Insulin sensitivity, lipids, and blood pressure in young American blacks

Contribution of a genetic risk score to ethnic differences in fatty liver disease

BACKGROUND AND AIMS

Susceptibility to fatty liver disease (FLD) varies among individuals and between racial/ethnic groups. Several genetic variants influence FLD risk, but whether these variants explain racial/ethnic differences in FLD prevalence is unclear. We examined the contribution of genetic risk factors to racial/ethnic-specific differences in FLD.

METHODS

A case-control study comparing FLD patients (n = 1194) and population-based controls (n = 3120) was performed. Patient characteristics, FLD risk variants (PNPLA3-rs738409 + rs6006460, TM6SF2-rs58542926, HSD17B13-rs80182459 + rs72613567, MBOAT7/TMC4-rs641738, and GCKR-rs1260326) and a multi-locus genetic risk score (GRS) were examined. The odds of FLD for individuals with different risk factor burdens were determined.

RESULTS

Hispanics and Whites were over-represented (56% vs. 38% and 36% vs. 29% respectively) and Blacks under-represented (5% vs. 23%) among FLD patients, compared to the population from which controls were selected (p < .001). Among cases and controls, Blacks had a lower and Hispanics a greater, net number of risk alleles than Whites (p < .001). GRS was associated with increased odds of FLD (OR_Q5vsQ1  = 8.72 [95% CI = 5.97-13.0], p = 9.8 × 10^-28 ), with the association being stronger in Hispanics (OR_Q5vsQ1  = 14.8 [8.3-27.1]) than Blacks (OR_Q5vsQ1  = 3.7 [1.5-11.5], P-interaction = 0.002). After accounting for GRS, the odds of FLD between Hispanics and Whites did not differ significantly (OR = 1.06 [0.87-1.28], p = .58), whereas Blacks retained much lower odds of FLD (OR = 0.21, [0.15-0.30], p < .001).

CONCLUSIONS

Blacks had a lower and Hispanics a greater FLD risk allele burden than Whites. These differences contributed to, but did not fully explain, racial/ethnic differences in FLD prevalence. Identification of additional factors protecting Blacks from FLD may provide new targets for prevention and treatment of FLD.

Postprandial endothelial function does not differ in women by race: an insulin resistance paradox?

Insulin resistance is associated with endothelial dysfunction. Because African-American women are more insulin-resistant than white women, it is assumed that African-American women have impaired endothelial function. However, racial differences in postprandial endothelial function have not been examined. In this study, we test the hypothesis that African-American women have impaired postprandial endothelial function compared with white women. Postprandial endothelial function following a breakfast (20% protein, 40% fat, and 40% carbohydrate) was evaluated in 36 (18 African-American women, 18 white women) age- and body mass index (BMI)-matched (age: 37 ± 11 yr; BMI: 30 ± 6 kg/m(2)) women. Endothelial function, defined by percent change in brachial artery flow-mediated dilation (FMD), was measured at 0, 2, 4, and 6 h following a meal. There were no significant differences between the groups in baseline FMD, total body fat, abdominal visceral fat, and fasting levels of glucose, insulin, total cholesterol, low-density lipoprotein cholesterol, or serum estradiol. Although African-American women were less insulin-sensitive [insulin sensitivity index (mean ± SD): 3.6 ± 1.5 vs. 5.2 ± 2.6, P = 0.02], both fasting triglyceride (TG: 56 ± 37 vs. 97 ± 49 mg/dl, P = 0.007) and incremental TG area under the curve (AUC(0-6hr): 279 ± 190 vs. 492 ± 255 mg·dl(-1)·min(-1)·10(-2), P = 0.008) were lower in African-American than white women. Breakfast was associated with a significant increase in FMD in whites and African-Americans, and there was no significant difference in postprandial FMD between the groups (P > 0.1 for group × time interactions). Despite being insulin-resistant, postprandial endothelial function in African-American women was comparable to white women. These results imply that insulin sensitivity may not be an important determinant of racial differences in endothelial function.

