Age-related changes in insulin sensitivity and β-cell function among European American and African American women

Abstract

Type 2 diabetes is more prevalent among African American (AA) than European American (EA) women for reasons that are unknown. Ethnic differences in physiological processes related to insulin sensitivity and secretion, and age-related changes in these processes, may play a role. The purpose of this study was to identify ethnicity- and age-related differences in insulin sensitivity and β-cell responsivity among AA and EA females, and to determine whether these differences are independent of body composition and fat distribution. Healthy, normoglycemic females aged 7–12 yr (n=62), 18–32 yr (n=57), and 40–70 yr (n=49) were recruited for entry into this study. Following an overnight fast, insulin sensitivity (S_I), intravenous glucose tolerance (Kg), acute C-peptide secretion (X0), and basal, first-phase, second-phase, and total β-cell responsivity to glucose (PhiB, Phi1, Phi2, and PhiTOT, respectively) were measured by an intravenous glucose tolerance test. Total % body fat was assessed by dual-energy X-ray absorptiometry, and intra-abdominal adiposity (IAAT) by computed tomography. Main effects of age group and ethnicity were measured with ANCOVA, adjusting for %fat, IAAT, and S_I as indicated. AA had lower S_I, and higher Kg, X0, Phi1, and PhiTOT (P<0.05), which remained after adjustment for %fat and IAAT. Greater X0, Phi1, and PhiTOT among AA were independent of S_I. Advancing age was associated with greater Phi2 among both EA and AA. To conclude, inherent ethnic differences in β-cell function exist independently of adiposity and insulin sensitivity. Future research should examine whether ethnic differences in β-cell physiology contribute to disparities in type 2 diabetes risk.

Introduction

The prevalence of type 2 diabetes (T2D) is higher among African American (AA) versus European American (EA) women, and this ethnic difference is greatest during post-menopausal years ¹. To date, the underlying reason for this disproportionate burden on AA women is unknown. The greater prevalence of obesity among AA women may explain some, but not all, of the ethnic difference in the prevalence of T2D ^{2, 3}. It is possible that changes in insulin secretion or action with age disproportionately compromise glucose tolerance among AA. Aging is associated with declines in glucose tolerance, insulin sensitivity, and insulin secretion ⁴, although the extent to which these changes are due to adiposity is not clear. Both total and intra-abdominal adipose tissues commonly accumulate with age and may impair aspects of glucose metabolism. Information is lacking on whether ethnic differences exist in age-related changes to physiological processes regulating glucose metabolism, and to what extent these differences are associated with body composition.

Previous work from our group and others has shown that insulin sensitivity is lower among AA versus EA, both among children and adults ^5–7. In many cases, this difference was independent of body composition, suggesting an inherent physiological basis. It is possible that the life-long burden of depressed insulin sensitivity may result in a strain on the β-cell due to the continuous need for compensatory insulin secretion. Whether age is related to a decline in β-cell responsivity among AA has not been examined.

Little research has been conducted to examine β-cell responsivity in AA. Among healthy glucose-tolerant subjects, AA show greater insulin concentration immediately following a glucose challenge compared to EA ^8–12. Previous work from our group has shown that this greater acute insulin response to glucose (AIRg) among AA was independent of their lower insulin sensitivity ^{6, 9}. The extent to which higher post-challenge insulin reflects secretion versus clearance is not entirely clear. Using C-peptide analysis, we and others have shown that insulin clearance is reduced among AA ^{7, 9}. Among children, at least a portion of the greater post-challenge insulin in AA appears due to greater first-phase insulin secretion ⁹. However, C-peptide data in adults suggest that insulin secretion may not differ with ethnicity ⁷. Previous work however, has largely ignored patterns of insulin secretion beyond the first 10 minutes during which AIRg is typically measured. It would be of interest to compare the pattern of insulin secretion among AA and EA, to determine if there are underlying ethnic differences in the pattern of β-cell responsivity, as assessed with robust methodology, and whether such differences are independent of confounding factors such as body composition and insulin sensitivity.

