Insulin and atherosclerosis: villain, accomplice, or innocent bystander?

The roles of insulin resistance, hyperinsulinemia, and thiazolidinediones in cardiovascular disease

Although it is difficult to distinguish between the relative effects of insulin resistance and hyperinsulinemia, insulin resistance is clearly associated with significantly increased cardiovascular and cerebrovascular risk. This effect is consistent across the spectrum of worsening glycemic control, from the onset of impaired glucose tolerance to the development of clinical diabetes. It is more difficult to discriminate between the roles of elevated circulating insulin and proinsulin levels; the association between insulin levels and cardiovascular risk is weak. The thiazolidinediones (TZDs) significantly improve insulin sensitivity and exert numerous effects on the vascular bed, including improved endothelial function, decreased vascular inflammation, decreased plasma free fatty acid levels, improved dyslipidemic profiles, and inhibition of vascular smooth muscle proliferation. These findings provide increasing evidence to suggest that the TZDs may have a beneficial effect on atherosclerosis and may reduce the incidence and severity of adverse cardiovascular outcomes. These effects remain to be substantiated by the results of large outcomes studies to evaluate the impact of glycemic control and reversal of insulin resistance on cardiovascular events.

Fasting insulin levels independently associated with coronary heart disease in non-diabetic Turkish men and women

BACKGROUND

Levels of plasma insulin have been recognized as a weak risk indicator for coronary or cardiovascular risk in the general population with ethnic background and gender modifying this relationship. We assessed whether insulin concentrations are associated with or would serve as a marker of prevalent coronary heart disease risk in a cross-sectional study of a population having low cholesterol levels (just under 5 mmol/l) but higher prevalence of components of the metabolic syndrome.

METHODS

In 688 participants of the Turkish Adult Risk Factor Survey in 2001, plasma insulin values as well as other risk variables were evaluated, and coronary heart disease was diagnosed based on clinical findings and Minnesota coding of resting electrocardiograms. Nearly equal numbers of men and women (>30 years of age) constituted the population sample from the two largest regions of Turkey. Concentrations of insulin were determined by the chemiluminescent immunometric method.

RESULTS

Geometric mean value was 50 pmol/l (interquartile range 37-68 pmol/l), without revealing a significant difference in genders. Fasting insulin was correlated in both genders with many variables, notably those involving central obesity, triglycerides, blood pressure, physical inactivity and, inversely, with high-density lipoprotein (HDL)-cholesterol. In a regression model, waist circumference and body mass index were strongly associated with log insulin, after controlling for age and presence of coronary heart disease. The age- and obesity-adjusted odds ratio for coronary heart disease in the highest as opposed to the lowest quartile was 2-fold in both genders (P<0.05). Even after adjustment for dyslipidemia, blood pressure, glucose intolerance, physical activity and smoking status, an over 2-fold increased coronary heart disease risk still persisted with regard to hyperinsulinemia (>or=10 mU/l, 69.5 pmol/l). When C-reactive protein which was correlated with fasting insulin only in women, was added to the model, the impact of hyperinsulinemia on coronary heart disease risk remained unchanged.

CONCLUSION

Hyperinsulinemia (i) may provide information on the coronary heart disease likelihood over and above that provided by the other risk factors, including HDL-cholesterol, and (ii) may contribute, within the frame of insulin resistance, to the coronary heart disease risk independently of the classical risk factors.

Association of serum apolipoprotein A-I (but not high-density lipoprotein cholesterol) with healed myocardial infarction in men independent of serum insulin and C-peptide