Endothelin-1 response to glucose and insulin among African Americans

Endothelin-1 (ET-1) is implicated in the pathogenesis of hypertension. In vitro studies demonstrate that ET-1 is upregulated by insulin and glucose. The purpose of this study was to determine the effects of insulin and glucose on ET-1 levels in young adult African Americans, a population with a high burden of hypertension and diabetes. Plasma and urine ET-1 levels were measured before and after an oral glucose tolerance test (OGTT) and insulin clamp procedure in 288 participants. Subjects were classified according to glucose tolerance and blood pressure (BP) status. Plasma and urine ET-1 were not significantly different among the glucose tolerance groups. There was a trend toward increased plasma ET-1 among those with diabetes compared with impaired glucose tolerance and normal glucose tolerance; however, this was not statistically significant (P = .085). According to BP status, plasma ET-1 was highest among the high BP group compared with the normal BP group (P = .01). After glucose challenge, plasma ET-1 levels decreased and urine ET-1 increased in all three BP groups (P = .037). Our data show that plasma ET-1 is higher among young adult African Americans with hypertension compared with normotension. Urine ET-1 levels increased in response to glucose challenge, possibly indicating early renal injury.

Association of a polymorphic variant of the adiponectin gene with insulin resistance in african americans

Hypertension, type 2 diabetes, and obesity are common complex disorders that contribute to cardiovascular (CV) disease. Insulin resistance increases CV risk and is present in these disorders. Adiponectin, a protein secreted by adipocytes with metabolic and vascular protective effects, is lower in obesity and insulin resistance. Several single nucleotide polymorphisms (SNP) have been identified in the adiponectin (ADIPOQ) gene. Associations of ADIPOQ polymorphisms with diabetes and obesity have been described in Caucasians and Asians. The purpose of this study was to determine if genetic variants of ADIPOQ are associated with insulin resistance and CV risk in African Americans. Metabolic traits (lipids, glucose, insulin, and insulin sensitivity) and blood pressure were measured in 273 African Americans. DNA was examined by DNA sequence analysis and SNPs of candidate genes including ADIPOQ were studied. Statistica analyses were performed by regression of the quantitative trait phenotypes on the groups defined by the SNP genotypes, adjusting for age, sex, and body mass index (BMI). SNP 712 (rs3774261) ofthe/lD/POQgene showed significant association with insulin resistance (p= 0.001). Despite the relatively small sample, our results indicate that genes that regulate adipocyte function may have a regulatory role in the expression of metabolic traits in obesity-associated chronic disease.

Insulin resistance predicts future deterioration of glucose tolerance in nondiabetic young African Americans

Insulin resistance has been linked to the development of type 2 diabetes mellitus and increased cardiovascular risk in several high-risk populations. The purpose of this study was to determine if insulin resistance measured by insulin clamp can predict deterioration of glucose metabolism and increased cardiovascular risk in nondiabetic young adult African Americans. Nondiabetic young African American men (n = 60) and women (n = 114) were enrolled. Measurements obtained included blood pressure, anthropometrics, plasma lipids, oral glucose tolerance test, and insulin sensitivity by insulin clamp. Participants were reexamined 8 years later. The relationship between insulin sensitivity and glucose metabolism was analyzed using a 2-way analysis of variance with body mass index at the initial examination as a covariate. After adjusting for the significant difference of body mass index between the insulin-resistant and insulin-sensitive groups, insulin resistance predicted statistically significant worsening glucose metabolism, developing diabetes, and increasing risk factors for cardiovascular disease.

Association of age and sex with cardiovascular risk factors and insulin sensitivity in overweight children and adolescents