The purpose of this study was to identify the independent effects of ethnicity and age on insulin sensitivity and elements of insulin secretion using robust methodology based on C-peptide analysis. Our goal also was to determine whether observed ethnic differences were independent of confounding factors such as body composition and body fat distribution, and in the case of β-cell function, independent of insulin sensitivity. We hypothesized that both AA ethnicity and advancing age would be associated with lower insulin sensitivity, lower glucose tolerance, and reduced ability of β-cells to compensate for lower insulin sensitivity, but that these effects would be partially mediated by adiposity.

Methods and Procedures

Participants

African American and EA girls and women were recruited as three age groups: 7–12 years (prepubertal), 18–32 years (premenopausal), and 40–70 years (postmenopausal). Exclusion criteria included type 1 or type 2 diabetes, polycystic ovary disease, disorders of glucose or lipid metabolism, hypertension, use of medication that could affect body composition or glucose metabolism (including oral contraceptives and postmenopausal hormone replacement therapy), use of tobacco, alcohol consumption in excess of 400 grams per week, history of hypoglycemic episodes, and a medical history that counter-indicated inclusion in the study. All participants had normal glucose tolerance ¹³. A minimum body weight of 20 kg was required for the girls in order to minimize risks associated with blood sampling. Pubertal status of the girls was evaluated using the criteria of Marshall and Tanner ¹⁴; and adult women were confirmed to be premenopausal if they experienced regular menstrual cycles, and postmenopausal if they had not had a cycle in the past 12 months. Serum FSH was used to verify postmenopausal status (FSH>35 IU/ml). Participants were informed of the experimental design, and written consent was obtained. The study was approved by the Institutional Review Board for Human Use at the University of Alabama at Birmingham (UAB). Data regarding free fatty acids¹⁵, metabolic syndrome components⁵, and adiposity-insulin associations¹⁶, have been published from this subject population.

Protocol

Participants underwent all testing during an overnight in-patient visit at UAB’s General Clinical Research Center (GCRC). Premenopausal women were tested within 10 days of the beginning of their menstrual cycle, during the follicular phase. Prior to admission, adult participants were asked to consume at least 250 grams carbohydrates for 3 days, and were provided with a list of common foods and their carbohydrate content; child participants were asked to consume an amount of carbohydrate that was proportional to their smaller body size and energy requirements. Participants were admitted to the GCRC on the evening prior to testing and were given a standard meal consisting of 50% energy from carbohydrate, 30% energy from fat, and 20% energy from protein. Participants then fasted until the morning testing (approximately 12 hours). Blood pressure was determined (Dinamap Pro 200 automated cuff: GE Medical Systems, Waukesha, WI) upon waking, on two consecutive mornings, while participants laid in a supine position. Following this, a fasting blood draw was obtained, and the intravenous glucose tolerance test was initiated. After completion of the glucose tolerance test, participants were given a late breakfast/lunch. Child participants were accompanied by at least one parent during testing.

Intravenous glucose tolerance test (IVGTT)

Insulin sensitivity and β-cell responsivity to glucose were determined during an intravenous glucose tolerance test (IVGTT). Flexible catheters were placed in the antecubital spaces of both arms. Three blood samples were taken over a 15 min period to determine basal glucose and insulin (the average of the values was used for basal concentrations). At time zero, glucose (50% dextrose, 300 mg/kg) was given intravenously, and at 20 min post-glucose injection, insulin (0.02 Units/kg) was infused over a 5-min period. For adults, blood samples (2.0 ml) were collected at the following times (min) relative to glucose administration: 2, 3, 4, 5, 6, 8, 10, 12, 15, 19, 20, 21, 22, 24, 26, 28, 30, 35, 40, 45, 50, 55, 60, 70, 80, 100, 120, 140, 180, 210, 240, 300. For children, a reduced sampling protocol was used, where blood was drawn at baseline (two samples) and at 2, 3, 4, 5, 6, 8, 10, 12, 15, 19, 20, 21, 22, 24, 26, 28, 30, 35, 40, 50, 60, 70, and 240 min post glucose injection (total of 25 samples). Serum was stored at −85°C until analysis.

