Ethnic differences in glucose disposal, hepatic insulin sensitivity, and endogenous glucose production among African American and European American women

Proinsulin-to-C-Peptide Ratio as a Marker of β-Cell Function in African American and European American Adults

OBJECTIVE

The primary purpose of the current study was to test the hypothesis that the proinsulin-to-C-peptide (PI-to-CP) ratio, as an index of proinsulin secretion, would be higher and associated with indices of β-cell function in African American adults relative to European American adults without type 2 diabetes.

RESEARCH DESIGN AND METHODS

Participants were 114 African American and European American adult men and women. A 2-h oral glucose tolerance test was conducted to measure glucose, insulin, C-peptide, and proinsulin and derive indices of β-cell response to glucose. The Matsuda index was calculated as a measure of insulin sensitivity. The disposition index (DI), the product of insulin sensitivity and β-cell response, was calculated for each phase of β-cell responsivity. Pearson correlations were used to investigate the relationship of the PI-to-CP ratio with each phase of β-cell response (basal, Φb; dynamic, Φd; static, Φs; total, Φtot), disposition indices (DId, DIs, DItot), and insulin sensitivity. Multiple linear regression analysis was used to evaluate independent contributions of race, BMI, and glucose tolerance status on PI-to-CP levels before and after adjustment for insulin sensitivity.

RESULTS

African American participants had higher fasting and 2-h PI-to-CP ratios. The fasting PI-to-CP ratio was positively associated with Φb, and the fasting PI-to-CP ratio and 2-h PI-to-CP ratio were inversely associated with DId and insulin sensitivity only in African American participants.

CONCLUSIONS

The PI-to-CP ratio could be useful in identifying African American individuals at highest risk for β-cell dysfunction and ultimately type 2 diabetes.

Intersectionality in Alzheimer's Disease: The Role of Female Sex and Black American Race in the Development and Prevalence of Alzheimer's Disease

It is well known that vascular factors and specific social determinants of health contribute to dementia risk and that the prevalence of these risk factors differs according to race and sex. In this review, we discuss the intersection of sex and race, particularly female sex and Black American race. Women, particularly Black women, have been underrepresented in Alzheimer's disease clinical trials and research. However, in recent years, the number of women participating in clinical research has steadily increased. A greater prevalence of vascular risk factors such as hypertension and type 2 diabetes, coupled with unique social and environmental pressures, puts Black American women particularly at risk for the development of Alzheimer's disease and related dementias. Female sex hormones and the use of hormonal birth control may offer some protective benefits, but results are mixed, and studies do not consistently report the demographics of their samples. We argue that as a research community, greater efforts should be made to not only recruit this vulnerable population, but also report the demographic makeup of samples in research to better target those at greatest risk for the disease.

Associations of atrial natriuretic peptide with measures of insulin and adipose depots

Low concentrations of natriuretic peptides (NPs) have been associated with greater risk for Type 2 diabetes (T2D). African American individuals (AA) have lower NP levels and are disproportionately burdened by T2D. The purpose of this study was to test the hypothesis that higher post-challenge insulin in AA adults is associated with lower plasma N-terminal pro-atrial natriuretic peptide (NT-proANP). A secondary purpose was to explore associations between NT-proANP and adipose depots. Participants were 112 AA and European American (EA) adult men and women. Measures of insulin were obtained from an oral glucose tolerance test and hyperinsulinemic-euglycemic glucose clamp. Total and regional adipose depots were measured from DXA and MRI. Multiple linear regression analysis was used to assess associations of NT-proANP with measures of insulin and adipose depots. Lower NT-proANP concentrations in AA participants was not independent of 30-min insulin area under the curve (AUC). NT-proANP was inversely associated with 30-min insulin AUC in AA participants, and with fasting insulin and HOMA-IR in EA participants. Thigh subcutaneous adipose tissue and perimuscular adipose tissue were positively associated with NT-proANP in EA participants. Higher post-challenge insulin may contribute to lower ANP concentrations in AA adults.

Association between vitamin D serum levels and insulin resistance assessed by HOMA-IR among non-diabetic adults in the United States: Results from NHANES 2007-2014

Insulin resistance, a pathological response to insulin hormone in insulin-dependent cells, is characterized by the presence of high glucose and insulin concentrations. The homeostasis model of insulin resistance (HOMA-IR) is one of the most used indexes to estimate insulin resistance by assessing the fasting glucose and insulin levels. An association was observed between vitamin D levels and insulin resistance, which varied in different ethnic groups, and there is some evidence that vitamin D supplementation could contribute to the improvement of insulin resistance. This study assessed the association between 25-hydroxyvitamin D (25[OH]D) concentration and HOMA-IR in American adults aged 20 years and older, without diabetes and other chronic diseases that can influence insulin resistance. The data from the National Health and Nutrition Examination Survey (NHANES) 2007–2014 were used by exploiting the free and publicly-accessible web datasets. Linear regression models were performed to evaluate the association between serum 25(OH)D concentration and HOMA-IR, and a negative association was observed, which remained significant following the adjustment for age, gender, race/ethnicity, education, body mass index (BMI), physical activity, the season of examination, current smoking, hypertension, the use of drugs which can influence insulin resistance, serum bicarbonates, triglycerides, and calcium and phosphorus levels. Only in non-Hispanic Blacks was this inverse association between vitamin D and HOMA-IR not observed in the fully adjusted model. Further studies are needed to explain the mechanisms of the observed ethnic/racial differences in the association of vitamin D levels with HOMA-IR.

Ectopic fat deposition in populations of black African ancestry: A systematic review and meta-analysis

AIMS

In populations of black African ancestry (BA), a paradox exists whereby lower visceral adipose tissue is found despite their high risk for type 2 diabetes (T2D). This systematic review investigates ethnic differences in other ectopic fat depots (intrahepatic lipid: IHL; intramyocellular lipid: IMCL and intrapancreatic lipid; IPL) to help contextualise their potential contribution to T2D risk.

METHODS

A systematic literature search was performed in December 2020 to identify studies reporting at least one ectopic fat comparison between BA and one/more other ethnicity. For IHL, a meta-analysis was carried out with studies considered comparable based on the method of measurement.

RESULTS

Twenty-eight studies were included (IHL: n = 20; IMCL: n = 8; IPL: n = 4). Meta-analysis of 11 studies investigating IHL revealed that it was lower in BA populations vs pooled ethnic comparators (MD -1.35%, 95% CI -1.55 to -1.16, I² = 85%, P < 0.00001), white European ancestry (MD -0.94%, 95% CI -1.17 to -0.70, I² = 79%, P < 0.00001), Hispanic ancestry (MD -2.06%, 95% CI -2.49 to -1.63, I² = 81%, P < 0.00001) and South Asian ancestry comparators (MD -1.92%, 95% CI -3.26 to -0.57, I² = 78%, P = 0.005). However, heterogeneity was high in all analyses. Most studies found no significant differences in IMCL between BA and WE. Few studies investigated IPL, however, indicated that IPL is lower in BA compared to WE and HIS.

CONCLUSION

The discordance between ectopic fat and greater risk for T2D in BA populations raises questions around its contribution to T2D pathophysiology in BA.