To determine the effect of age and sex on cardiovascular risk factor expression in overweight children, data from clinical records of 497 overweight children (2-18 years of age) were examined. Data included average blood pressure (BP), fasting lipids, glucose, and insulin. The sample was stratified by age (younger than 11 and 11 years and older) and analyzed by sex. Subjects with an average BP > or = 90th percentile were classified as having high BP. Insulin and glucose were used in equations to estimate insulin sensitivity. Among subjects 11 years and older (n = 268), 52.6% of males had high BP compared with 32.6% of females (P < .001). Mean high-density lipoprotein cholesterol was lowest in the males 11 years and older compared with the females and younger males (P < .01). Triglyceride levels trended higher in males independent of age. In multivariate analyses, high BP was most strongly associated with age and severity of overweight while triglyceride level was most associated with sex and insulin resistance. The prevalence of high BP and dyslipidemia in overweight children is high. Overweight males 11 years and older have a higher prevalence of high BP and low high-density lipoprotein cholesterol than females and younger males. Greater cardiovascular risk factor expression in overweight males 11 years and older may explain the earlier appearance of cardiovascular disease end points in overweight men.

Plasma lipid concentrations in nondiabetic African American adults: associations with insulin resistance and the metabolic syndrome

Despite higher rates of cardiovascular disease, African Americans have a more favorable lipid profile. The purpose of the study was to examine the association between plasma lipid concentrations and insulin resistance in African Americans and to determine if insulin resistance is present at a lower triglyceride (TG) threshold than is used for metabolic syndrome criteria. Data were examined on 185 nondiabetic African American men (n = 61) and women (n = 124), mean age, 39.8 years. Measurements included blood pressure, anthropometrics, oral glucose tolerance test, and insulin sensitivity (M) by insulin clamp. The relationship between lipids and insulin sensitivity was analyzed by correlation analysis and by comparing TG levels among tertiles of M. Despite relatively low mean (+/- SD) TG level (87.8 +/- 55.2 mg/dL), there were statistically significant correlations of M with TG (r = -0.23, P < .002), high-density lipoprotein cholesterol (HDL-C; r = 0.19, P < .01), and TG/HDL-C ratio (r = -0.23, P < .002). The correlations were strongest in men. Subjects with TG in an intermediate range (110-149 mg/dL) had insulin resistance equivalent to that of the high-TG group (>/=150 mg/dL). In African Americans, TG levels below the current metabolic syndrome threshold criterion are associated with insulin resistance.

Correlation of oral glucose tolerance test-derived estimates of insulin sensitivity with insulin clamp measurements in an African-American cohort

The purpose of this study was to determine which measures obtained from an oral glucose tolerance test (OGTT) are the best estimates of insulin sensitivity measured directly using the euglycemic hyperinsulinemic clamp procedure. Data were examined from a study conducted on 307 young adult African-American men and women. An OGTT with insulin measurements was conducted after a 12-hour overnight fast. The euglycemic hyperinsulinemic clamp was used to measure insulin-stimulated glucose uptake (M) directly. Pearson's correlation analyses were performed to examine the relationship of OGTT-derived parameters with insulin sensitivity measured using the clamp. There were consistent statistically significant correlations between calculated estimates of insulin sensitivity (fasting insulin/fasting glucose, summed insulin/summed glucose, homeostasis model assessment [HOMA], Quantitative Insulin Sensitivity Check Index [QUICKI]) with insulin sensitivity measured by the insulin clamp (P <.001). The calculated estimates that correlated most strongly with clamp measured insulin sensitivity were QUICKI and the logarithm of summed insulin during the OGTT. These data indicate that fasting and OGTT-derived plasma insulin and glucose concentrations can be used to estimate insulin sensitivity in young adult African-Americans when it is not feasible to conduct the insulin clamp procedure. Calculated indices that include log transformation of plasma insulin concentration improve the estimation of insulin sensitivity.

Obesity and high blood pressure: a clinical phenotype for the insulin resistance syndrome in African Americans

The high prevalence of insulin resistance syndrome in African Americans predisposes this population to higher morbidity and mortality from cardiovascular disease. To test the hypothesis that the combination of obesity and high blood pressure (BP) represents the physical phenotype of insulin resistance syndrome, 337 African-American men and women aged 32+/-4 years were examined and classified into four groups (nonobese-normal BP, nonobese-high BP, obese-normal BP, obese-high BP), according to presence or absence of obesity and high BP. Mean values of glucose, insulin, lipids, urinary albumin excretion, and clamp-derived insulin sensitivity were determined for each group. Prevalence of prediabetes (24.4%), diabetes (19.2%), and insulin resistance syndrome (87.2%) were highest in the obese-high BP group (p<0.001). Mean triglycerides, urinary albumin excretion, fasting glucose, fasting insulin, and insulin resistance were highest in the obese-high BP group (p<0.001). Subjects with both obesity and high BP showed greater expression of lipid and glucose abnormalities, higher urinary albumin excretion, and greater prevalence of prediabetes, undetected type 2 diabetes, and insulin resistance syndrome.

