Insulin-resistant prediabetic subjects have more atherogenic risk factors than insulin-sensitive prediabetic subjects: implications for preventing coronary heart disease during the prediabetic state

BACKGROUND

Subjects who convert to type 2 diabetes mellitus have increased cardiovascular risk factors relative to nonconverters. However, it is not known whether these atherogenic changes in the prediabetic state are predominantly due to insulin resistance, decreased insulin secretion, or both.

METHODS AND RESULTS

We examined this issue in the 7-year follow-up of the San Antonio Heart Study, in which 195 of 1734 subjects converted to type 2 diabetes. At baseline, converters had significantly higher body mass index, waist circumference, triglyceride concentration, and blood pressure and lower HDL cholesterol than nonconverters. Atherogenic changes in converters were markedly attenuated (and no longer significant) after adjustment for the homeostasis model assessment of insulin resistance (HOMA IR, a surrogate for insulin resistance); in contrast, the differences in risk factors between converters and nonconverters increased after adjustment for the ratio of early insulin increment to early glucose increment (DeltaI(30-0)/DeltaG(30-0)) during an oral glucose tolerance test (a surrogate for insulin secretion). We also compared converters who had a predominant insulin resistance (high HOMA IR and high DeltaI(30-0)/DeltaG(30-0)) (n=56) and converters who had a predominant decrease in insulin secretion (low HOMA IR and low DeltaI(30-0)/DeltaG(30-0)) (n=31) with nonconverters (n=1539). Only the converters who were insulin resistant had higher blood pressure and triglyceride levels and lower HDL cholesterol levels than nonconverters.

CONCLUSIONS

Our data suggest that atherogenic changes in the prediabetic state are mainly seen in insulin-resistant subjects and that strategies to prevent type 2 diabetes might focus on insulin-sensitizing interventions rather than interventions that increase insulin secretion because of potential effects on cardiovascular risk.

Birth weight and the metabolic syndrome: thrifty phenotype or thrifty genotype?

BACKGROUND

Inverse correlations have been reported between birth weight and the Metabolic Syndrome (abdominal obesity, insulin resistance, hyperinsulinemia, glucose intolerance, dyslipidemia, and hypertension). These correlations are thought to reflect primarily nutritional inadequacies during fetal and early life. We explored familial influences on these correlations.

METHODS

Using birth weight data on 602 subjects from 65 pedigrees, we partitioned phenotypic correlations into familial and non-familial. The former are usually regarded as reflecting primarily genetic influences, although they may also reflect environmental influences that are shared by family members, and the latter reflect random environmental influences.

RESULTS

A consistent pattern of positive familial and inverse non-familial correlations were observed. The strongest familial correlations were between birth weight and fasting insulin (r = 0.58, p = 0.002), leptin (r = 0.48, p = 0.021), split proinsulin (r=0.51, p = 0.090), and heart rate (r = 0.39, p = 0.037). An inverse familial correlation was observed with HDL cholesterol (r = -0.28, p = 0.018). Non-familial correlations were weaker and only two-- subscapular-to-triceps skinfold ratio and fasting insulin--were statistically significant.

CONCLUSION

Since the familial and non-familial correlations were in opposite directions, we attribute the former to the pleiotropic effects of genes. Specifically, we conclude that genes that increase birth weight also worsen the Metabolic Syndrome in adult life. Since the inverse correlations reported in the literature reflect mainly cohorts born in the early part of the 20th century, improved maternal nutrition since then may have allowed the expression of genetic influences in our participants, all of whom were born after 1950.

Do Mexican Americans really have low rates of cardiovascular disease?