Low serum levels of high-density lipoprotein (HDL) cholesterol or apolipoprotein A-I and high serum levels of insulin increase the risk of coronary heart disease (CHD) and can indicate insulin resistance. We tested the strength, independence, and interactions of associations between HDL cholesterol (or apolipoprotein A-I), insulin (or C-peptide), glucose, and CHD in 95 male nondiabetic patients with CHD who were <60 years old, in 92 probands from the PROCAM study, and in 61 non-cardiologic patients; all subjects were matched by age, body mass index, and smoking habits. Systemic hypertension (odds radio [OR] 2.8, 95% confidence intervals [CI] 1.6 to 4.8), high serum levels of glucose (OR 2.3, 95% CI 1.6 to 4.8), insulin (OR 2.1, 95% CI 1.3 to 3.6), and C-peptide (OR 4.1, 95% CI 2.2 to 7.5) as well as low serum levels of HDL cholesterol (OR 2.0, 95% CI 1.1 to 3.5) or apolipoprotein A-I (OR 3.9, 95% CI 2.1 to 7.1) had significant associations with CHD. At multivariate analysis, systolic blood pressure, glucose, apolipoprotein A-I, and C-peptide, but not HDL cholesterol and insulin, had consistent independent associations with CHD. Thus, the combined measurement of apolipoprotein A-I and C-peptide may improve the identification of nondiabetic patients at increased risk for CHD.

Fibrinolytic factors, serum lipid and C-reactive protein predicting cardiac events in Japanese patients with coronary atherosclerotic lesions

Although disturbances of the fibrinolytic system and serum lipid, and the presence of inflammation, may be risk factors for coronary artery disease (CAD), few reports have investigated these relationships in Japanese patients. Data on 106 patients (79 men and 27 women, mean age 62.3 years) with atherosclerotic lesions on the coronary angiogram were evaluated prospectively to identify whether the factors were useful in predicting the risk of coronary events during a follow-up of 50+/-4 months. Of the 106 patients who were followed, 11 patients had coronary events (4 acute myocardial infarction and 7 unstable angina pectoris). In univariate Cox analyses, a high level of tissue-plasminogen activator (t-PA), apolipoprotein CII, C-reactive protein (CRP), and a low level of high-density lipoprotein-cholesterol (HDL-C) was each associated with a significant increase in the risk of future cardiac events. The stepwise model of Cox proportional hazards analysis selected only a high level of t-PA and CRP as predictors of cardiac events. Controlling for any risk factor did not lower the relation between t-PA and the risk of cardiac events, whereas the relative risk of cardiac events in CRP was not significant when controlled for HDL-C. Thus, in prospective data obtained from a cohort of Japanese patients with coronary atherosclerotic lesions, the elevation of t-PA was an independent predictor of subsequent cardiac events. The prognostic role of CRP in cardiac events was related to a low level of HDL-C.

Hyperinsulinemia predicts coronary heart disease risk in healthy middle-aged men: the 22-year follow-up results of the Helsinki Policemen Study

BACKGROUND

The Helsinki Policemen Study is one of the first prospective epidemiological studies demonstrating an association of hyperinsulinemia to the risk of coronary heart disease (CHD). The aim of the present study was to investigate the predictive value of hyperinsulinemia with regard to CHD risk during a 22-year follow-up of the Helsinki Policemen Study population.

METHODS AND RESULTS

The study was based on a cohort of 970 men who were 34 to 64 years of age and free of CHD, other cardiovascular disease, and diabetes. Risk factor measurements at baseline examination included an oral glucose tolerance test (OGTT) with blood glucose and plasma insulin measurements at 0, 1, and 2 hours. Area under the plasma insulin response curve (AUC insulin) during OGTT was used as a composite variable reflecting plasma insulin levels. During the 22-year follow-up, 164 men had a major CHD event (CHD death or nonfatal myocardial infarction). Age-adjusted hazard ratios for a major CHD event comparing men in the highest AUC insulin quintile with those in the combined 4 lower quintiles during 5-, 10-, 15-, and 22-year follow-up periods were 3.29 (95% CI, 1.56 to 6.91), 2.72 (95% CI, 1.67 to 4.42), 2.14 (95% CI, 1.43 to 3.21), and 1.61 (95% CI, 1.14 to 2.27), respectively. Further adjustment for other risk factors attenuated these hazard ratios to 2.36 (95% CI, 1.00 to 5.57), 2.29 (95% CI, 1.31 to 4.02), 1.76 (95% CI, 1.09 to 2.82), and 1.32 (95% CI, 0.89 to 1.97), respectively.

CONCLUSIONS

Hyperinsulinemia predicted CHD risk in Helsinki policemen over the 22-year follow-up, and to a large extent independently of other CHD risk factors, but its predictive value diminished with lengthening follow-up time.