Differences in gut microbiome by insulin sensitivity status in Black and White women of the National Growth and Health Study (NGHS): A pilot study

The prevalence of overweight and obesity is greatest amongst Black women in the U.S., contributing to disproportionately higher type 2 diabetes prevalence compared to White women. Insulin resistance, independent of body mass index, tends to be greater in Black compared to White women, yet the mechanisms to explain these differences are not completely understood. The gut microbiome is implicated in the pathophysiology of obesity, insulin resistance and cardiometabolic disease. Only two studies have examined race differences in Black and White women, however none characterizing the gut microbiome based on insulin sensitivity by race and sex. Our objective was to determine if gut microbiome profiles differ between Black and White women and if so, determine if these race differences persisted when accounting for insulin sensitivity status. In a pilot cross-sectional analysis, we measured the relative abundance of bacteria in fecal samples collected from a subset of 168 Black (n = 94) and White (n = 74) women of the National Growth and Health Study (NGHS). We conducted analyses by self-identified race and by race plus insulin sensitivity status (e.g. insulin sensitive versus insulin resistant as determined by HOMA-IR). A greater proportion of Black women were classified as IR (50%) compared to White women (30%). Alpha diversity did not differ by race nor by race and insulin sensitivity status. Beta diversity at the family level was significantly different by race (p = 0.033) and by the combination of race plus insulin sensitivity (p = 0.038). Black women, regardless of insulin sensitivity, had a greater relative abundance of the phylum Actinobacteria (p = 0.003), compared to White women. There was an interaction between race and insulin sensitivity for Verrucomicrobia (p = 0.008), where among those with insulin resistance, Black women had four fold higher abundance than White women. At the family level, we observed significant interactions between race and insulin sensitivity for Lachnospiraceae (p = 0.007) and Clostridiales Family XIII (p = 0.01). Our findings suggest that the gut microbiome, particularly lower beta diversity and greater Actinobacteria, one of the most abundant species, may play an important role in driving cardiometabolic health disparities of Black women, indicating an influence of social and environmental factors on the gut microbiome.

β-cell function in black South African women: exploratory associations with insulin clearance, visceral and ectopic fat

The role of ectopic fat, insulin secretion and clearance in the preservation ofβ-cell function in black African women with obesity who typically present with hyperinsulinaemia is not clear. We aim to examine the associations between disposition index (DI, an estimate of β-cell function), insulin secretion and clearance and ectopic fat deposition. This is a cross-sectional study of 43 black South African women (age 20-35 years) with obesity (BMI 30-40 kg/m2) and without type 2 diabetes that measured the following: DI, insulin sensitivity (SI), acute insulin response (AIRg), insulin secretion rate (ISR), hepatic insulin extraction and peripheral insulin clearance (frequently sampled i.v. glucose tolerance test); pancreatic and hepatic fat, visceral adipose tissue (VAT) and abdominal s.c. adipose tissue (aSAT) volume (MRI), intra-myocellular (IMCL) and extra-myocellular fat content (EMCL) (magnetic resonance spectroscopy). DI correlated positively with peripheral insulin clearance (β 55.80, P = 0.002). Higher DI was associated with lower VAT, pancreatic fat and soleus fat, but VAT explained most of the variance in DI (32%). Additionally, higher first phase ISR (P = 0.033) and lower hepatic insulin extraction (P = 0.022) were associated with lower VAT, independent from SI, rather than with ectopic fat. In conclusion, peripheral insulin clearance emerged as an important correlate of DI. However, VAT was the main determinant of a lower DI above ectopic fat depots. Importantly, VAT, but not ectopic fat, is associated with both lower insulin secretion and higher hepatic insulin extraction. Prevention of VAT accumulation in young black African women should, therefore, be an important target for beta cell preservation.

Disparities in cardio metabolic risk between Black and White women with polycystic ovary syndrome: a systematic review and meta-analysis

OBJECTIVE

We conducted a systematic review and meta-analysis to summarize and quantitatively pool evidence on cardiometabolic health disparities between Black and White women with polycystic ovary syndrome in the United States in response to the call for further delineation of these disparities in the international evidence-based guideline for the assessment and management of polycystic ovary syndrome.

DATA SOURCES

Databases of MEDLINE, Web of Science, and Scopus were searched initially through March 05, 2020, and confirmed on September 11, 2020.

STUDY ELIGIBILITY CRITERIA

Observational studies documenting cardiometabolic risk profile (glucoregulatory, lipid profile, anthropometric, and blood pressure status) in Black and White women with polycystic ovary syndrome were included. Studies on children (<17 years old) and pregnant or menopausal-aged women (>50 years) were excluded. The primary outcome was fasting glucose. Furthermore, data on major cardiovascular events (stroke, coronary heart disease, heart failure) and mortality rate (cardiovascular death, total mortality) were evaluated.

METHODS

Data were pooled by random-effects models and expressed as mean differences and 95% confidence intervals. Studies were weighted based on the inverse of the variance. Heterogeneity was evaluated by Cochran Q and I2 statistics. Study methodologic quality was assessed by the Newcastle-Ottawa scale.

RESULTS

A total of 11 studies (N=2851 [652 Black and 2199 White]) evaluated cardiometabolic risk profile and all had high quality (Newcastle-Ottawa scale score of ≥8). No studies reported on cardiovascular events and mortality rate. Black women had comparable fasting glucose (-0.61 [-1.69 to 2.92] mg/dL; I2=62.5%), yet exhibited increased fasting insulin (6.76 [4.97-8.56] μIU/mL; I2=59.0%); homeostatic model assessment of insulin resistance (1.47 [0.86-2.08]; I2=83.2%); systolic blood pressure (3.32 [0.34-6.30] mm Hg; I2=52.0%); and decreased triglyceride (-32.56 [-54.69 to -10.42] mg/dL; I2=68.0%) compared with White women (all, P≤.03). Groups exhibited comparable total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and diastolic blood pressure (all, P≥.06).

CONCLUSIONS

Black women with polycystic ovary syndrome have a greater tendency for an adverse cardiometabolic risk profile (increased insulin, homeostatic model assessment of insulin resistance, and systolic blood pressure) despite lower triglycerides than White women. Our observations support the consideration of these disparities for diagnostic, monitoring, and management practices in Black women and for future guideline recommendations. Given the heterogeneity among studies, future research should address the relative contributions of biologic, environmental, socioeconomic, and healthcare factors to the observed disparities. Furthermore, longitudinal research is required to address patient-pressing complications, including cardiovascular events and mortality rate in Black women with polycystic ovary syndrome as a high-risk yet understudied population.

Endogenous Glucose Production in Critical Illness

Regulation of endogenous glucose production (EGP) by hormonal, neuronal, and metabolic signaling pathways contributes to the maintenance of euglycemia under normal physiologic conditions. EGP is defined by the generation of glucose from substrates through glycogenolysis and gluconeogenesis, usually in fasted states, for local and systemic use. Abnormal increases in EGP are noted in patients with diabetes mellitus type 2, and elevated EGP may also impact the pathogenesis of nonalcoholic fatty liver disease and congestive heart failure. In this narrative review, we performed a literature search in PubMed to identify recently published English language articles characterizing EGP in critical illness. Evidence from preclinical and clinical studies demonstrates that critical illness can disrupt EGP through multiple mechanisms including increased systemic inflammation, counterregulatory hormone and catecholamine release, alterations in the hypothalamic-pituitary axis, insulin resistance, lactic acidosis, and iatrogenic insults such as vasopressors and glucocorticoids administered as part of clinical care. EGP contributes to hyperglycemia in critical illness when abnormally elevated and to hypoglycemia when abnormally depressed, each of which has been independently associated with increased mortality. Increased EGP may also promote protein catabolism that could worsen critical illness myopathy and impede recovery. Better understanding of the mechanisms and factors contributing to dysregulated EGP in critical illness may help in the development of therapeutic strategies that promote euglycemia, reduce intensive care unit-associated catabolism, and improve patient outcomes.

Insulin resistance contributes to racial disparities in breast cancer prognosis in US women

Background

Racial disparities in breast cancer survival between Black and White women persist across all stages of breast cancer. The metabolic syndrome (MetS) of insulin resistance disproportionately affects more Black than White women. It has not been discerned if insulin resistance mediates the link between race and poor prognosis in breast cancer. We aimed to determine whether insulin resistance mediates in part the association between race and breast cancer prognosis, and if insulin receptor (IR) and insulin-like growth factor receptor (IGF-1R) expression differs between tumors from Black and White women.