Laboratory analyses

Concentrations of lipids, glucose, insulin, and C-peptide were analyzed in the Core Laboratory of UAB’s GCRC, Clinical Nutrition Research Center (CNRC), and Diabetes Research and Training Center (DRTC). Total cholesterol, HLD-C, and triglycerides (TG) were measured with the Ektachem DT II System following methods previously reported⁵, and LDL-C was estimated with the Friedewald formula¹⁷. Glucose was measured in 10 μl of sera using an Ektachem DT II System (Johnson and Johnson Clinical Diagnostics). This analysis had a mean intra-assay coefficient of variation (CV) of 0.61%, and a mean inter-assay CV of 1.45%. Insulin was assayed in duplicate 100 μl aliquots with reagents from Linco Research Inc. (St. Charles, MO); assay sensitivity was 3.35 μIU/ml; mean intra-assay CV was 3.49%; and mean interassay CV was 5.57%. C-peptide was assayed in duplicate 25 ul aliquots with double-antibody radioimmunoassay reagents (Diagnostic Products Corporation, Los Angeles, CA); assay sensitivity was 0.318 ng/mL; mean intraassay CV was 3.57%; and mean interassay CV was 5.59%.

Estimate of insulin sensitivity, glucose tolerance, and β-cell responsivity to glucose

Insulin sensitivity (S_I) and glucose effectiveness (Sg: or fractional glucose turnover at basal insulin¹⁸) were derived from glucose and insulin values using minimal modeling ¹⁹. Intravenous glucose tolerance (Kg; %/min) was calculated as the inverse slope of time versus ln glucose from min 8–19. Assessment of insulin secretion and β-cell function were performed by measurement of C-peptide in order to isolate insulin secretion from insulin clearance ^20–22. Mathematical modeling of C-peptide and glucose data collected during the IVGTT can be used to derive indices of β-cell responsivity specific to basal, first-, and second-phase β-cell response to glucose. The major outcome variables of interest were the acute C-peptide release immediately following glucose administration (X0); and basal (PhiB), first-phase (Phi1), second-phase (Phi2), and total (PhiTOT) β-cell responsivity to glucose. As described by Cobelli and colleagues ²³, PhiB is believed to reflect non-stimulated C-peptide release adjusted for glucose concentration; Phi1, the “readily releasable” C-peptide per unit increase in glucose concentration; Phi2, the C-peptide release from new insulin secretory granules provided in response to, and proportionate to, a given glucose concentration; and PhiTOT reflects a global index of overall β-cell responsivity. Finally, since β-cell function needs to be interpreted in light of the prevailing insulin sensitivity, a Disposition Index (DI) was calculated (S_I x PhiTOT). If an individual’s β-cells respond to a decrease in insulin sensitivity by adequately increasing insulin secretion, the product of β-cell function and insulin sensitivity (i.e. DI) is unchanged reflecting normal glucose tolerance. In contrast, if there is not an adequate compensatory increase in β-cell function to compensate for decreased insulin sensitivity, the individual develops glucose intolerance. Thus, since among glucose tolerant individuals β-cell function is inversely related to insulin sensitivity, DI is almost constant in a healthy homogeneous population ^{23, 24}.

Body composition and fat distribution

Total body fat mass, total lean tissue mass, and percent body fat (%fat) were determined by dual-energy X-ray absorptiometry (Lunar Prodigy; (GE Healthcare Lunar, Madison, WI) in the Department of Nutrition Sciences at UAB. Participants were scanned in light clothing while lying flat on their backs with arms at their sides. Intra-abdominal adipose tissue (IAAT) and subcutaneous abdominal adipose tissue (SAAT) were analyzed by computed tomography scanning ^{25, 26} with a HiLight/Advantage Scanner (General Electric, Milwaukee) located in the UAB Department of Radiology. Participants were scanned in the supine position with arms stretched above their heads. A 5mm scan at the level of the umbilicus (approximately the L4-L5 intervertebral space) was taken. Scans were analyzed for cross-sectional area (cm²) of adipose tissue using the density contour program with Hounsfield units for adipose tissue set at −190 to −30. All scans were analyzed by the same individual. The CV for repeat cross-section analysis of scans among 40 subjects in our laboratory is less than 2% ²⁶.