Vasculopathy and insulin resistance in the JCR:LA-cp rat

The JCR:LA-cp rat is one of a number of strains incorporating the autosomal recessive cp gene that induces obesity. This strain is unique in the development of not only a profound insulin resistance, but an accompanying cardiovascular disease that correlates strongly with hyperinsulinemia. The hyperinsulinemia develops rapidly after 4 weeks of age, with an age at half-maximum of 5.5 weeks. This reflects postprandial plasma insulin levels that peak at 1000 mU/l in a standardized meal tolerance test. Defective acetylcholine-mediated vascular relaxation develops with a 1-week lag over the developing hyperinsulinemia. The frequency of staining for the vascular adhesion molecules, VCAM-1 and ICAM, does not show either age or genotype variation, although plasma levels do show an age variation. Treatment of the rats with the alpha-glucosidase inhibitor, miglitol (Bay m1099), obviates the exaggerated postprandial glucose and, especially, the insulin responses of the cp/cp rat. This causes an improvement in insulin sensitivity, prevention of the impaired vascular relaxation, and reduction in plasma levels of advanced glycated end-products. Arterial wall morphology, as visualized by both scanning and transmission electron microscopy, shows abnormal endothelium, adherent macrophages, and activated migrating smooth muscle cells in the intima. Oil-Red-O staining reveals lipid deposits in the intimal spaces, as confirmed by the presence of foam cells. The lesions resemble fatty streaks or modest atherosclerosis in man, rather than the extensive cholesterol-laden lesions seen in familial hypercholesterolemia or cholesterol-fed rabbit models. The lean rats of the strain show similar, but less marked, intimal abnormalities. The vasculopathy in this animal model appears to be precipitated by the developing hyperinsulinemia, but also requires an underlying abnormality of vascular smooth muscle and possibly also of the endothelium.

Hyperinsulinemia and cardiovascular disease in elderly men: the Honolulu Heart Program

Hyperinsulinemia has been associated with cardiovascular disease (CVD), but whether this relation is independent of other CVD risk factors is uncertain. Most studies have focused on coronary heart disease (CHD), but few have included peripheral vascular disease (PVD) and stroke. Moreover, evidence in elderly and minority populations is limited. Between 1991 and 1993, 3562 elderly (71 to 93 years) Japanese-American men from the Honolulu Heart Program were examined and had fasting insulin levels measured. Hyperinsulinemia, defined as a fasting insulin > or =95th percentile among nonobese men with normal glucose tolerance and no diabetic history or medication use, was observed in 22% of the population. Subjects with hyperinsulinemia had a more adverse CVD risk factor profile and had higher age-adjusted prevalences of CHD, angina, PVD, thromboembolic stroke, and hemorrhagic stroke compared with those without hyperinsulinemia. Age-adjusted fasting insulin levels but not 2-hour levels were also significantly elevated (P<.01) in those with prevalent CVD compared with those without. In logistic regression analyses, adjustment for multiple CVD risk factors attenuated the relations of hyperinsulinemia with CHD, angina, and PVD to nonsignificant levels, whereas those involving thromboembolic and hemorrhagic stroke were strengthened and remained significant (odds ratios=2.27 and 7.53, 95% confidence intervals=1.25 to 4.13 and 1.65 to 34.25, respectively). When multivariate analyses were restricted to nondiabetic subjects, associations were slightly weaker and in general nonsignificant. Nondiabetic men with thromboembolic stroke were twice as likely to have hyperinsulinemia as those who were stroke-free, although this association was of borderline significance (odds ratio= 1.99, 95% confidence interval=0.95 to 4.17, P=.069). In subjects with elevated total cholesterol levels, somewhat stronger associations were observed for PVD and stroke but not for CHD. Although further prospective studies are indicated, particularly for PVD and stroke, these cross-sectional results are consistent with an indirect role for insulin in CVD, wherein hyperinsulinemia or an underlying insulin-resistant state may adversely affect other CVD risk factors or serve as a marker for an atherogenic or thrombogenic state.

Efficacy of 24-week monotherapy with acarbose, metformin, or placebo in dietary-treated NIDDM patients: the Essen-II Study

PURPOSE

To compare the therapeutic potential of acarbose, metformin, or placebo as first line treatment in patients with non-insulin-dependent diabetes mellitus (NIDDM).