In this article we challenge the conclusion made from vital statistics that Hispanic Americans have lower all-cause and cardiovascular disease (CVD) mortality than non-Hispanic whites. There is reason to believe that vital statistics underascertain minority, and in particular Hispanic, deaths. Cohort studies minimize many of these limitations. In the San Antonio Heart Study risk factor distributions predicted higher all-cause and CVD mortality among Mexican Americans than among non-Hispanic whites. Follow-up of the cohort confirmed a mortality ratio of 1.38 for all-cause and 1.30 for CVD mortality for Mexican Americans vs non-Hispanic whites. This excess risk was confined to U.S.-born Mexican Americans, since immigrants from Mexico had very low mortality despite low socioeconomic status. We attribute this latter finding to a "healthy migrant effect."

A quantitative scale of acanthosis nigricans

OBJECTIVE

To develop and validate a scale for acanthosis nigricans (AN).

RESEARCH DESIGN AND METHODS

Subjects were participants from the San Antonio Family Diabetes Study and the San Antonio Family Heart Study. A total of 406 subjects were independently examined for AN by at least two observers. Five locations were examined: the neck, axilla, elbows, knuckles, and knees. Interobserver concordance and kappa statistics were calculated to determine replicability of the scale. Comparisons of diabetes-related risk factors by AN score were also calculated.

RESULTS

Only the neck had consistently high kappa statistics, and thus, other locations were excluded from further analyses. Elevated AN was strongly associated with elevated fasting insulin and BMI in both diabetic and nondiabetic subjects. Elevated AN was also strongly associated with elevated fasting glucose, systolic blood pressure, and diastolic blood pressure, and with decreased HDL in nondiabetic subjects. In diabetic subjects, elevated AN was associated with elevated total cholesterol.

CONCLUSIONS

We have developed a scale for AN that is easy to use, has high interobserver reliability in Mexican Americans, and correlates well with fasting insulin and BMI. This scale will permit longitudinal and cross-sectional evaluation of AN and will permit the evaluation of AN as a trait in genetic studies.

Normal variation in leptin levels in associated with polymorphisms in the proopiomelanocortin gene, POMC

We previously reported that our genome-scanning initiative had detected a highly significant linkage (log odds ratio = 4.95; P = 9 x 10(-7)) between a quantitative trait locus (QTL) on chromosome 2 and leptin levels in Mexican American families. We now have typed additional microsatellite markers in this region, increasing this log odds ratio score to 7.46 (P = 2 x 10(-9)). This region of chromosome 2 contains a strong positional candidate gene, POMC. The POMC gene codes for POMC, the prohormone from which alphaMSH, ACTH, and beta-endorphin are derived. Studies by others have shown that POMC-derived products are involved in the regulation of appetite and obesity. We have used polymorphisms in POMC to map its location within the 95% confidence interval of the peak for the linkage signal for the QTL. We also constructed POMC haplotypes using these polymorphisms and have found a significant association with normal variation in leptin levels (P = 0.001). We conclude that variation in POMC is associated with normal variation in serum leptin levels, providing further evidence that POMC may be the leptin QTL previously identified in Mexican American families.

Rapid rise in the incidence of type 2 diabetes from 1987 to 1996: results from the San Antonio Heart Study

BACKGROUND

The prevalence of type 2 diabetes has increased in the early part of the 20th century, particularly in developing countries. There is now evidence that the prevalence also continues to increase in developed countries, including the United States. However, it is unknown whether this increase is due to a rise in the incidence of diabetes or to decreasing diabetic mortality or both.

METHODS

Participants in the San Antonio Heart Study, who were nondiabetic at baseline and who returned for a 7- to 8-year follow-up examination, were examined for secular trends in the incidence of type 2 diabetes. Risk factors for diabetes, such as obesity, were also examined. Patients were enrolled in the San Antonio Heart Study from 1979 to 1988 and 7- to 8-year incidence of diabetes was determined from 1987 to 1996.

