of a history of hemiparesis or hemiplegia, and was
confirmed by one neurologist. Subjects were defined as having diabetes
mellitus if their fasting plasma glucose concentrations were >7.77
mmol/L or they were receiving oral hypoglycemic agents or insulin
injections. A BMI >27 kg/m2 and a WHR >0.94
were considered abnormal because these were 90th percentile values for
the study population. The quartiles for fasting insulin, specified by
sex in the study population, were established with the following
cutoffs:
16.50 pmol/L, 16.5029.99 pmol/L, 29.9952.45 pmol/L, and
52.45 pmol/L for men; and
27.27 pmol/L, 27.2742.33 pmol/L,
42.3367.84 pmol/L, and
67.84 pmol/L for women. The top
quartile concentration was defined as hyperinsulinemia. Homeostasis
model assessment (HOMA) was used to identify insulin resistance
syndrome, using the formula: [fasting insulin (mIU/L) x
fasting glucose (mg/dL) x 0.05551]/22.5 (26).
statistical analysis
We first compared the distribution of fasting serum insulin by age
and sex. The distribution of the insulin concentration appeared
highly skewed to the right and justified the use of geometric means.
Sex- and age-specific quartiles for fasting insulin were defined. The
interaction between fasting insulin and the atherosclerotic risk factor
was examined for each hyperinsulinemia quartile. All subjects
were divided according to the sex-specific quartiles for insulin.
Because of the skewed distribution of the insulin index indicators in
each gender, a Spearman rank correlation analysis was performed to
measure the linear relationship between covariates and three indicators
of insulin resistance syndrome. A multiple logistic regression model
was constructed to estimate the odds ratios and 95% confidence
intervals of covariates to predict the occurrence of hyperinsulinemia.
When the glucose and insulin concentrations for fasting subjects were
plotted, the localized regression method of Cleveland and Delvin(32) was adapted using S-plus 3.3 (33). Data
analysis was performed using the SAS release 6.11 software(34).
| Results |
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The geometric means and 95% confidence intervals for variables with
right-skewed distributions, displayed by sex, are listed in Table 2
. In coagulation profiles, women had higher fibrinogen and PAI-1
concentrations, whereas men had higher tPA concentrations. In the
comparisons of indicators of insulin resistance syndrome, women had
higher fasting and postloading insulin concentrations, and higher
insulin resistance by HOMA.
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The distribution of fasting serum insulin was highly skewed to the
right and was 0.14279.83 pmol/L in men and 0.36866.24 pmol/L in
women; women had higher concentrations than men (Fig. 1
). In men, serum fasting insulin decreased as age increased,
whereas for women, serum fasting insulin increased as age increased
(Fig. 2
).
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The the relationship between serum insulin and fasting glucose is
presented using scatter plots in Fig. 3
. In the fasting state, insulin concentrations increased
progressively as glucose concentrations increased in both genders, and
then plateaued for women, but increased slightly for men.
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The mean values of clinical and metabolic variables, according to
quartile concentrations of fasting insulin for the study population,
are shown in Table 3
. As fasting insulin concentrations increased, fasting glucose,
postloading glucose, insulin, systolic and diastolic blood pressure,
BMI, WHR, cholesterol, triglycerides, LDL-C, and apo B increased
significantly (P <0.001). The coagulation factors,
including fibrinogen, PAI-1, and tPA, also increased significantly
(P <0.001) as fasting insulin concentrations increased.
Serum Lp(a), HDL-C, and apo A1 decreased significantly as fasting
insulin increased.
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Fasting insulin concentrations were significantly positively
correlated with insulin resistance by HOMA method (
= 0.97;
P <0.0001). The relationships between these two indicators
of insulin resistance syndrome and various atherosclerotic profiles,
according to Spearman correlation coefficients, are shown in Table 4
. The correlation coefficients were consistent within the two
groups for fasting insulin concentrations and insulin resistance index
by HOMA. When fasting insulin concentrations were considered,
significant positive correlations were found for BMI, WHR, blood
pressures, cholesterol, triglycerides, LDL-C, apo B, PAI-1, and
tPA in both sexes, and significant negative correlations were found for
HDL-C and apo A1 in both sexes. Fasting insulin concentrations also
exhibited a significant negative association with age and Lp(a)
concentration for men, but not for women. Fibrinogen concentration was
not associated with fasting insulin concentrations for both sexes.
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The abilities of the odds ratios and 95% confidence intervals of
various covariates to predict the occurrence of hyperinsulinemia are
shown in Table 5
. We found that obesity, high LDL-C, and low HDL-C were
independent factors. Sex and hypertension were not significant
covariates for hyperinsulinemia. As age increased, the odds ratios for
the occurrence of hyperinsulinemia decreased, although not
significantly.