Diabetes in African Americans: unique pathophysiologic features

Type 2 diabetes is an increasing public health problem among African Americans, especially children. Several features make type 2 diabetes among African Americans unique. First, African-American adults with type 2 diabetes, or Flatbush diabetes, present with diabetic ketoacidosis. Patients are insulin resistant with acute, severe defects in insulin secretion and no islet cell autoantibodies. Following treatment, some insulin secretory capacity is recovered and ketoacidosis generally does not recur. The second is remission in African Americans with type 2 diabetes. Recovery of glucose homeostasis, accompanied by recovery of beta-cell function, follows intensive glycemic regulation. Finally, among African Americans with diabetes who are not obese, normal insulin sensitivity is not uncommon. Such individuals do not have the increased cardiovascular risk of insulin-resistant individuals. Differences in visceral, not subcutaneous, adipose tissue volume appear to determine insulin sensitivity. Understanding the unique physiologic and clinical features of African Americans is critical in designing appropriate treatment strategies.

Insulin resistance in African Americans

Hypertension and diabetes mellitus are the leading contributors to end-stage renal disease. African Americans suffer higher rates of renal failure as well as other vascular morbidities associated with hypertension and diabetes. Insulin resistance is strongly associated with hypertension. Insulin resistance is a component of diabetes and also precedes the clinical expression of type 2 diabetes. The relationship of blood pressure with insulin resistance, or impaired insulin action, occurs in African Americans and can be detected at young ages prior to the clinical expression of hypertension or diabetes. Through its relationship with hypertension, diabetes, and hyperlipidemia, insulin resistance is associated with endothelial dysfunction. The interface of insulin resistance with endothelial dysfunction may begin to explain the role of insulin resistance in vascular and renal pathology. The injury process, subsequent to both hypertension and diabetes, appears to be mediated by alterations tissue regulatory factors, and include vasoactive peptides such as angiotensin II, endothelin, and growth factors. Understanding the determinants that up-regulate the aberrant pathways and the early phases of these processes will be necessary to formulate strategies to effectively achieve renal protection and reduce the rates of renal failure in African Americans.

Glucose tolerance and cardiovascular risk in young adult African Americans

BACKGROUND

Patients with type 2 diabetes have higher rates of cardiovascular events. Among African Americans, there is a higher prevalence of both cardiovascular disease and type 2 diabetes. Few studies have examined longitudinally the change in glucose tolerance in younger adult African Americans.

METHODS

To examine the longitudinal relationship of glucose tolerance with other cardiovascular risk factors, 30 African American men and women aged 20 to 43 years were examined twice at an interval of 4 to 5 years. Cardiovascular risk factors, glucose tolerance, and insulin sensitivity (determined from euglycemic hyperinsulinemic clamp procedure) were assessed at each examination. Known diabetics were excluded from initial enrollment. The relationship of glucose tolerance status (normal, impaired, or diabetic glucose tolerance) to body mass index, blood pressure, cholesterol, and insulin sensitivity were further investigated.

RESULTS

Initial oral glucose tolerance test identified 24 of 130 (18.5%) subjects with impaired glucose tolerance and 2 of 130 (1.5%) subjects with diabetes. Of the remaining 104 subjects with normal glucose tolerance, subsequent 5-year examination detected 31 (29.8%) with impaired glucose tolerance and 5 (4.8%) with diabetes. Those who later developed diabetes had higher mean systolic blood pressure (133 versus 121, P = 0.037) at exam 1. By exam 2, those with abnormal glucose tolerance had worse cardiovascular risk profiles and increased insulin resistance (P < 0.001).