Methods

We conducted a cross-sectional, multi-center study across ten hospitals. Self-identified Black women and White women with newly diagnosed invasive breast cancer were recruited. The primary outcome was to determine if insulin resistance, which was calculated using the homeostatic model assessment of insulin resistance (HOMA-IR), mediated the effect of race on prognosis using the multivariate linear mediation model. Demographic data, anthropometric measurements, and fasting blood were collected. Poor prognosis was defined as a Nottingham Prognostic Index (NPI) > 4.4. Breast cancer pathology specimens were evaluated for IR and IGF-1R expression by immunohistochemistry (IHC).

Results

Five hundred fifteen women were recruited (83% White, 17% Black). The MetS was more prevalent in Black women than in White women (40% vs 20%, p < 0.0001). HOMA-IR was higher in Black women than in White women (1.9 ± 1.2 vs 1.3 ± 1.4, p = 0.0005). Poor breast cancer prognosis was more prevalent in Black women than in White women (28% vs 15%. p = 0.004). HOMA-IR was positively associated with NPI score (r = 0.1, p = 0.02). The mediation model, adjusted for age, revealed that HOMA-IR significantly mediated the association between Black race and poor prognosis (β = 0.04, 95% CI 0.005–0.009, p = 0.002). IR expression was higher in tumors from Black women than in those from White women (79% vs 52%, p = 0.004), and greater IR/IGF-1R ratio was also associated with higher NPI score (IR/IGF-1R >  1: 4.2 ± 0.8 vs IR/IGF-1R = 1: 3.9 ± 0.8 vs IR/IGF-1R < 1: 3.5 ± 1.0, p < 0.0001).

Conclusions

In this multi-center, cross-sectional study of US women with newly diagnosed invasive breast cancer, insulin resistance is one factor mediating part of the association between race and poor prognosis in breast cancer.

Type 2 diabetes does not account for ethnic differences in exercise capacity or skeletal muscle function in older adults

AIMS/HYPOTHESIS

The aim of this study was to compare exercise capacity, strength and skeletal muscle perfusion during exercise, and oxidative capacity between South Asians, African Caribbeans and Europeans, and determine what effect ethnic differences in the prevalence of type 2 diabetes has on these functional outcomes.

METHODS

In total, 708 participants (aged [mean±SD] 73 ± 7 years, 56% male) were recruited from the Southall and Brent Revisited (SABRE) study, a UK population-based cohort comprised of Europeans (n = 311) and South Asian (n = 232) and African Caribbean (n = 165) migrants. Measurements of exercise capacity using a 6 min stepper test (6MST), including measurement of oxygen consumption ([Formula: see text]) and grip strength, were performed. Skeletal muscle was assessed using near infrared spectroscopy (NIRS); measures included changes in tissue saturation index (∆TSI%) with exercise and oxidative capacity (muscle oxygen consumption recovery, represented by a time constant [τ]). Analysis was by multiple linear regression.

RESULTS

When adjusted for age and sex, in South Asians and African Caribbeans, exercise capacity was reduced compared with Europeans ([Formula: see text] [ml min-1 kg-1]: β = -1.2 [95% CI -1.9, -0.4], p = 0.002, and β -1.7 [95% CI -2.5, -0.8], p < 0.001, respectively). South Asians had lower and African Caribbeans had higher strength compared with Europeans (strength [kPa]: β = -9 [95% CI -12, -6), p < 0.001, and β = 6 [95% CI 3, 9], p < 0.001, respectively). South Asians had greater decreases in TSI% and longer τ compared with Europeans (∆TSI% [%]: β = -0.9 [95% CI -1.7, -0.1), p = 0.024; τ [s]: β = 11 [95% CI 3, 18], p = 0.006). Ethnic differences in [Formula: see text] and grip strength remained despite adjustment for type 2 diabetes or HbA1c (and fat-free mass for grip strength). However, the differences between Europeans and South Asians were no longer statistically significant after adjustment for other possible mediators or confounders (including physical activity, waist-to-hip ratio, cardiovascular disease or hypertension, smoking, haemoglobin levels or β-blocker use). The difference in ∆TSI% between Europeans and South Asians was marginally attenuated after adjustment for type 2 diabetes or HbA1c and was also no longer statistically significant after adjusting for other confounders; however, τ remained significantly longer in South Asians vs Europeans despite adjustment for all confounders.

CONCLUSIONS/INTERPRETATION

Reduced exercise capacity in South Asians and African Caribbeans is unexplained by higher rates of type 2 diabetes. Poorer exercise tolerance in these populations, and impaired muscle function and perfusion in South Asians, may contribute to the higher morbidity burden of UK ethnic minority groups in older age.

Ethnic differences in intrahepatic lipid and its association with hepatic insulin sensitivity and insulin clearance between men of black and white ethnicity with early type 2 diabetes

Intrahepatic lipid (IHL) is linked with reduced hepatic insulin sensitivity and insulin clearance. Despite their high risk for type 2 diabetes (T2D), there have been limited investigations of these relationships in black populations. We investigated these relationships in 18 white European (WE) and 18 black West African (BWA) men with T2D <5 years. They underwent magnetic resonance imaging to quantify IHL, a hyperinsulinemic euglycaemic clamp with [6,6 ² H₂ ] glucose infusion to assess hepatic insulin sensitivity and a hyperglycaemic clamp to assess insulin clearance. BWA men had lower IHL than WE men (3.7 [5.3] vs 6.6 [10.6]%, P = 0.03). IHL was inversely associated with basal hepatic insulin sensitivity in WE but not BWA men (BWA: r = -0.01, P = 0.96; WE: r = -0.72, P = 0.006) with a significant interaction by ethnicity (P_interaction  = 0.05); however, IHL was not associated with % suppression of endogenous glucose production by insulin in either ethnicity. IHL showed a trend to an association with insulin clearance in BWA only (BWA: r = -0.42, P = 0.09; WE: r = -0.14, P = 0.58). The lack of association between IHL and hepatic insulin sensitivity in BWA men indicates IHL may play a lesser detrimental role in T2D in BWA men.

Ethnic distinctions in the pathophysiology of type 2 diabetes: a focus on black African-Caribbean populations

Type 2 diabetes (T2D) is a global public health priority, particularly for populations of black African-Caribbean ethnicity, who suffer disproportionately high rates of the disease. While the mechanisms underlying the development of T2D are well documented, there is growing evidence describing distinctions among black African-Caribbean populations. In the present paper, we review the evidence describing the impact of black African-Caribbean ethnicity on T2D pathophysiology. Ethnic differences were first recognised through evidence that metabolic syndrome diagnostic criteria fail to detect T2D risk in black populations due to less central obesity and dyslipidaemia. Subsequently more detailed investigations have recognised other mechanistic differences, particularly lower visceral and hepatic fat accumulation and a distinctly hyperinsulinaemic response to glucose stimulation. While epidemiological studies have reported exaggerated insulin resistance in black populations, more detailed and direct measures of insulin sensitivity have provided evidence that insulin sensitivity is not markedly different to other ethnic groups and does not explain the hyperinsulinaemia that is exhibited. These findings lead us to hypothesise that ectopic fat does not play a pivotal role in driving insulin resistance in black populations. Furthermore, we hypothesise that hyperinsulinaemia is driven by lower rates of hepatic insulin clearance rather than heightened insulin resistance and is a primary defect rather than occurring in compensation for insulin resistance. These hypotheses are being investigated in our ongoing South London Diabetes and Ethnicity Phenotyping study, which will enable a more detailed understanding of ethnic distinctions in the pathophysiology of T2D between men of black African and white European ethnicity.