RESULTS

A significant secular trend in the 7- to 8-year incidence of type 2 diabetes was observed in Mexican Americans (5.7% for participants enrolled in 1979 to 15.7% for participants enrolled in 1988). In non-Hispanic whites, the incidence increased from 2.6% for participants enrolled in 1980 to 9.4% for participants enrolled in 1988 (P = .07) . After adjusting for age and sex, the secular trend remained significant in Mexican Americans and borderline significant in non-Hispanic whites. This indicates that between 1987 and 1996 the 7- to 8-year incidence of type 2 diabetes approximately tripled in both ethnic groups. The overall secular trend also remained significant after adjusting for additional risk factors for diabetes, such as obesity. A rising secular trend in obesity was also observed.

CONCLUSIONS

There has been a significant increasing secular trend in the incidence of type 2 diabetes in Mexican Americans and a borderline significant trend in non-Hispanic whites participating in the San Antonio Heart Study. Unlike other cardiovascular risk factors such as lipid levels, cigarette smoking, and blood pressure, which are either declining or under progressively better medical management and control, and unlike cardiovascular mortality, which is also declining, obesity and type 2 diabetes are exhibiting increasing trends. Thus, obesity and diabetes could easily become the preeminent US public health problem.

Prevalence of hypertension in a Mexican population according to the Sixth Report of the Joint National Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure

BACKGROUND

Hypertension in Mexico represents a challenging public health problem. The National Survey on Chronic Diseases published in 1993 reported that hypertension affects more than 10 million Mexicans. No information has been published regarding the prevalence of hypertension in Mexico using the new diagnostic criteria established by the Sixth Report of the Joint National Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure (JNC VI).

METHODS

The Mexico City Diabetes Study is a prospective study designed to estimate the prevalence and incidence of cardiovascular risk factors in a low-income area. The survey included 941 men and 1341 non-pregnant women aged 35-64 years. Blood pressure measurements were performed using a random zero sphygmomanometer. The diagnostic criteria for hypertension were those recommended by the JNC VI.

RESULTS

The crude prevalence of hypertension was 17.2% and 18.1% in men and women, respectively. We found significant associations between hypertension and obesity, body fat distribution, very-low-density lipoprotein cholesterol, fasting and 2-h post-glucose in both sexes, and between hypertension and total cholesterol, low-density lipoprotein cholesterol and triglycerides levels in women. In 40% of hypertensive men and 23% of women, hypertension was undiagnosed and untreated. Of the previously diagnosed hypertensive individuals, 38% of men and 30% of women reported not taking antihypertensive medicine. The prevalence++ of associated risk factors in this population is 12.3% for tobacco consumption, 22.4% for diabetes, 49.8% for hypertriglyceridemia and 40.9% for hypercholesterolemia.

CONCLUSIONS

Hypertension occurs in 18% of this population. There is a high prevalence of undiagnosed and untreated cases. Associated cardiovascular risk factors are highly prevalent.

Diabetes and hypertension in Mexican American families: relation to cardiovascular risk

There is a strong familial predisposition to type 2 diabetes, hypertension, and cardiovascular disease. The authors evaluated the association between a family history of these diseases and a large panel of cardiovascular risk factors in 1,431 Mexican American subjects who were enrolled in the San Antonio Family Heart Study in San Antonio, Texas. The baseline phase of the study covered 1992-1996. Diabetes and hypertension were diagnosed according to standard clinical criteria, while cardiovascular disease was defined as a history of heart attack or heart surgery. The prevalence of diabetes, hypertension, and cardiovascular disease in this population was 15%, 12%, and 3%, respectively. For each unaffected subject, the authors computed a family history score based on the presence or absence of disease in parents and older siblings, and correlations between cardiovascular risk factors and family history scores were estimated by using likelihood-based variance component methods. Diabetes family history score was significantly correlated with a broad panel of cardiovascular risk factors, including glucose and insulin, obesity, blood pressure, triglycerides, and total cholesterol. Hypertension family history score was significantly correlated with glucose, blood pressure, body mass index, waist circumference, total cholesterol, and triglycerides. These results support the idea that genes that confer a risk for diabetes, and to a lesser extent hypertension, adversely alter the cardiovascular risk profile long before the manifestation of clinical disease.