CONCLUSION

Conversion to abnormal glucose tolerance is relatively frequent in young adult African Americans. Deterioration in glucose tolerance may be preceded by higher systolic blood pressure and is accompanied by worsening of other cardiovascular risk factors and insulin resistance.

The association of left ventricular mass with cardiovascular risk factors in African American women

BACKGROUND

African American women have disproportionately high rates of myocardial infarction and stroke. Left ventricular hypertrophy is an independent risk factor for cardiovascular disease. Increases in left ventricular mass (LVM) may precede the expression of hypertension. The purpose of this study was to determine whether LVM is related to cardiovascular risk variables in healthy, premenopausal African American women.

METHODS

Normotensive or borderline hypertensive nondiabetic African American women (N = 52; mean age, 31 years) underwent anthropometric and blood pressure measurements, oral glucose tolerance test, euglycemic clamp, fasting lipid profile, and two-dimensional echocardiography. LVM was calculated by the cube root formula and adjusted for height [LVM index (LVMI)].

RESULTS

LVMI correlated with body mass index (r = .36, P = 0.009), systolic blood pressure (r = .44, P = 0.001), diastolic blood pressure (r = .43, P = 0.002), and central body fat (r = .42, P = 0.002). LVMI also directly correlated with lipoprotein (a) (r = .34, P = 0.02). Significant independent relationships of other metabolic variables with LVMI were not detected.

DISCUSSION

These data show that increased LVMI is associated with body mass index and central obesity, but not with lipids, insulin resistance, or insulin sensitivity. LVMI is also associated with blood pressure before the expression of severe hypertension in healthy, premenopausal African American women.

Blood pressure increase with impaired glucose tolerance in young adult american blacks

Hypertension and non-insulin-dependent diabetes mellitus are more prevalent in blacks than whites. The convergence of these 2 disorders augments the expression and severity of cardiovascular disease. The purpose of this study was to determine whether alterations in glucose metabolism are related to an increase in blood pressure (BP). This study was conducted on 304 nondiabetic blacks (mean age=32 years). Measurements in all subjects included BP, anthropometric measures, oral glucose tolerance test, insulin clamp to measure insulin sensitivity, and plasma lipids. The sample was stratified according to plasma glucose on oral glucose tolerance test to normal glucose tolerance (NGT), impaired glucose tolerance (IGT), and diabetes mellitus (DM). A 2-way ANOVA was performed to determine differences between the metabolic groups. With the use of American Diabetic Association criteria, 20.4% of the samples were classified as IGT and 5.9% were diabetic. A significant increase in BP existed from NGT to IGT to DM, which was stronger in women than men (systolic blood pressure in women: NGT=122, IGT=127, and DM=140 mm Hg, P<0.001) with a significant linear trend (P<0.001). With the use of body mass index as a covariate, the group difference in BP remained significant (P=0.006). Measures of insulin sensitivity demonstrated significant metabolic group differences (P<0.001) with a linear trend (P<0.001) of decreasing insulin sensitivity from NGT to DM. These results indicate that early alterations in glucose metabolism effects an upward shift in BP. The higher BP in IGT and DM may be due to vascular endothelial cell resistance to insulin action.