PATIENTS AND METHODS

Ninety-six patients with NIDDM (35-70 years of age, body mass index (BMI) < or = 35 kg/m2, insufficiently treated with diet alone, glycated hemoglobin (HbA1c; 7% to 11%) were randomized into 3 groups and treated for 24 weeks with acarbose, 3 x 100 mg/day, or metformin, 2 x 850 mg/day, or placebo. Efficacy, based on HbA1c (primary efficacy criterion), fasting blood glucose (BG) and insulin, 1 hour postprandial BG and insulin (after standard meal test), postprandial insulin increase, plasma lipid profile, and tolerability, based on subjective symptoms and laboratory values were determined every 6 weeks. Analysis of covariance was performed for endvalues with adjustment on baseline values. Ninety-four patients were valid for efficacy evaluation.

RESULTS

Both active drugs showed the same improvement of efficacy criteria compared with placebo. Baseline adjusted means at endpoint were as follows: BG, fasting and 1 hour postprandial, 9.2 mM and 10.9 mM with placebo, 7.6 mM and 8.7 mM with acarbose, and 7.8 mM and 9.0 mM with metformin; HbA1c was 9.8% with placebo, 8.5% with acarbose, and 8.7% with metformin. Comparisons: acarbose versus placebo and metformin versus placebo were statistically significant, but not acarbose versus metformin. No effect on fasting insulin could be observed. Relative postprandial insulin increase was 1.90 with placebo, 1.09 with acarbose, and 1.03 with metformin. Comparisons: acarbose versus placebo and metformin versus placebo were statistically significant, but not acarbose versus metformin. With respect to lipid profile, acarbose was superior to metformin. Low-density lipoprotein (LDL)/high-density lipoprotein (HDL) cholesterol ratio increased by 14.4% with placebo, was unchanged with metformin, but decreased by 26.7% with acarbose. Comparisons: acarbose versus placebo and acarbose versus metformin were statistically significant, but not metformin versus placebo. Slight body weight changes were observed with acarbose (-0.8 kg) and metformin (-0.5 kg), but not with placebo. Acarbose led to mild or moderate intestinal symptoms in 50% of the patients within the first 4 weeks, but in only 13.8% of the patients within the last 4 weeks.

CONCLUSIONS

Acarbose and metformin are effective drugs for the first line monotherapy of patients with NIDDM. With respect to plasma lipid profile, especially HDL cholesterol, LDL cholesterol and LDL/HDL cholesterol ratio acarbose may be superior to metformin.

Fetal and postnatal growth to age 2 years by mother's country of birth

The objective of this study was to measure the physical growth of fetuses and infants in an inner city health district in the north of England and to compare their growth profiles according to mother's country of birth (British Isles or Indian subcontinent). The study was part of the Central Manchester Child Growth Project, a prospective longitudinal study of fetal and postnatal growth and development in a sample from the geographically-defined Central Manchester Health District. Data were collected from the beginning of the second trimester of pregnancy to the age of 2 years. One-hundred seventy-four singleton infants born at term ( > or = 37 weeks) had serial antenatal cephalometry every 3 weeks from the beginning of the second trimester and had serial head, length and weight measurements at birth and at the ages of 6, 13, 26, 52 and 104 weeks. Infants of Indian-born mothers tended to be lighter at birth than those of locally-born mothers, but the difference was not due to lower accumulation of soft tissue. Body length from 6 to 52 weeks in both groups of infants was similar. The major finding was the reduced head size in infants of Indian-born mothers, the difference being significant among boys, evident from mid-pregnancy and persisting postnatally to age 2 years. Reduced fetal growth is associated with the development of cardiovascular disease in adulthood, mortality from ischaemic heart disease being specifically linked with head size at birth. The reduced head size of boys of Indian-born mothers is of interest because male immigrants from the Indian subcontinent who live in England have an increased incidence of non-insulin dependent diabetes and a substantial excess mortality (standardised mortality ratio 313 at ages 20-29) from ischaemic heart disease.