Human pedigree-based quantitative-trait-locus mapping: localization of two genes influencing HDL-cholesterol metabolism

Common disorders with genetic susceptibilities involve the action of multiple genes interacting with each other and with environmental factors, making it difficult to localize the specific genetic loci responsible. An important route to the disentangling of this complex inheritance is through the study of normal physiological variation in quantitative risk factors that may underlie liability to disease. We present an analysis of HDL-cholesterol (HDL-C), which is inversely correlated with risk of heart disease. A variety of HDL subphenotypes were analyzed, including HDL particle-size classes and the concentrations and proportions of esterified and unesterified HDL-C. Results of a complete genomic screen in large, randomly ascertained pedigrees implicated two loci, one on chromosome 8 and the other on chromosome 15, that influence a component of HDL-C-namely, unesterified HDL2a-C. Multivariate analyses of multiple HDL phenotypes and simultaneous multilocus analysis of the quantitative-trait loci identified permit further characterization of the genetic effects on HDL-C. These analyses suggest that the action of the chromosome 8 locus is specific to unesterified cholesterol levels, whereas the chromosome 15 locus appears to influence both HDL-C concentration and distribution of cholesterol among HDL particle sizes.

[Incidence and progression of diabetic retinopathy in low income population of Mexico City]

Diabetic retinopathy is one of the leading causes of preventable blindness in working age population. Diabetes mellitus and this microvascular complication affects frequently Mexican population and presents itself in severe clinical forms. There are no incidence studies of diabetic retinopathy in Mexico. The four year incidence and progression of diabetic retinopathy were investigated in low income diabetic patients of Mexico City. In the follow up phase we studied 164 patients, 76.6% of the patients studied at baseline, 63 were men and 101 women. All participants had a complete ophthalmological exam and seven field stereo photographs. All photographs were graded using internationally accepted criteria in the reading center of our institution. The four year incidence of any level of retinopathy was 22.5%. Worsening of retinopathy occurred in 20.6% and the proliferative diabetic retinopathy stage was reached in 4.5%. Incidence of diabetic retinopathy was associated to age at diagnosis of diabetes mellitus of less than 45 years and progression was associated to duration of disease of more than ten years. The four year incidence of macular edema was 8.8%. These data are important to plan strategies for prevention of blindness and the implementation of optimal care of diabetic patients in our country.

Genetic determinants of variation in gallbladder disease in the Mexican-American population

Since there have not been any studies that quantify the influence of genetic factors on gallbladder disease (GBD) in humans using information from families, we utilized pedigree data to explore the genetic control of variation in liability to GBD. Using an extension of a variance components approach, we performed genetic analyses of GBD using information from 32 low-income Mexican-American families with two slightly different general models incorporating several sex-specific GBD risk factors. After evaluating the relative magnitudes of the covariate effects from these two models, we identified a parsimonious model including only significant predictors of GBD. According to this model, heritability for GBD was high (h2 = 0.44+/-0.18), after accounting for the significant effects of age, leptin in both sexes, total cholesterol, and HDL cholesterol in males only. We have shown quantitatively that variation in GBD is under strong genetic control. However, there are two major limitations to our findings: (1) since GBD was defined by a self-reported clinical history rather than an ultrasound examination, the prevalence of GBD could have been underestimated; and (2) since our design did not allow for shared environmental effects, our estimate of heritability may have been inflated.