Hyperinsulinism and sex hormones in young adult African Americans

Hyperinsulinemia is a risk factor for cardiovascular disease, and is linked with non-insulin-dependent diabetes mellitus (NIDDM), hyperlipidemia, obesity, and hypertension. Sex hormones also play a role in the metabolic alterations associated with the risk for cardiovascular disease. A reduction in sex hormone-binding globulin (SHBG) may be predictive of future NIDDM particularly in women. The postmenopausal decline in estrogen is also associated with an increase in risk factor expression in women. Since African Americans experience a greater prevalence of NIDDM, obesity, and hypertension, conditions associated with hyperinsulinemia, the purpose of this study was to determine if alterations in sex hormone levels are associated with the plasma insulin concentration in young adult African Americans, and to determine if there are sex differences in the effect of insulin on lipids and sex hormones. In a sample of 221 nondiabetic African American men (n = 105) and women (n = 116) with a mean age of 31 years, we examined the relationship of the plasma insulin concentration with the body mass index (BMI), blood pressure, plasma lipids, and sex hormones, including free testosterone, estradiol, and SHBG. Plasma insulin increased with the BMI and other measures of adiposity (P<.001) in men and women. Significant correlations of insulin with plasma lipids were also present in both sexes. There was a significant inverse correlation of insulin with SHBG in both men (r = .28, P = .007) and women (r = .27, P = .02). There was a significant direct correlation of insulin with free testosterone in women (r = .032, P<.001). Stepwise multiple regression analyses with insulin as the dependent variable detected the BMI, triglyceride, and apolipoprotein A1 as significant contributors to the plasma insulin concentration in men. In women, the multiple regression model detected percent body fat, low-density lipoprotein (LDL) cholesterol, and free testosterone as significant contributors to plasma insulin. These data on young African Americans demonstrate a significant relationship between hyperinsulinemia and obesity, atherogenic lipid status, and lower SHBG. In the premenopausal women, the lower SHBG is linked with higher free testosterone, favoring a condition of relative androgen excess.

Racial differences in renal kallikrein excretion: effect of the ovulatory cycle

BACKGROUND

Renal kallikrein excretion is diminished in essential hypertension, especially in African-Americans, and evidence exists for a major gene effect on the kallikrein phenotype. In addition, urinary kallikrein excretion differs by gender, with ovulating females having greater kallikrein excretion than males or postmenopausal females. Recent studies have shown that renal kallikrein excretion varies in females during the ovulatory cycle, with levels rising during the luteal phase and returning during the follicular phase to levels that are similar to those of males. In family studies, gender differences in urinary kallikrein excretion were present in white subjects, but not black subjects. We therefore hypothesized dysregulation of kallikrein biosynthetic responses in African-Americans.

METHODS

We determined urinary kallikrein activity [chromogenic substrate S2266 (D-val-leu-arg-paranitroanilide) assay; in microU/mg creatinine] in white (N = 15) and black (N = 11) ovulating females during the ovulatory cycle. Serum progesterone, estrogen, plasma renin activity as well as urinary aldosterone, and urinary electrolytes were determined to investigate changes between mid-follicular and mid-luteal phases in the two groups.

RESULTS

White and black groups were matched for age, body mass index, blood pressure, heart rate and renal function. Ovulatory cycle phases were confirmed by serum progesterone determinations, which increased significantly in whites and blacks to a comparable degree [0.84 +/- 0.14 nmol/liter (mid-follicular) to 29.77 +/- 4.70 nmol/liter (mid-luteal) in whites, 0.67 +/- 0.08 nmol/liter (mid-follicular) to 28.62 +/- 5.83 nmol/liter (mid-luteal) in blacks; P < 0.001 for cycle effect, P = NS for race effect and race X cycle interaction]. Urinary kallikrein activity increased from 623 +/- 86 microU/mg creatinine (mid-follicular) to 948 +/- 142 microU/mg creatinine (mid-luteal) in whites, but did not change in blacks during the ovulatory cycle [239 +/- 73 microU/mg creatinine (mid-follicular] to 244 +/- 41 microU/mg creatinine (mid-luteal)]. Two-way ANOVA revealed significant effects on urinary kallikrein for race (P < 0.001), cycle (P < 0.05), and race X cycle interaction (P < 0.05). Thus, white females had higher urinary kallikrein than black females, and demonstrated a significant increase in urinary kallikrein excretion during the ovulatory cycle, whereas no significant change in urinary kallikrein activity was seen in the black group. Enzyme kinetic studies and mixing studies demonstrated that these racial differences in renal kallikrein excretion were quantitative, rather than due to qualitative differences in the renal kallikrein enzyme or due to the presence of a kallikrein inhibitor.

CONCLUSIONS

These results suggest pronounced blunting of menstrual cycle changes in urinary kallikrein excretion in black females. Blunted urinary kallikrein responses during the ovulatory cycle are consistent with dysregulation of renal kallikrein biosynthetic responses in African-Americans, a group at increased risk for hypertension.