Glucose effectiveness and its components in relation to body mass index

BACKGROUND

Obesity is known to induce a deterioration of insulin sensitivity (S_I ), one of the insulin-dependent components of glucose tolerance. However, few studies investigated whether obesity affects also the insulin-independent component, that is glucose effectiveness (S_G ). This cross-sectional study aimed to analyse S_G and its components in different body mass index (BMI) categories.

MATERIALS AND METHODS

Three groups of subjects spanning different BMI (kg m^-2 ) categories underwent a 3-h frequently sampled intravenous glucose tolerance test: Lean (LE; 18.5 ≤ BMI < 25, n = 73), Overweight (OW; 25 ≤ BMI < 30, n = 90), and Obese (OB; BMI ≥ 30, n = 41). OB has been further divided into two subgroups, namely Obese I (OB-I; 30 ≤ BMI < 35, n = 27) and Morbidly Obese (OB-M; BMI ≥ 35, n = 14). Minimal model analysis provided S_G and its components at zero (GEZI) and at basal (BIE) insulin.

RESULTS

Values for S_G were 1.98 ± 1.30 × 10^-2 ·min^-1 in all subjects grouped and 2.38 ± 1.23, 1.84 ± 0.82, 1.59 ± 0.61 10^-2 ·min^-1 in LE, OW and OB, respectively. In all subjects grouped, a significant inverse linear correlation was found between the log-transformed values of S_G and BMI (r = -0.3, P < 0.0001). S_G was significantly reduced in OW and OB with respect to LE (P < 0.001) but no significant difference was detected between OB and OW (P = 0.35) and between OB-I and OB-M (P = 0.25). Similar results were found for GEZI. BIE was not significantly different among NW, OW and OB (P = 0.11) and between OB-I and OB-M (P ≥ 0.07).

CONCLUSIONS

S_G and its major component GEZI deteriorate in overweight individuals compared to those in the normal BMI range, without further deterioration when BMI increases above 30 kg m^-2 .

Black African men with early type 2 diabetes have similar muscle, liver and adipose tissue insulin sensitivity to white European men despite lower visceral fat

AIMS/HYPOTHESIS

Type 2 diabetes is more prevalent in black African than white European populations although, paradoxically, black African individuals present with lower levels of visceral fat, which has a known association with insulin resistance. Insulin resistance occurs at a tissue-specific level; however, no study has simultaneously compared whole body, skeletal muscle, hepatic and adipose tissue insulin sensitivity between black and white men. We hypothesised that, in those with early type 2 diabetes, black (West) African men (BAM) have greater hepatic and adipose tissue insulin sensitivity, compared with white European men (WEM), because of their reduced visceral fat.

METHODS

Eighteen BAM and 15 WEM with type 2 diabetes underwent a two-stage hyperinsulinaemic-euglycaemic clamp with stable glucose and glycerol isotope tracers to assess tissue-specific insulin sensitivity and a magnetic resonance imaging scan to assess body composition.

RESULTS

We found no ethnic differences in whole body, skeletal muscle, hepatic or adipose tissue insulin sensitivity between BAM and WEM. This finding occurred in the presence of lower visceral fat in BAM (3.72 vs 5.68 kg [mean difference -1.96, 95% CI -3.30, 0.62]; p = 0.01). There was an association between skeletal muscle and adipose tissue insulin sensitivity in WEM that was not present in BAM (r = 0.78, p < 0.01 vs r = 0.25 p = 0.37).

CONCLUSIONS/INTERPRETATION

Our data suggest that in type 2 diabetes there are no ethnic differences in whole body, skeletal muscle, hepatic and adipose tissue insulin sensitivity between black and white men, despite differences in visceral adipose tissue, and that impaired lipolysis may not be contributing to skeletal muscle insulin resistance in men of black African ethnicity.

A Review of Diabetes Prediction Equations in African Descent Populations

Background: Predicting undiagnosed diabetes is a critical step toward addressing the diabetes epidemic in populations of African descent worldwide. Objective: To review characteristics of equations developed, tested, or modified to predict diabetes in African descent populations. Methods: Using PubMed, Scopus, and Embase databases, a scoping review yielded 585 research articles. After removal of duplicates (n = 205), 380 articles were reviewed. After title and abstract review 328 articles did not meet inclusion criteria and were excluded. Fifty-two articles were retained. However, full text review revealed that 44 of the 52 articles did not report findings by AROC or C-statistic in African descent populations. Therefore, eight articles remained. Results: The 8 articles reported on a total of 15 prediction equation studies. The prediction equations were of two types. Prevalence prediction equations (n = 9) detected undiagnosed diabetes and were based on non-invasive variables only. Non-invasive variables included demographics, blood pressure and measures of body size. Incidence prediction equations (n = 6) predicted risk of developing diabetes and used either non-invasive variables or both non-invasive and invasive. Invasive variables required blood tests and included fasting glucose, high density lipoprotein-cholesterol (HDL), triglycerides (TG), and A1C. Prevalence prediction studies were conducted in the United States, Africa and Europe. Incidence prediction studies were conducted only in the United States. In all these studies, the performance of diabetes prediction equations was assessed by area under the receiver operator characteristics curve (AROC) or the C-statistic. Therefore, we evaluated the efficacy of these equations based on standard criteria, specifically discrimination by either AROC or C-statistic were defined as: Poor (0.50 - 0.69); Acceptable (0.70 - 0.79); Excellent (0.80 - 0.89); or Outstanding (0.90 - 1.00). Prediction equations based only on non-invasive variables reported to have poor to acceptable detection of diabetes with AROC or C-statistic 0.64 - 0.79. In contrast, prediction equations which were based on both non-invasive and invasive variables had excellent diabetes detection with AROC or C-statistic 0.80 - 0.82. Conclusion: Equations which use a combination of non-invasive and invasive variables appear to be superior in the prediction of diabetes in African descent populations than equations that rely on non-invasive variables alone.

Physiology of Glycemic Recovery and Stabilization After Hyperinsulinemic Euglycemic Clamp in Healthy Subjects

BACKGROUND

The hyperinsulinemic euglycemic clamp (HEC) is the gold standard for measuring insulin sensitivity, but glycemic recovery and stabilization after the procedure have not been well studied. Here, we assessed the physiological determinants of postclamp recovery.

METHODS

We analyzed data from 207 healthy subjects [102 African American (AA) and 105 European American (EA)] who underwent HEC in the Pathobiology of Prediabetes in a Biracial Cohort study. At the end of HEC, insulin infusion was stopped, and dextrose (20%) infusion was tapered and stopped when plasma glucose stabilized ≥20 mg/dL above the preclamp value (∼100 mg/dL). Glucose recovery time (GRT) was defined as the interval from cessation of insulin infusion to discontinuation of dextrose infusion. Insulin clearance was calculated under basal and clamp conditions.

RESULTS

The mean (± SD) age and body mass index were 46.3 ± 9.96 years and 30.7 ± 8.43 kg/m2, respectively. Plasma glucose (mg/dL) was 92.2 ± 6.26 preclamp and 124.2 ± 26.9 postclamp. The median GRT (minutes) was 65 (range, 30 to 270); mean GRT was 77.1 ± 42.7 (men: 82.9 ± 45.5; women: 74.4 ± 42.3; AA, 82.0 ± 49.6; EA, 72.3 ± 34.2; P > 0.1 for sex or race). The 90th percentile for GRT was 119 minutes. In regression models, significant predictors of GRT were age (P = 0.03), weight (P = 0.009), 2-hour plasma glucose (P = 0.0002), insulin sensitivity (P = 0.03), disposition index (P = 0.017), and basal insulin clearance (P = 0.02).

CONCLUSIONS

In our biracial cohort, glycemic recovery after hyperinsulinemic clamp was independent of sex or race but was significantly predicted by age, weight, and glucose tolerance and by insulin sensitivity, secretion, and clearance. We recommend that monitoring be maintained for ∼2 hours postclamp to ensure adequate glycemic stabilization.