Statistical Analysis

To examine ethnicity and age group differences in S_I, Kg, Sg, indices of β-cell function, DI, and body fat, two-way ANOVA were conducted, with Bonferroni post hoc tests of any significant age group differences. To examine whether ethnic- and age-related differences in metabolic measures were independent of adiposity, ANCOVA were performed, adjusting for both %fat and IAAT. Both measures were used as covariates because both were significant determinants of S_I, Kg, PhiB, Phi2, and DI, with %fat being a stronger predictor for prepubertal girls, and IAAT becoming predictive of these variables among postmenopausal women. Finally, to examine whether ethnic- and age-related differences in indices of β-cell function were independent of differences S_I, ANCOVA with adjustment for S_I were conducted on each index of β-cell insulin secretion.

Values for S_I, Kg, Sg, X0, PhiB, Phi1, Phi2, Phi_TOT, DI, lipids, and IAAT, were log₁₀ transformed prior to statistical analysis to ensure a normal distribution. All statistical tests were two-sided and were performed using a Type I error rate of 0.05. All statistical analyses were performed using SPSS (version 10; SPSS Inc., Chicago, IL).

Results

Descriptive characteristics of EA and AA participants are shown in Table 1 by ethnicity and age group. An ethnicity x age group interaction was observed for IAAT, with AA exhibiting a relatively smaller increase between prepubertal and premenopausal age groups, but greater increase between premenopausal and postmenopausal age groups, as compared to EA women. Main effects of ethnicity were found for BMI and IAAT (P<0.05), with EA having lower BMI but greater IAAT than AA. A main effect of age group was noted for BMI and %fat (P<0.05), whereby advancing age was associated increased fat (P<0.001).

Table 1.

Descriptive statistics by age and ethnic group. Data are mean ± SD.

	Prepubertal	Premeno	Postmeno	Main effects*
EA	29	32	31
Age (yr)	10.3 ± 1.5	25.9 ± 3.4	55.7 ± 4.2	---
BMI (kg/m2)	18.2 ± 2.4	25.8 ± 5.9	26.1 ± 5.0	Age, ethnicity
BMI z-score	0.28 ± 0.91	---	---
%fat	26.0 ± 7.7	36.5 ± 8.5	38.8 ± 8.7	Age
IAAT	32.8 ± 13.0	68.4 ± 44.8	117.9 ± 58.8	Age, ethnicity, Age group by ethnicity interaction
Systolic BP	105.0 ± 9.9	110.8 ± 10.1	122.5 ± 15.6	Age
Diastolic BP	59.5 ± 6.5	60.4 ± 6.8	64.3 ± 10.4	Age, ethnicity
Fasting glucose (mg/dL)	96.6 ± 7.3	90.8 ± 6.2	95.6 ± 9.1	Age
Fasting insulin (uU/mL)	10.6 ± 4.6	9.8 ± 4.9	10.8 ± 7.5	Ethnicity
Total cholesterol	151.7 ± 23.2	148.3 ± 28.7	199.9 ± 32.9	Age, Age group by ethnicity interaction
LDL	90.6 ± 27.4	86.3 ± 24.3	118.8 ± 30.6	Age
HDL	44.5 ± 11.2	42.1 ± 12.9	53.8 ± 25.6	Age
TC/HDL	3.3 ± 1.0	3.5 ± 1.2	3.7 ± 1.8	---
Triglycerides	65.0 ± 31.7	80.5 ± 50.7	96.1 ± 76.6	Age, ethnicity
AA	33	25	18
Age (yr)	9.8 ± 1.6	25.1 ± 3.3	55.6 ± 5.0	---
BMI (kg/m2)	19.4 ± 4.1	27.2 ± 6.2	31.2 ± 5.3	---
BMI z-score	0.60 ± 1.07	---	---
%fat	24.4 ± 9.9	36.0 ± 10.9	44.6 ± 7.3	---
IAAT	27.7 ± 17.9	41.6 ± 22.9	122.4 ± 40.8	---
Systolic BP	105.4 ± 11.7	116.2 ± 10.4	124.9 ± 15.9	---
Diastolic BP	59.8 ± 8.4	64.0 ± 7.5	70.5 ± 11.5	---
Fasting glucose (mg/dL)	93.2 ± 6.1	89.3 ± 7.7	98.5 ± 8.4	---
Fasting insulin (uU/mL)	13.3 ± 6.8	11.4 ± 4.0	11.4 ± 5.4	---
Total cholesterol	154.1 ± 28.3	158.8 ± 33.2	178.9 ± 38.1	---
LDL	89.4 ± 32.8	95.5 ± 32.2	109.3 ± 37.3	---
HDL	49.2 ± 14.0	49.2 ± 14.0	50.7 ± 18.7	---
TC/HDL	3.0 ± 1.0	3.2 ± 1.0	3.5 ± 1.4	---
Triglycerides	54.7 ± 32.9	53.5 ± 24.3	71.6 ± 28.9	---