Linkage of type 2 diabetes mellitus and of age at onset to a genetic location on chromosome 10q in Mexican Americans

Since little is known about chromosomal locations harboring type 2 diabetes-susceptibility genes, we conducted a genomewide scan for such genes in a Mexican American population. We used data from 27 low-income extended Mexican American pedigrees consisting of 440 individuals for whom genotypic data are available for 379 markers. We used a variance-components technique to conduct multipoint linkage analyses for two phenotypes: type 2 diabetes (a discrete trait) and age at onset of diabetes (a truncated quantitative trait). For the multipoint analyses, a subset of 295 markers was selected on the basis of optimal spacing and informativeness. We found significant evidence that a susceptibility locus near the marker D10S587 on chromosome 10q influences age at onset of diabetes (LOD score 3.75) and is also linked with type 2 diabetes itself (LOD score 2.88). This susceptibility locus explains 63.8%+/-9.9% (P=. 000016) of the total phenotypic variation in age at onset of diabetes and 65.7%+/-10.9% (P=.000135) of the total variation in liability to type 2 diabetes. Weaker evidence was found for linkage of diabetes and of age at onset to regions on chromosomes 3p, 4q, and 9p. In conclusion, our strongest evidence for linkage to both age at onset of diabetes and type 2 diabetes itself in the Mexican American population was for a region on chromosome 10q.

Variant in sulfonylurea receptor-1 gene is associated with high insulin concentrations in non-diabetic Mexican Americans: SUR-1 gene variant and hyperinsulinemia

The high-affinity sulfonylurea receptor (SUR1) gene regulates insulin secretion and may play a role in type 2 diabetes. A silent variant in exon 31 of SUR1 (AGG-->AGA) was detected by single-strand conformational polymorphism and genotypes were determined for 396 Mexican American subjects (289 non-diabetic). The normal and mutant alleles were designated G and A, respectively. Among non-diabetics, those with the AA genotype had higher fasting insulin values than those with the AG and GG genotypes (113.4 pmol/l for AA vs 82.8 pmol/l for AG/GG, P=0.043). Similar results were observed for 2-h insulin (849.6 pmol/l for AA vs 498.6 pmol/l for AG/GG, P=0.0003) and for the proinsulin to specific insulin ratio (0.068 for AA vs 0.056 for AG/GG, P=0.030). Specific insulin levels also differed significantly across the three genotypic classes (P=0.021). No differences in fasting glucose, body mass index, or waist circumference according to genotype were noted. Two-hour glucose was modestly higher in individuals with the AA genotype. Since we have previously reported linkage between SUR1 and hyperglycemia, the present association between a SUR1 variant and hyperinsulinemia in normal individuals from a high diabetes risk ethnic group raises the possibility of primary insulin hypersecretion as an antecedent of type 2 diabetes in at least some individuals from this population.

Leptin concentrations are associated with higher proinsulin and insulin concentrations but a lower proinsulin/insulin ratio in non-diabetic subjects

INTRODUCTION

Leptin (the product of the human OB gene) is increased in obese individuals, suggesting resistance to its effect. However, there is considerable variability in leptin levels at each level of body mass index (BMI), suggesting that genetic and environmental factors may regulate leptin concentrations. Previous data have suggested that leptin levels are associated with insulin resistance and in a few reports with impaired insulin secretion. We examined whether non-diabetic subjects, with elevated specific insulin and proinsulin levels, had increased leptin levels.

METHODS

We used a radioimmunoassay (RIA) to measure serum leptin levels in 197 non-diabetic Mexican Americans and non-Hispanic whites from the San Antonio Heart Study. We also evaluated whether leptin levels were associated with impaired insulin secretion or increased beta cell stress, as evaluated by the fasting proinsulin/insulin ratio. (Higher fasting proinsulin/insulin ratios are thought to reflect impaired insulin secretion.)

RESULTS

Leptin levels were significantly correlated with fasting specific insulin (r=0.55, P<0.001) and proinsulin (r=0.57, P<0.001) and inversely with the proinsulin/insulin ratio (r= -0.154, P=0.035) after adjustment for age, gender, ethnicity and 2 h glucose. These associations were similar in men and women and in Mexican Americans and in non-Hispanic whites. After further adjustment for BMI and waist-to-hip ratio (WHR), leptin levels remained significantly correlated with specific insulin (r=0.31, P<0.001) and proinsulin (r=0.24, P<0.001) although the magnitude of the associations were considerably attenuated.