Gluconeogenesis and risk for fasting hyperglycemia in Black and White women

Black women, compared with White women, have high rates of whole-body insulin resistance but a lower prevalence of fasting hyperglycemia and hepatic steatosis. This dissociation of whole-body insulin resistance from fasting hyperglycemia may be explained by racial differences in gluconeogenesis, hepatic fat, or tissue-specific insulin sensitivity. Two groups of premenopausal federally employed women, without diabetes were studied. Using stable isotope tracers, [2H2O] and [6,62-H2]glucose, basal glucose production was partitioned into its components (gluconeogenesis and glycogenolysis) and basal whole-body lipolysis ([2H5]glycerol) was measured. Indices of insulin sensitivity, whole-body (SI), hepatic (HISIGPR), and adipose tissue, were calculated. Hepatic fat was measured by proton magnetic resonance spectroscopy. Black women had less hepatic fat and lower fractional and absolute gluconeogenesis. Whole-body SI, HISIGPR, and adipose tissue sensitivity were similar by race, but at any given level of whole-body SI, Black women had higher HISIGPR. Therefore, fasting hyperglycemia may be a less common early pathological feature of prediabetes in Black women compared with White women, because gluconeogenesis remains lower despite similar whole-body SI.

Pathogenic Mechanisms of Prediabetes in Obese vs. Very Obese African American Women: Implications for Diabetes Prevention

BACKGROUND

The influence of obesity on the development of prediabetes among African American women (AAW) remains uncertain. Thus, we investigated whether the pathogenic mechanisms of prediabetes differ in obese (OB, BMI<35 kg/m²) and very obese (VOB, BMI>35 kg/m²) AAW.

SUBJECTS/METHODS

We recruited 26-OB and 41-VOB, AAW with prediabetes, mean age (46.3 ± 10.3 years), A1C (5.9 ± 0.4%) and BMI (38.3 ± 8.2 kg/m²). OGTT and FSIVGT were performed in each subject. Body composition (% body fat) was measured using DEXA. Si, Sg acute insulin response to glucose (AIRg) and disposition index (DI) were calculated using minimal model method.

RESULTS

Mean BMI (32.6 ± 1.9 vs. 42.8 ± 5.5 kg/m²) and %body fat (44.7 ± 2.0 vs. 49.6 ± 2.2%) were significantly (p = 0.0001) lower in OB vs VOB. Mean fasting and post-glucose challenge, (glucose, insulin, c-peptide) levels were significantly (p = 0.03-0.0001) lower in OB vs VOB. Mean Si and Sg was not different. Mean AIRg tended to be higher (808 ± 776 vs. 535 ± 443 (x min [uU/L] ^-1), p = 0.106) whereas DI was greater (1999 ± 1408 vs. 1511 ± 1033, (×10^-2 x min^-1), p = 0.01) in OB vs VOB subjects.

CONCLUSION

We found that OB and VOB AAW had similar Si and Sg, but VOB showed attenuated AIRg and DI. These parameters should be considered when developing primary prevention programs in AAW with prediabetes.

Ethnic differences in insulin secretory function between black African and white European men with early type 2 diabetes

AIM

To test the hypothesis that men of black (West) African ethnicity (black African men [BAM]) with early type 2 diabetes (T2D) would have greater insulin secretory deficits compared with white European men (WEM), following prediabetic hypersecretion.

METHODS

In 19 BAM and 15 WEM, matched for age, body mass index and duration of diabetes, we assessed and modelled insulin secretory responses to hyperglycaemia stimulated intravenously (hyperglycaemic clamp) and orally (meal tolerance test).

RESULTS

With similar post-challenge glucose responses, BAM had lower second-phase C-peptide responses to intravenous glucose (BAM 70.6 vs WEM 115.1 nmol/L/min [ratio of geometric mean 0.55, 95% confidence interval {CI} 0.37, 0.83]; P = .006) and to oral glucose (BAM 65.4 vs WEM 88.5 nmol/L/min [mean difference -23.2, 95% CI -40.0, -6.3]; P = .009). Peripheral insulin response in BAM to oral glucose was preserved (BAM 47.4 vs WEM 59.4 nmol/L/min [ratio of geometric mean 0.89, 95% CI 0.59, 1.35]; P = .566), with relative reductions in insulin clearance (BAM 506.2 vs WEM 630.1 mL/m² BSA/min [mean difference -123.9, 95% CI -270.5, 22.6]; P = .095), associated with enhanced incretin responses (gastric inhibitory polypeptide incremental area under the curve: BAM 46.8 vs WEM 33.9 μg/L/min [mean difference 12.9, 95% CI 2.1, 23.7]; P = .021).

CONCLUSIONS

In early T2D, BAM had significantly lower insulin secretory responses to intravenous and oral stimulation than WEM. Lower insulin clearance, potentially driven by increased incretin responses, may act to preserve peripheral insulin concentrations. Tailoring early management strategies to reflect distinct ethnic-specific pathophysiology may improve outcomes in this high-risk population.

Parental History of Type 2 Diabetes Abrogates Ethnic Disparities in Key Glucoregulatory Indices

CONTEXT

There are ethnic differences in glucoregulation and prevalence of type 2 diabetes, but studies on the role of genetics in modifying ethnic effects in normoglycemic African-Americans and Caucasians are limited. Therefore, we investigated glucoregulation in normoglycemic African-Americans and Caucasians with or without parental diabetes.

DESIGN

Fifty subjects with parental diabetes (from the Pathobiology of Prediabetes in a Biracial Cohort Study) and 50 subjects without parental diabetes were matched in age, sex, ethnicity, and body mass index (BMI). Subjects underwent a 75-g oral glucose tolerance test (OGTT), physical examination, anthropometry, biochemistries, indirect calorimetry and assessment of body composition, insulin sensitivity by euglycemic clamp (Si-clamp), and β-cell function by Disposition index.

RESULTS

The mean age was 40.5 ± 11.6 years, BMI 28.7 ± 5.9 kg/m2, fasting plasma glucose 90.2 ± 5.9 mg/dL, and 2-hour postglucose 120.0 ± 26.8 mg/dL. Offspring with parental diabetes showed higher glycemic excursion during OGTT-area under the curve-glucose (16,005.6 ± 2324.7 vs 14,973.8 ± 1819.9, P < 0.005), lower Si-clamp (0.132 ± 0.068 vs 0.162 ± 0.081 µmol/kg fat-free mass/min/pmol/L, P < 0.05), and lower Disposition index (8.74 ± 5.72 vs 11.83 ± 7.49, P < 0.05). Compared with lean subjects without parental diabetes, β cell function was lower by ∼30% in lean subjects with parental diabetes, ∼40% in obese subjects without parental diabetes, and ∼50% in obese individuals with parental diabetes (P < 0.0001). African-Americans without parental diabetes had ∼40% lower insulin sensitivity (P < 0.001), twofold higher acute insulin secretion (P < 0.001), but ∼30% lower Disposition index (P < 0.01) compared with Caucasians without parental diabetes. Remarkably, there were no significant differences by ethnicity in these glucoregulatory measures among subjects with parental diabetes.

CONCLUSION

Offspring with parental diabetes harbor substantial impairments in glucoregulation compared with individuals without parental diabetes. Ethnic disparities in glucoregulation were abrogated by parental diabetes.

Clinical and Histologic Characterization of Nonalcoholic Steatohepatitis in African American Patients

OBJECTIVE

There has been a widespread misconception among physicians that African Americans are protected from developing nonalcoholic steatohepatitis (NASH). However, a formal histologic and metabolic comparison against well-matched Caucasians has never been performed.