^*P<0.05.

There was a main effect of ethnicity on S_I (P<0.001) and Kg with AA having lower S_I but higher Kg (Figure 1). Main effects of ethnicity (greater among AA) were also found for X0, Phi1, and PhiTOT (P<0.001; Figure 2). Adjustment for %fat and IAAT weakened the main effect of ethnicity on S_I and Kg slightly (Figure 1B), but did not attenuate the main effect of ethnicity on indices of β-cell responsivity (Figure 2).

Figure 1.

Insulin sensitivity (S_I): A) unadjusted, B) adjusted for total % fat and IAAT; intravenous glucose tolerance (Kg): C) unadjusted, D) adjusted for total % fat and IAAT; and disposition index (DI): E) unadjusted, F) adjusted for total % fat and IAAT; by age and ethnicity. *** P<0.001, * P<0.05, ⊗ 0.05<P<0.10.

Figure 2.

Indices of β-cell responsivity unadjusted (left column) and adjusted for total %fat and IAAT (right column), by age and ethnicity. ***P<0.001, ** P<0.01, * P<0.05.

Given that the greater β-cell responsivity among AA women may have been a compensatory response for reduced S_I, we investigated whether the effect of ethnicity on each β-cell index remained significant after adjusting for S_I (Figure 3). The main effect of ethnicity on X0, Phi1, and PhiTOT remained significant after adjustment for S_I (P<0.05). Adjustment for S_I also uncovered a main effect of ethnicity on Phi2 (P<0.01), and a similar trend for PhiB (P=0.053), both being lower among AA.

Figure 3.

Indices of β-cell responsivity vs. S_I, by ethnicity. A) S_I versus X0; significant effects of ethnicity and S_I (both P<0.001). B) S_I versus PhiB, adjusted for age group; P for ethnicity = 0.0533 and P for S_I <0.001. C) S_I versus Phi2, adjusted for age group; significant effects of ethnicity and S_I (both P<0.001). ○ EA ● AA.