CONCLUSION

We conclude that specific insulin and proinsulin are positively related to leptin levels and that these associations are to some degree independent of obesity and body fat distribution. Thus, subjects with increased insulin levels may be relatively resistant to the effects of leptin. However, leptin levels are associated with a decrease in the fasting proinsulin/insulin ratio suggesting that leptin levels are not associated with an impairment in insulin secretion.

Reversion from type 2 diabetes to nondiabetic status. Influence of the 1997 American Diabetes Association criteria

OBJECTIVE

To determine the incidence and the rate of reversion of type 2 diabetes to a nondiabetic status in the 7- to 8-year follow-up of the San Antonio Heart Study, and to determine the influence of the recent 1997 American Diabetes Association (ADA) criteria for diabetes on these rates. Individuals who revert have been problematic for those developing criteria for the diagnosis of type 2 diabetes. Few studies have addressed this issue using 1979 National Diabetes Data Group/1980 World Health Organization (WHO) criteria.

RESEARCH DESIGN AND METHODS

We studied 3,682 Mexican-American and non-Hispanic white men and nonpregnant women who completed both the baseline and follow-up examination of the San Antonio Heart Study. Incidence and reversion rates were calculated using both the 1980 WHO and the 1997 ADA criteria. Risk factors for reversion were identified, and the best fitting model using multiple logistic regression was determined using both the 1980 WHO and the 1997 ADA criteria.

RESULTS

Using the 1997 ADA criteria, the age-adjusted incidences of type 2 diabetes for Mexican-American men and women were 10.8 and 12.2%, respectively. For non-Hispanic white men and women, the age-adjusted incidence rates were 5.5 and 5.1%, respectively. Similar age-adjusted incidences were recorded using the 1980 WHO criteria. The reversion rate for individuals with type 2 diabetes was 11.5% using the 1980 WHO criteria and 12.5% using the 1997 ADA criteria. These rates were not significantly different. Numerous risk factors for reversion were identified. The best fitting model, after controlling for age, sex, and ethnicity, included baseline 2-h glucose level, baseline HDL cholesterol, and previous diagnosis of diabetes. The models were the same for both the 1980 WHO and the 1997 ADA criteria.

CONCLUSIONS

There was no significant difference in the incidence or the reversion rates for diabetic subjects using either 1980 WHO or 1997 ADA criteria. In addition, the risk factors for reversion were very similar using either set of criteria. The revision of the ADA criteria did not have a significant influence on reversion in this study.

Leptin concentrations do not predict weight gain: the Mexico City Diabetes Study

BACKGROUND

Leptin, a hormone which is produced by adipose tissue, has been shown to inhibit food intake and increase energy expenditure. In humans, leptin levels are correlated with body fat. In addition, leptin levels decline in subjects who lose weight. Yet few data exist on whether leptin levels predict weight change, except for a recent report suggesting that low leptin levels predict weight gain in very obese middle-aged Pima Indians.

METHODS

We have examined the association between baseline leptin levels and subsequent weight gain over 3.25 y in 180 non-diabetic participants in the Mexico City Diabetes Study.

RESULTS

At baseline, the correlation between leptin levels and body mass index (BMI) was 0.712 in men and 0.691 in women (both P < 0.001). Subjects were matched on age (+/- 2 y), gender and BMI (+/- 2 kg/m2) at baseline. Baseline BMI was 25.3 kg/m2 in men and 27.2 kg/m2 in women. Baseline leptin levels (ng/ml) did not predict weight gain in either men (weight gainers: 4.3; weight stable: 5.8; and weight losers: 5.2) or women (weight gainers: 17.4; weight stable: 17.7; and weight losers: 17.4).

CONCLUSIONS

We conclude that baseline leptin levels did not predict weight change in moderately obese individuals.