Ability of alternative indices of insulin sensitivity to predict cardiovascular risk: comparison with the "minimal model". Insulin Resistance Atherosclerosis Study (IRAS) Investigators

INTRODUCTION

Although recognition of insulin sensitivity as a risk factor for cardiovascular disease is growing, a deeper understanding of its role is impeded by the cost and complexity of currently available measures. This report evaluates previously described alternative indices of insulin sensitivity with the goal of identifying a reliable, but logistically simpler, alternative.

METHODS

Data from 1460 participants in the Insulin Resistance Atherosclerosis Study (IRAS) were used to assess the proportion of the relationship between a recognized measure of insulin sensitivity (Bergman's SI) and cardiovascular risk factors that is contained in each of nine alternative measures.

RESULTS

A number of the alternative indices contained a substantial proportion of the information available in Bergman's SI. The Galvin's index and the homeostasis model were most promising. However, there remained a significant amount of the information in Bergman's SI that was not contained in any of the alternative indices.

DISCUSSION

There are simpler alternative indices of insulin sensitivity for use in epidemiological studies, but each alternative is associated with some loss of information. It may be possible that this loss can be overcome with an increased sample size; however, using the alternative indices may also confound the assessment of insulin sensitivity with other underlying factors (i.e., hyperinsulinemia). The alternative indices are not recommended for the clinical assessment of insulin sensitivity for an individual patient or subject.

The relation of fasting insulin to blood pressure in a multiethnic population: the Miami Community Health Study

PURPOSE

The aim of this study was to examine the associations among fasting insulin, adiposity, waist girth, and blood pressure among a nondiabetic multiethnic population.

METHODS

A cross-sectional study was performed among 25-44-year-old African-Americans (n = 159), Cuban-Americans (n = 128), and non-Hispanic whites (n = 207) selected from Dade County, Florida. Fasting insulin levels were correlated with resting blood pressure level within each ethnic group. The separate effects of percentage body fat and waist girth on the association between blood pressure and insulin were analyzed in multiple linear regression and analysis of covariance.

RESULTS

Fasting insulin was positively associated with systolic (r = 0.26-0.39; P < 0.01) and diastolic blood pressure (r = 0.19-0.30; P = 0.10 to P < 0.001) among women of all ethnic groups and among non-Hispanic white men (r = 0.27; P < 0.05). Stepwise linear regression analyses revealed statistically significant associations between systolic and diastolic blood pressure and fasting insulin level in non-Hispanic whites independent of other covariates, including sex and percentage body fat (P < 0.001). Fasting insulin was also independently and significantly related to systolic blood pressure among African-Americans (P = 0.02). Among Cuban-Americans, sex and percentage body fat were the main correlates of blood pressure level. Analysis of covariance revealed a relationship between insulin and blood pressure that was independent of waist girth among men and women.

CONCLUSIONS

Fasting insulin level and blood pressure were positively associated among African-Americans and non-Hispanic whites. This association was not entirely due to the common association with percentage body fat or waist girth.

Sex hormone-binding globulin and insulin resistance in African-American women

Sex hormone-binding globulin (SHBG) binds testosterone, determining the level of free, biologically active hormone, and is a sensitive indicator of androgen status in women. SHBG is strongly correlated with high-density lipoprotein (HDL), central obesity, and insulin sensitivity in Caucasian and Mexican-American women, thereby acting as a biologic marker for cardiovascular disease risk. The purpose of this study was to determine if SHBG was a significant correlate of metabolic cardiovascular risk factors in African-American women. Eighty-one nondiabetic, normotensive African-American women were enrolled (mean age, 30 years). After excluding women on oral contraceptives (n = 19), 62 women were examined during the follicular phase of the menstrual cycle. All subjects underwent an oral glucose tolerance test (OGTT) and a euglycemic-hyperinsulinemic insulin clamp, and the lipid and sex hormone levels were measured. Correlation analyses showed a significant correlation between SHBG and the following variables in women: central obesity, body mass index (BMI), HDL cholesterol, apolipoprotein B (apoB), insulin sensitivity adjusted for lean mass (M'), and the sum of insulin during the OGTT. The strongest correlates of SHBG in women were measures of insulin resistance (r = .421, P < .001). SHBG appears to be a biologic marker for insulin resistance, which is linked to cardiovascular risk, in African-American women.