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| Discussion |
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The fasting and postloading insulin measurements for the 2165 native Taiwanese men and women from the community-based CCCC study provided a unique opportunity to observe how atherosclerotic risk factors are associated with insulin resistance syndrome in the Chinese population. The fasting insulin concentrations in this population had a right-skewed distribution, a pattern similar to that found for other ethnic groups (3)(9). The median concentrations and intervals for fasting insulin for this study population seemed similar to those for Caucasians, higher on average than those for the Japanese (9), but lower than those values for African Americans(9)(13)(18)(26). Although fasting insulin concentrations were likely lower in females than in males for Caucasians, the concentrations were higher in females than in males for both the Japanese and the Chinese. However, triglycerides were fairly high for women in our study, compared with Caucasian women. The relationship was similar for the men in our study compared with Caucasian men. This study also demonstrated the different patterns in the age distribution of fasting insulin between genders: the concentration decreased as age increased in men, but increased gradually in women, peaking for the ages 6574 years. This was a sex- and age-interactive phenomenon not reported previously.
The patterns of the relationships between fasting insulin and fasting glucose were similar for both genders. In men, there was a persistent positive relationship between glucose and insulin, whereas in women, a small hump was noted for insulin when the fasting glucose was ~8.22 mmol/L, and then the concentration for insulin became flat. The hump of the curve in women may indicate when pancreatic ß-cell decompensation occurs (13)(38).
Hypertension shares an important role in insulin resistance syndrome(1)(2)(23). In this study, hyperinsulinemia was associated with higher systolic and diastolic blood pressure in both genders, similar to patterns for Chinese, Japanese, and Caucasians in other studies(9)(8)(39). In the study by Saad et al. (8), insulin concentrations and resistance were related to blood pressure in Caucasians, but not in Pima Indians or African Americans.
Obesity is strongly related to insulin resistance. In particular, individuals with a preponderance of abdominal fat deposition tend to be more insulin-resistant and have several metabolic abnormalities that place them at risk for an abnormal blood lipid profile (40). Compared with adipocytes located in the gluteo-femoral regions, abdominal adipocytes are more sensitive to lipolytic hormones(41). In this study, BMI and WHR increased as the insulin quartile increased, which is consistent with previous reports(19)(37)(42)(43).
The relationship between hyperinsulinemia and dyslipidemia, especially
in subjects with low HDL-C concentrations and hypertriglyceridemia, is
well-recognized in clinical practice and population studies(3)(17)(44)(45). In the
present study, the concentrations of cholesterol, triglycerides,
LDL-C, and apo B increased significantly as insulin concentrations
increased for both genders. The concentrations of HDL-C and apo A1
decreased significantly as insulin concentrations increased. Different
patterns between genders may exist in different countries. In a Western
study, cholesterol concentrations were positively correlated with
insulin concentrations only in men (46). It is noteworthy
that in our study, Lp(a) concentrations decreased as insulin increased,
similar to findings in a Japanese study (9). However, the
correlation was significant only for men, not for women (
= -0.14
vs -0.04 in men and women, respectively). This suggests that different
mechanisms of hyperinsulinemia and dyslipidemia may exist between
genders.
The association of coagulation factors and insulin resistance syndrome varied. Several studies have demonstrated increased plasma PAI-1 concentrations for patients with hyperinsulinemia(26)(47)(48), suggesting that insulin resistance syndrome is susceptible to atherothrombosis events. The fibrinogen concentration was also associated with hyperinsulinemia, but with a weaker correlation. The positive correlation between tPA and insulin in this study was different from that reported in the Northern Sweden MONICA study (36), which showed low tPA activity in hyperinsulinemic status. The diversity in population structure may be explained by racial differences. Other factors, such as dietary habits and physical activity, may be taken into consideration to explain the differences in the association.
Lifestyle influences of insulin resistance, including dietary intake and physical activity, are important to the pathogenesis of insulin resistance syndrome (22)(40). In this community-based study, inquiries concerning dietary habit and physical activity will be undertaken later. Controlling obesity and increased exercise are effective means of preventing diabetes and CHD. Further intervention treatments on the populations at risk for CHD are advised.
In conclusion, the present study demonstrated that various atherosclerotic and coagulation risk factors were strongly associated with fasting insulin concentrations. Insulin resistance syndrome may put these participants at risk for CHD.
| Acknowledgments |
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| Footnotes |
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1 Nonstandard abbreviations: CHD, coronary heart disease; CCCC, the Chin-Shan Community Cardiovascular Cohort; BMI, body mass index; WHR, waist-to-hip ratio; LDL-C, LDL-cholesterol; HDL-C, HDL-cholesterol; apo A1, apoprotein A1; apo B, apoprotein B; Lp(a), lipoprotein(a); tPA, tissue plasminogen activator; PAI-1, plasminogen activator inhibitor; and HOMA, homeostasis model assessment. ![]()
| References |
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The following articles in journals at HighWire Press have cited this article:
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K.-L. Chien, M.-F. Chen, H.-C. Hsu, W.-T. Chang, T.-C. Su, Y.-T. Lee, and F. B. Hu Plasma Uric Acid and the Risk of Type 2 Diabetes in a Chinese Community Clin. Chem., February 1, 2008; 54(2): 310 - 316. [Abstract] [Full Text] [PDF] |
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T.-C. Su, J.-S. Jeng, K.-L. Chien, F.-C. Sung, H.-C. Hsu, and Y.-T. Lee Hypertension Status Is the Major Determinant of Carotid Atherosclerosis: A Community-Based Study in Taiwan Stroke, October 1, 2001; 32(10): 2265 - 2271. [Abstract] [Full Text] [PDF] |
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