RESEARCH DESIGN AND METHODS

Sixty-seven African American patients were matched 2:1 to Caucasians (n = 134) for age, sex, BMI, hemoglobin A_1c, and prevalence of type 2 diabetes mellitus (T2DM). Screening for NASH included measurement of intrahepatic triglyceride content by proton MRS (¹H-MRS), followed by a liver biopsy if patients had hepatic steatosis. Insulin resistance was estimated during an oral glucose tolerance test using the Matsuda Index.

RESULTS

Compared with Caucasians, African American patients had a lower intrahepatic triglyceride content (mean ± SD 6.1 ± 6.8% vs. 9.4 ± 7.5%, P = 0.007) and the presence of nonalcoholic fatty liver disease (NAFLD) was less common (25.0% vs. 51.9%, P = 0.003). However, prevalence of NASH was not different between ethnicities in patients with NAFLD (57.1% vs. 73.3%, P = 0.12). Moreover, they showed similar severity in each of the individual histologic parameters (inflammation, ballooning, and fibrosis). Among patients with NAFLD, insulin resistance was similar between both ethnic groups (Matsuda Index: 3.3 ± 1.8 vs. 3.1 ± 1.9, P = 0.61; adipose tissue insulin resistance [Adipo-IR] index: 5.7 ± 4.6 vs. 6.4 ± 4.7 mmol/L ⋅ µU/mL, P = 0.53) but appeared to be worse in African American versus Caucasian patients without NAFLD (Matsuda Index: 4.9 ± 3.6 vs. 7.0 ± 4.9, P = 0.11; Adipo-IR: 3.9 ± 2.8 vs. 2.7 ± 2.3 mmol/L ⋅ µU/mL, P = 0.06). African American patients also had lower plasma triglycerides and higher HDL cholesterol, independent of the severity of intrahepatic triglyceride.

CONCLUSIONS

Although African Americans have lower intrahepatic triglyceride accumulation, once NAFLD develops, NASH occurs as frequently, and as severe, as in Caucasian patients. Therefore, African Americans with NAFLD should be screened for NASH with the same degree of clinical resolve as in Caucasian patients.

Reversing the tide - diagnosis and prevention of T2DM in populations of African descent

Populations of African descent are at the forefront of the worldwide epidemic of type 2 diabetes mellitus (T2DM). The burden of T2DM is amplified by diagnosis after preventable complications of the disease have occurred. Earlier detection would result in a reduction in undiagnosed T2DM, more accurate statistics, more informed resource allocation and better health. An underappreciated factor contributing to undiagnosed T2DM in populations of African descent is that screening tests for hyperglycaemia, specifically, fasting plasma glucose and HbA_1c, perform sub-optimally in these populations. To offset this problem, combining tests or adding glycated albumin (a nonfasting marker of glycaemia), might be the way forward. However, differences in diet, exercise, BMI, environment, gene-environment interactions and the prevalence of sickle cell trait mean that neither diagnostic tests nor interventions will be uniformly effective in individuals of African, Caribbean or African-American descent. Among these three populations of African descent, intensive lifestyle interventions have been reported in only the African-American population, in which they have been found to provide effective primary prevention of T2DM but not secondary prevention. Owing to a lack of health literacy and poor glycaemic control in Africa and the Caribbean, customized lifestyle interventions might achieve both secondary and primary prevention. Overall, diagnosis and prevention of T2DM requires innovative strategies that are sensitive to the diversity that exists within populations of African descent.

Protective effect of black relative to white race against non-alcoholic fatty liver disease in patients with severe obesity, independent of type 2 diabetes

Severe obesity (body mass index ⩾35 kg m^-2) and type 2 diabetes (T2D) are potent and additive risk factors for non-alcoholic fatty liver disease (NAFLD), including non-alcoholic steatohepatitis (NASH). Scant available evidence indicates that black relative to white patients with severe obesity are less susceptible to NAFLD, but it is unclear if T2D abolishes this apparent racial disparity. Therefore, we compared biopsy-proven NAFLD and its progression between black (n=71) and white (n=155) patients with severe obesity stratified by presence or absence of T2D. Although prevalence of T2D was similar between races (37%, P>0.9), whites were significantly more likely than blacks to have NAFLD, NASH and advanced fibrosis (defined as bridging fibrosis and/or cirrhosis). Importantly, T2D was associated with increased odds of NAFLD, NASH and advanced fibrosis (defined as bridging fibrosis or cirrhosis) in whites only (P<0.05). In turn, a higher proportion of blacks than whites with T2D were free of NAFLD (58 versus 22%, P<0.01). These preliminary findings question translation of the powerful interconnection between T2D and NAFLD in whites with severe obesity to blacks and point to an important role of race in the pathophysiology and treatment of these diseases.

Hepatic but Not Extrahepatic Insulin Clearance Is Lower in African American Than in European American Women

African Americans (AAs) tend to have higher plasma insulin concentrations than European Americans (EAs); the increased insulin concentrations have been attributed to increased secretion and/or decreased insulin clearance by liver or other tissues. This work characterizes the contributions of hepatic versus extrahepatic insulin degradation related to ethnic differences between AAs and EAs. By using a recently developed mathematical model that uses insulin and C-peptide measurements from the insulin-modified, frequently sampled intravenous glucose tolerance test (FSIGT), we estimated hepatic versus extrahepatic insulin clearance in 29 EA and 18 AA healthy women. During the first 20 min of the FSIGT, plasma insulin was approximately twice as high in AAs as in EAs. In contrast, insulin was similar in AAs and EAs after the 20-25 min intravenous insulin infusion. Hepatic insulin first-pass extraction was two-thirds lower in AAs versus EAs in the overnight-fasted state. In contrast, extrahepatic insulin clearance was not lower in AAs than in EAs. The difference in insulin degradation between AAs and EAs can be attributed totally to liver clearance. The mechanism underlying reduced insulin degradation in AAs remains to be clarified, as does the relative importance of reduced liver clearance to increased risk for type 2 diabetes.

Ethnic differences in glucose effectiveness and disposition index in overweight/obese African American and white women with prediabetes: A study of compensatory mechanisms

OBJECTIVE

Prediabetes, a major precursor of type 2 diabetes, varies among ethnic populations. Therefore, we compared the pathophysiologic mechanisms of prediabetes in overweight/obese African American (AA) and White American (WA) women.

SUBJECTS AND METHODS

We recruited 95 women (67 AA, 28 WA) with prediabetes. Standard OGTT and FSIVGTT were performed in each subject. Insulin sensitivity (Si), glucose effectiveness (Sg), beta cell function (acute insulin response to glucose (AIRg) and disposition index (DI: Si×AIRg) were calculated using Bergman's Minmod.

RESULTS

Mean BMI was greater in AA vs WA with prediabetes (38.3±8.2vs 34.6±8.5kg/m², p=0.05). Mean fasting serum glucose, and insulin levels were lower in AA vs WA. Similarly, mean peak serum glucose levels were lower while peak insulin levels were higher at 30 and 60minutes in AA vs WA. In contrast, mean fasting and peak serum c-peptide levels at 60 and 90minutes were significantly lower in AA vs WA. Mean AIRg was higher but not significantly different in AA vs WA (633±520.92 vs 414.8±246.8, p=0.193). Although, Si (2.93±3.25vs 44 2.50±1.76 (×10^-4×min^-1 [μU/ml]^-1), p=0.448) was not different, DI was significantly higher in AA vs WA (1381±1126 vs 901.9±477.1, p=0.01). In addition, mean Sg was significantly higher in AAvs WA (2.51±1.17 vs 1.97±0.723 (×10^-2/min), p=0.02).

CONCLUSIONS

We found that in overweight/obese prediabetic AA and WA women with similar Si, the mean Sg and DI were significantly higher in AA. We conclude that the pathophysiologic mechanisms of prediabetes differ in the overweight/obese AA and WA women.