The relationship in African-Americans of sex differences in insulin-mediated suppression of nonesterified fatty acids to sex differences in fasting triglyceride levels

Insulin is a potent antilipolytic hormone that promotes the deposition of fat and decreases the release of nonesterified fatty acids (NEFA) from adipose tissue. The purpose of this study was to investigate in African-Americans (AAs) sex differences in insulin-mediated suppression of plasma NEFA and fasting triglyceride (TG) levels. Ninety AAs, 44 men and 46 women with a mean age of 34 +/- 8 years were classified by body mass index (BMI) into three groups: non-obese (22 men and 18 women), obese (12 men and 10 women), and severely obese (10 men and 18 women). In each BMI group, women versus men had greater percent body fat (non-obese, 30 +/- 6 v 18 +/- 6, P < .001; obese, 36 +/- 3 v 26 +/- 2, P < .001; and severely obese, 39 +/- 4 v 29 +/- 4, P < .001). An oral glucose tolerance test (OGTT) was performed with fasting TG levels and plasma insulin and NEFA concentrations obtained at 0, 30, 60, and 120 minutes. In women, insulin-mediated NEFA suppression was similar in each of the three BMI groups (non-obese, 85% +/- 14%; obese, 88% +/- 11%; and severely obese, 87% +/- 10%; P = .8). In men, the percent suppression of NEFA declined with increasing obesity (non-obese, 83% +/- 14%; obese, 71% +/- 21%; and severely obese, 68% +/- 16%; P = .04). Changes in NEFA suppression were reflected in the fasting TG levels. TG levels in women were similar in each BMI group (non-obese, 71 +/- 39 mg/dL; obese; 69 +/- 21; severely obese, 79 +/- 30; P = .7). In contrast, fasting TG levels for men were higher in the higher BMI groups. Plasma TG levels in men were 87 +/- 41 mg/dL for obese, 113 +/- 65 for obese, and 169 +/- 81 for severely obese (P = .001). These data demonstrate sex differences in insulin-mediated NEFA metabolism. In AA women, the maintenance of sensitivity to insulin-mediated suppression of NEFA regardless of the degree of obesity may contribute to the normal plasma TG levels. For AA men, the resistance to insulin-mediated suppression of NEFA in the higher BMI categories may allow more NEFA to be released from adipose tissue into the circulation and available to the liver for synthesis into TG-containing lipoproteins.

The genetic basis of atherosclerosis

Atherogenesis is a complex process that involves the contributions of several pathophysiological sub-systems. The dissection of the genetic component of atherosclerosis has become possible using current molecular technologies and analytical methods. Genetic factors are considered to determine the limits under which atherosclerosis develops and environmental factors are considered to position an individual's risk within these limits. Atherosclerosis proceeds through a well-characterized series of pathological stages that involve key cell types and the expression of particular gene products. Reductionist experimental models have helped to produce a list of several hundred candidate genes for the study of the genetic component of atherosclerosis. Within certain families and isolated communities the effect of a single candidate gene upon atherosclerosis susceptibility may be profound, as in the case of mutations in the gene encoding the low-density lipoprotein receptor, which produce familial hypercholesterolemia and premature atherosclerosis. However, particular candidate genes have small effects on atherosclerosis or to one of its intermediate phenotypes, in whole populations. In addition, pleiotropy and epistasis can confound the identification of the genetic component of atherosclerosis. Despite these limitations, it might still be possible to use genetic information clinically in order to classify individuals who are susceptible to atherosclerosis, especially if as yet undiscovered candidate genes are found to be important determinants of disease. However, it will be impossible to predict the onset of a clinical manifestation of atherosclerosis in a particular person. This is due to the confounding influence of other forces, such as variations in interindividual environmental landscape, non-linear interactions between genes and environment, and even the possible influence of biological chaos.