Effects of diabetes and level of glycemia on all-cause and cardiovascular mortality. The San Antonio Heart Study

OBJECTIVE

Although the level of hyperglycemia is clearly a risk factor for microvascular complications in diabetic patients, its role in macrovascular complications remains controversial. We followed 4,875 subjects (65% Mexican-American) for 7-8 years to investigate the effects of diabetes and hyperglycemia on all-cause and cardiovascular disease (CVD) mortality. These end points were also analyzed according to quartiles of baseline fasting plasma glucose among diabetic participants.

RESEARCH DESIGN AND METHODS

The Cox proportional hazards model was used to estimate the relative risks (RRs) for all-cause and CVD mortality.

RESULTS

Diabetes was significantly associated with increased all-cause mortality (RR [95% CI] = 2.1 [1.3-3.5] in men; 3.2 [1.9-5.4] in women) and increased CVD mortality (3.2 [1.4-7.1] in men; 8.5 [2.8-25.2] in women). Among diabetic subjects, those in quartile 4 had a 4.2-fold greater risk of all-cause mortality (P < 0.001) and a 4.7-fold greater risk of CVD mortality (P = 0.01) than those in quartiles 1 and 2 combined. After further adjustment for other potential risk factors, subjects in quartile 4 had a 4.9-fold greater risk of all-cause mortality and a 4.9-fold greater risk of CVD mortality than those in quartiles 1 and 2. In addition, hypertension, current smoking, and cholesterol > 6.2 mmol/l were significant predictors of CVD mortality using Cox models.

CONCLUSIONS

We conclude that diabetes is a predictor of both all-cause and CVD mortality in the general population and that both hyperglycemia and common CVD risk factors are important predictors of all-cause and CVD mortality in diabetic subjects.

The effect of glycemic control on the incidence of macrovascular complications of type 2 diabetes

Of all the complications of diabetes mellitus, macrovascular complications, ie, large-vessel atherosclerosis, account for the largest share of morbidity, mortality, and health care expenditures. Whereas there is now highly persuasive evidence that glycemic control reduces the risk of microvascular complications in type 1 diabetes, and probably in type 2 diabetes as well, such evidence is unavailable for macrovascular complications. Prospective epidemiologic studies, however, indicate that poor glycemic control enhances cardiovascular risk, and a number of biochemical mechanisms have been advanced to explain this phenomenon. However, data from animal studies, in vitro studies, and prospective epidemiologic studies suggest that endogenous insulin or insulin resistance may be atherogenic. Thus, a dilemma exists for insulin treatment, although the weight of evidence still favors its aggressive use. For persons whose glycemia can be adequately controlled with oral agents, the use of agents such as metformin and troglitazone--which do not raise, and may even lower, insulin concentrations--may offer an advantage. Definitive clinical trials on the benefits and risks of insulin therapy related to macrovascular complications are lacking and urgently needed.

A paired sibling analysis of the beta-3 adrenergic receptor and obesity in Mexican Americans

The beta3 adrenergic receptor, located on chromosome 8, is a regulator of energy expenditure and lipolysis. A missense mutation in this gene, characterized by the replacement of tryptophan by arginine at codon 64 (Trp64Arg), is associated with obesity in some studies. We examined the effect of this variant on obesity in Mexican Americans, using a paired sibling design to minimize variability due to genetic background and a previously identified major susceptibility locus for obesity. We identified 45 sib-pairs that were concordant (identical by descent) for a locus on chromosome 2 which we have shown previously to be tightly linked to obesity in this population. The Trp64Arg variant, detected by PCR-restriction fragment length polymorphism analysis, was present in one sibling within each of the 45 sib-pairs. Presence of the variant was associated with significantly higher values in body mass index (P = 0.04), fat mass (P = 0.04), and waist circumference (P = 0.05). We conclude that the Trp64Arg variant is associated with obesity in this Mexican American population. The paired sibling design probably enhanced our ability to detect the effects of this variant by allowing us to account for variation attributable to another obesity susceptibility locus and to background genes.