A Common CD36 Variant Influences Endothelial Function and Response to Treatment with Phosphodiesterase 5 Inhibition

CONTEXT

The scavenger receptor CD36 influences the endothelial nitric oxide-cGMP pathway in vitro. Genetic variants that alter CD36 level are common in African Americans (AAs), a population at high risk of endothelial dysfunction.

OBJECTIVE

To examine if the minor allele (G) of coding CD36 variant rs3211938 (G/T) which reduces CD36 level by approximately 50% influences endothelial function, insulin sensitivity (IS), and the response to treatment with the nitric oxide-cGMP potentiator sildenafil.

DESIGN

IS (frequently sampled iv glucose tolerance) and endothelial function (flow mediated dilation [FMD]) were determined in age- and body mass index-matched obese AA women with or without the G allele of rs3211938 (protocol 1). Effect of chronic sildenafil treatment on IS and FMD was tested in AA women with metabolic syndrome and with/without the CD36 variant, using a randomized, placebo-controlled trial (protocol 2).

SETTING

Two-center study.

PARTICIPANTS

Obese AA women.

INTERVENTION

A total of 20-mg sildenafil citrate or placebo thrice daily for 4 weeks.

MAIN OUTCOME

IS, FMD.

RESULTS

G allele carriers have lower FMD (P = .03) and cGMP levels (P = .01) than noncarriers. Sildenafil did not improve IS, mean difference 0.12 (95% confidence interval [CI], -0.33 to 0.58; P = .550). However, there was a significant interaction between FMD response to sildenafil and rs3211938 (P = .018). FMD tended to improve in G carriers, 2.9 (95% CI, -0.9 to 6.8; P = .126), whereas it deteriorated in noncarriers, -2.6 (95% CI, -5.1 to -0.1; P = .04).

CONCLUSIONS

The data document influence of a common genetic variant on susceptibility to endothelial dysfunction and its response to sildenafil treatment.

Hyperglycemic clamp-derived disposition index is negatively associated with metabolic syndrome severity in obese subjects

OBJECTIVE

Metabolic syndrome is associated with insulin resistance and increased future risk of type 2 diabetes. This study investigates the relationship between insulin secretion, insulin resistance and individual metabolic syndrome components in subjects without a prior diagnosis of diabetes.

RESEARCH DESIGN AND METHODS

We assessed insulin secretion during hyperglycemic clamps by infusing dextrose to maintain hyperglycemia (200mg/dL), followed by L-arginine administration. Studies in 98 individuals (mean age 45.3±1.2years, 56% female, 22% African-American, 49% with metabolic syndrome) were analyzed. We tested the association between the number of metabolic syndrome components and individual outcome variables using linear mixed-effects models to adjust for potential confounding effects of age, sex, and race.

RESULTS

Insulin sensitivity index was reduced in the presence of 1 or more metabolic syndrome components. Insulin sensitivity was independently associated with age, waist circumference, male gender and decreased HDL cholesterol. The acute insulin response was greater with two or more metabolic syndrome components, and late glucose-stimulated and L-arginine-stimulated insulin responses exhibited a similar trend. In contrast, the disposition index, a measure of beta cell compensation for insulin resistance, was linearly lower with the number of metabolic syndrome components, and was negatively associated with age, Caucasian race, waist circumference, fasting glucose, and decreased HDL cholesterol.

CONCLUSIONS

The insulin secretory response in metabolic syndrome is inadequate for the worsening insulin sensitivity, as demonstrated by a decline in disposition index. A dysfunctional insulin secretory response is evident in non-diabetic individuals and worsens with accumulation of metabolic syndrome components.

Disparities in Treatment Uptake and Outcomes of Patients with Obesity in the USA

There are clear and persistent disparities in obesity prevalence within the USA. Some of these disparities fall along racial/ethnic lines; however, there are a number of other social and demographic constructs where obesity disparities are present. In addition to differing rates of obesity across groups, there is growing evidence that subgroups of patients both seek out and respond to obesity treatment differently. This review article explores the epidemiology of obesity disparities, as well as the existing evidence around how different groups may respond to behavioral, medical, and surgical therapies, and potential reasons for differential uptake and response, from culture, to access, to physiology. We find that the vast majority of evidence in this area has focused on the observation that African Americans tend to lose less weight in clinical trials compared to non-Hispanic whites and mainly pertains to behavioral interventions. Moving forward, there will be a need for studies that broaden the notion of health disparity beyond just comparing African Americans and non-Hispanic whites. Additionally, a more thorough examination of the potential for disparate outcomes after medical and surgical treatments of obesity is needed, coupled with the careful study of possible physiologic drivers of differential treatment response.

Hepatic and Extrahepatic Insulin Clearance Are Differentially Regulated: Results From a Novel Model-Based Analysis of Intravenous Glucose Tolerance Data

Insulin clearance is a highly variable and important factor that affects circulating insulin concentrations. We developed a novel model-based method to estimate both hepatic and extrahepatic insulin clearance using plasma insulin and C-peptide profiles obtained from the insulin-modified frequently sampled intravenous glucose tolerance test. Data from 100 African immigrants without diabetes (mean age 38 years, body weight 81.7 kg, fasting plasma glucose concentration 83 mg/dL, and fasting insulin concentration 37 pmol/L) were used. Endogenous insulin secretion (calculated by C-peptide deconvolution) and insulin infusion rates were used as inputs to a new two-compartment model of insulin kinetics and hepatic and extrahepatic clearance parameters were estimated. Good agreement between modeled and measured plasma insulin profiles was observed (mean normalized root mean square error 6.8%), and considerable intersubject variability in parameters of insulin clearance among individuals was identified (the mean [interquartile range] for hepatic extraction was 25.8% [32.7%], and for extrahepatic insulin clearance was 20.7 mL/kg/min [11.7 mL/kg/min]). Parameters of insulin clearance were correlated with measures of insulin sensitivity and acute insulin response to glucose. The method described appears promising for future research aimed at characterizing variability in insulin clearance and the mechanisms involved in the regulation of insulin clearance.

Racial/Ethnic Differences in Insulin Resistance and Beta Cell Function: Relationship to Racial Disparities in Type 2 Diabetes among African Americans versus Caucasians

Both biological and sociocultural factors have been implicated in the well-documented racial disparity in incidence and prevalence of type 2 diabetes (T2D) between African Americans (AA) and non-Hispanic whites (NHW). This review examines the extent to which biological differences in glucose metabolism, specifically insulin resistance and beta cell function (BCF), contribute to this disparity. The majority of available data suggests that AA are more insulin resistant and have upregulated BCF compared to NHW. Increasing evidence implicates high insulin secretion as a cause rather than consequence of T2D; therefore, upregulated BCF in AA may specifically confer increased risk of T2D in this cohort. Racial disparities in the metabolic characteristics of T2D have direct implications for the treatment and health consequences of this disease; therefore, future research is needed to determine whether strategies to reduce insulin secretion in AA may prevent or delay T2D and lessen racial health disparities.