Vascular actions of insulin in health and disease

Insulin has well known metabolic effects. However, depending on the magnitude and duration of the insulin stimulus, this hormone can also produce vasodilation and vascular smooth muscle growth. The association of hyperinsulinemia with the metabolic disorders of obesity and non-insulin-dependent diabetes, as well as with the cardiovascular pathologies of hypertension and atherosclerosis, has led to suggestions that perhaps elevated insulin levels are causally related to these diseases. Alternatively, insulin resistance may develop following an increase in skeletal muscle vascular resistance, with or without hypertension, such that a reduction in skeletal muscle blood flow leads to an attenuated glucose delivery and uptake. These hypotheses are explored in this review by examining the effects of insulin on vascular smooth muscle tissue during both acute and prolonged exposure. An interaction among hyperinsulinemia, hyperglycemia, and hyperlipidemia associated with the insulin resistant state is described whereby insulin resistance can be both a cause and a result of elevated vascular resistance. The association between blood flow and insulin stimulated glucose uptake suggests that therapeutic intervention against the development of skeletal muscle vascular resistance should occur early in individuals generally predisposed to cardiovascular pathology in order to attenuate, or avoid, insulin resistance and its sequelae.

Prospective study of risk factors for development of non-insulin dependent diabetes in middle aged British men

OBJECTIVE

To determine the risk factors for noninsulin dependent diabetes in a cohort representative of middle aged British men.

DESIGN

Prospective study.

SUBJECTS AND SETTINGS

7735 men aged 40-59, drawn from one group practice in each of 24 towns in Britain. Known and probable cases of diabetes at screening (n = 158) were excluded.

MAIN OUTCOME MEASURES

Non-insulin dependent diabetes (doctor diagnosed) over a mean follow up period of 12.8 years.

RESULTS

There were 194 new cases of non-insulin dependent diabetes. Body mass index was the dominant risk factor for diabetes, with an age adjusted relative risk (upper fifth to lower fifth) of 11.6; 95% confidence interval 5.4 to 16.8. Men engaged in moderate levels of physical activity had a substantially reduced risk of diabetes, relative to the physically inactive men, after adjustment for age and body mass index (0.4; 0.2 to 0.7), an association which persisted in full multivariate analysis. A nonlinear relation between alcohol intake and diabetes was observed, with the lowest risk among moderate drinkers (16-42 units/week) relative to the baseline group of occasional drinkers (0.6; 0.4 to 1.0). Additional significant predictors of diabetes in multivariate analysis included serum triglyceride concentration, high density lipoprotein cholesterol concentration (inverse association), heart rate, uric acid concentration, and prevalent coronary heart disease.

CONCLUSION

These findings emphasise the interrelations between risk factors for non-insulin dependent diabetes and coronary heart disease and the potential value of an integrated approach to the prevention of these conditions based on the prevention of obesity and the promotion of physical activity.

Insulin in ischaemic heart disease: are associations explained by triglyceride concentrations? The Caerphilly prospective study

OBJECTIVE

To investigate the predictive value of fasting insulin concentrations for subsequent fatal or non-fatal ischaemic heart disease at five year follow up and to examine the associations between insulin and other indicators of risk.

DESIGN

A prospective population study among 2512 men aged 45 to 59 at recruitment.

SETTING

A whole population sample of men resident in Caerphilly, South Wales.

MEASUREMENTS

At recruitment fasting blood samples were taken for measurement of plasma lipids and serum insulin. Men were re-examined at a five year follow up and ischaemic heart disease events during this period were assessed from hospital notes, death certificates, and electrocardiograms.

MAIN RESULTS

Diabetic men and those men with a fasting blood glucose of > or = 8 mmol/l were excluded from all analyses. In a univariate analysis the incidence of ischaemic heart disease increased with increasing concentration of fasting insulin, such that for men in the top 20% of the insulin distribution the odds of developing ischaemic heart disease were 1.87 relative to men in the bottom 20%. On multivariate analysis this relation disappeared on adjusting for plasma triglycerides, body mass index, and evidence of ischaemic heart disease at recruitment.

CONCLUSION

In this population in South Wales there was no evidence that the fasting insulin concentration is an independent risk factor for ischaemic heart disease. The univariate association between insulin and incident disease was almost entirely explained by the association of both with triglycerides and body mass index.