Racial Disparities in the Pathogenesis of Type 2 Diabetes and its Subtypes in the African Diaspora: A New Paradigm

The global epidemic of diabetes has extended to the developing countries including Sub-Sahara Africa. In this context, blacks with type 2 diabetes in the African Diaspora continue to manifest 1.5-2 times higher prevalent rates than in their white counterparts. Previous studies have demonstrated that blacks with and without type 2 diabetes have alterations in hepatic and peripheral insulin sensitivity, beta-cell function, and hepatic insulin clearance as well as hepatic glucose dysregulation when compared to whites. In addition, non-diabetic blacks in the African Diaspora manifest multiple metabolic mediators that predict type 2 diabetes and its subtypes. These pathogenic modifiers include differences in subclinical inflammation, oxidative stress burden, and adipocytokines in blacks in the African Diaspora prior to clinical diagnosis. Consequently, blacks in the African Diaspora manifest subtypes of type 2 diabetes, including ketosis-prone diabetes and J type diabetes. Given the diversity of type 2 diabetes in blacks in the African Diaspora, we hypothesize that blacks manifest multiple early pathogenic defects prior to the diagnosis of type 2 diabetes and its subtypes. These metabolic alterations have strong genetic component, which appears to play pivotal and primary role in the pathogenesis of type 2 diabetes and its subtypes in blacks in the African Diaspora. However, environmental factors must also be considered as major contributors to the higher prevalence of type 2 diabetes and its subtypes in blacks in the African Diaspora. These multiple alterations should be targets for early prevention of type 2 diabetes in blacks in the African Diaspora.

comparative study of glucose homeostasis, lipids and lipoproteins, HDL functionality, and cardiometabolic parameters in modestly severely obese African Americans and White Americans with prediabetes: implications for the metabolic paradoxes

OBJECTIVE

To determine whether modestly severe obesity modifies glucose homeostasis, levels of cardiometabolic markers, and HDL function in African Americans (AAs) and white Americans (WAs) with prediabetes.

RESEARCH DESIGN AND METHODS

We studied 145 subjects with prediabetes (N = 61 WAs, N = 84 AAs, mean age 46.5 ± 11.2 years, mean BMI 37.8 ± 6.3 kg/m(2)). We measured fasting levels of lipids, lipoproteins, and an inflammatory marker (C-reactive protein [CRP]); HDL functionality (i.e., levels of paraoxonase 1 [PON1]); and levels of oxidized LDL, adiponectin, and interleukin-6 (IL-6). We measured serum levels of glucose, insulin, and C-peptide during an oral glucose tolerance test. Values for insulin sensitivity index (Si), glucose effectiveness index (Sg), glucose effectiveness at zero insulin (GEZI), and acute insulin response to glucose (AIRg) were derived using a frequently sampled intravenous glucose tolerance test (using MINMOD software).

RESULTS

Mean levels of fasting and incremental serum glucose, insulin, and C-peptide tended to be higher in WAs versus AAs. The mean Si was not different in WAs versus AAs (2.6 ± 2.3 vs. 2.9 ± 3.0 × 10(-4) × min(-1) [μU/mL](-1)). Mean values for AIRg and disposition index as well as Sg and GEZI were lower in WAs than AAs. WAs had higher serum triglyceride levels than AAs (116.1 ± 55.5 vs. 82.7 ± 44.2 mg/dL, P = 0.0002). Mean levels of apolipoprotein (apo) A1, HDL cholesterol, PON1, oxidized LDL, CRP, adiponectin, and IL-6 were not significantly different in obese AAs versus WAs with prediabetes.

CONCLUSIONS

Modestly severe obesity attenuated the ethnic differences in Si, but not in Sg and triglyceride levels in WAs and AAs with prediabetes. Despite the lower Si and PON1 values, AAs preserved paradoxical relationships between the Si and HDL/apoA1/triglyceride ratios. We conclude that modestly severe obesity has differential effects on the pathogenic mechanisms underlying glucose homeostasis and atherogenesis in obese AAs and WAs with prediabetes.

Racial differences in peripheral insulin sensitivity and mitochondrial capacity in the absence of obesity

CONTEXT

African-American women (AAW) have an increased risk of developing type 2 diabetes compared with Caucasian women (CW). Lower insulin sensitivity has been reported in AAW, but the reasons for this racial difference and the contributions of liver versus skeletal muscle are incompletely understood.

OBJECTIVE

We tested the hypothesis that young, nonobese AAW manifest lower insulin sensitivity specific to skeletal muscle, not liver, and is accompanied by lower skeletal muscle mitochondrial oxidative capacity.

PARTICIPANTS AND MAIN OUTCOME MEASURES

Twenty-two nonobese (body mass index 22.7 ± 3.1 kg/m(2)) AAW and 22 matched CW (body mass index 22.7 ± 3.1 kg/m(2)) underwent characterization of body composition, objectively assessed habitual physical activity, and insulin sensitivity with euglycemic clamps and stable-isotope tracers. Skeletal muscle biopsies were performed for lipid content, fiber typing, and mitochondrial measurements.

RESULTS

Peripheral insulin sensitivity was 26% lower in AAW (P < .01), but hepatic insulin sensitivity was similar between groups. Physical activity levels were similar between groups. Lower insulin sensitivity in AAW was not explained by total or central adiposity. Skeletal muscle triglyceride content was similar, but mitochondrial content was lower in AAW. Mitochondrial respiration was 24% lower in AAW and correlated with skeletal muscle insulin sensitivity (r = 0.33, P < .05).

CONCLUSION

When compared with CW, AAW have similar hepatic insulin sensitivity but a muscle phenotype characterized by both lower insulin sensitivity and lower mitochondrial oxidative capacity. These observations occur in the absence of obesity and are not explained by physical activity. The only factor associated with lower insulin sensitivity in AAW was mitochondrial oxidative capacity. Because exercise training improves both mitochondrial capacity and insulin sensitivity, we suggest that it may be of particular benefit as a strategy for diabetes prevention in AAW.

Effects of an overnight intravenous lipid infusion on intramyocellular lipid content and insulin sensitivity in African-American versus Caucasian adolescents

OBJECTIVE

To explain the predisposition for insulin resistance among African American (AA) adolescents, this study aimed to: 1) examine changes in intramyocellular lipid content (IMCL), and insulin sensitivity with intralipid (IL) infusion; and 2) determine whether the increase in IMCL is comparable between AA and Caucasian adolescents.

MATERIALS AND METHODS

Thirteen AA and 15 Caucasian normal-weight adolescents (BMI <85th) underwent a 3-h hyperinsulinemic-euglycemic clamp, on two occasions in random order, after an overnight 12-h infusion of: 1) 20% IL and 2) normal saline (NS). IMCL was quantified by (1)H magnetic resonance spectroscopy in tibialis anterior muscle before and after IL infusion.

RESULTS

During IL infusion, plasma TG, glycerol, FFA and fat oxidation increased significantly, with no race differences. Hepatic insulin sensitivity decreased with IL infusion with no difference between the groups. IL infusion was associated with a significant increase in IMCL, which was comparable between AA (Δ 105%; NS: 1.9±0.8 vs. IL: 3.9±1.6 mmol/kg wet weight) and Caucasian (Δ 86%; NS: 2.8±2.1 vs. IL: 5.2±2.4 mmol/kg wet weight), with similar reductions (P<0.01) in insulin sensitivity between the groups (Δ -44%: NS: 9.1±3.3 vs. IL: 5.1±1.8 mg/kg/min per μU/ml in AA) and (Δ -39%: NS: 12.9±6.0 vs. IL: 7.9±3.8 mg/kg/min per μU/ml in Caucasian) adolescents.

CONCLUSIONS

In healthy adolescents, an acute elevation in plasma FFA with IL infusion is accompanied by significant increases in IMCL and reductions in insulin sensitivity with no race differential. Our findings suggest that AA normal-weight adolescents are not more susceptible than Caucasians to FFA-induced IMCL accumulation and insulin resistance.

Obesity and african americans: physiologic and behavioral pathways

Although progress has been made to understand the association between physiological and lifestyle behaviors with regard to obesity, ethnic differences in markers of obesity and pathways towards obesity remain somewhat unexplained. However, obesity remains a serious growing concern. This paper highlights ethnic differences in African Americans and Caucasians that may contribute to the higher prevalence of obesity among African Americans. Understanding ethnic differences in metabolic syndrome criteria, functioning of the hypothalamic pituitary adrenal axis, variations in glucocorticoid sensitivity and insulin resistance, and physical activity and cardiovascular fitness levels may help to inform practical clinical and public health interventions and reduce obesity disparities.