A main effect of age was noted for Kg (P<0.001; Figure 1), with prepubertal girls having greater Kg than both groups of adult women (P<0.01), which did not differ from each other. The effect of age group on Kg remained statistically significant after adjustment for %fat and IAAT, although posthoc tests found that the prepubertal girls had greater Kg compared to only the premenopausal women (P<0.05) after adjustment for adiposity. No main effect of age on S_I was observed in the unadjusted data. After adjustment for %fat and IAAT, a main effect of age on S_I became evident (P<0.001), with posthoc tests revealing a graded increase in S_I with age, such that postmenopausal women had higher S_I than both of the younger age groups (P < 0.05), while premenopausal women tended to have higher S_I than did the prepubertal girls (P = 0.055; Figure 1B). A similar pattern was noted for DI, although this effect did not obtain statistical significance. A main effect of age was also noted for PhiB and Phi2 (P<0.01; Figure 2), whereby prepubertal girls had lower PhiB compared to postmenopausal women (P < 0.01), and lower Phi2 compared to both groups of adult women (P < 0.01). Adjustment for %fat and IAAT abolished the age group effect on PhiB (Figure 2), and weakened, but did not abolish the effect of age group on Phi2 (Figure 2). Adjustment for S_I did not reduce the age effect on PhiB and Phi2 (not shown).

Discussion

This study was conducted to determine if basic physiological processes related to glucose metabolism differed with ethnicity; whether age had a differential impact on aspects of β-cell function in AA versus EA; and the extent to which these differences could be attributed to adiposity. The main findings were that AA had lower insulin sensitivity, but a greater rate of glucose disposal, and greater indices of early insulin secretion, differences that were independent of body composition and fat distribution. Further, the greater early insulin secretion among AA was independent of lower S_I. Advancing age was associated with changes in β-cell function (greater basal and second phase responsivity) similarly in AA and EA. Adiposity explained greater PhiB but not Phi2 among older subjects, but masked an age-associated increase in S_I.

The lower insulin sensitivityamong AA shown here is consistent with reports from previous studies ^5–7, and was consistent across all age groups in this study. The fact that this difference remained after statistically adjusting for adiposity implies that the relative insulin resistance of AA women may be an inherent ethnic difference rather than being due to their greater adiposity. This supports a number of other studies which have also shown that insulin sensitivity is at least partially independent of adiposity ^{6, 7, 27, 28}.

The greater intravenous glucose tolerance among AA, however, suggests that women and girls in this healthy cohort are not only compensating for their reduced insulin sensitivity, but are able to dispose of glucose at an accelerated rate relative to EA. Given that no ethnicity or age effect was noted for Sg, the greater rate of glucose disposal among AA may have been due to greater insulin secretion. AA had greater glucose-stimulated first phase and total β-cell responses that were not attributed to adiposity, and were greater than that which would be predicted based on compensation for lower insulin sensitivity. Whether “excessive” insulin secretion confers risk for T2D is not clear. However, at least one other minority population known to have high risk for T2D also has greater early phase insulin secretion that is independent of their inherent insulin resistance ²⁹. Given that endoplasmic reticulum stress may contribute to β-cell apoptosis ^{30, 31}, it is possible that the excessive demand for insulin synthesis due to exaggerated early phase insulin secretion leads to a loss of β-cells and ultimately to T2D.

It is important to note that DI, or the ability of β-cells to compensate for variation in S_I, did not differ between AA and EA. Despite lower S_I among AA, greater overall insulin secretion (PhiTOT) across the test period appeared to compensate appropriately. Greater PhiTOT among AA was primarily due to ~2-fold greater Phi1. In contrast, PhiB and Phi2 were significantly lower among AA, suggesting that basic differences exist between AA and EA in the characteristics of the insulin secretory profile. Lower PhiB and Phi2 among AA were apparent only after adjusting for S_I, suggesting that these measures are elevated in some cases to compensate for lower S_I.

The ethnic differences in specific indices of β-cell responsivity shown here may reflect underlying differences in the physiology of β-cell function in AA versus EA. Insulin is continuously synthesized in pancreatic β-cells and is stored in secretory vesicles which, upon docking with the cell membrane, become “readily releasable” in response to a glucose challenge^{19, 23}. The mathematical models used here to describe β-cell function are hypothesized to reflect various aspects of these intra-cellular events^{19, 23}. One interpretation of greater X0 and Phi1 among AA is a greater pool of readily-releasable insulin. However, it is not possible to conclude that AA and EA differ with respect to cellular aspects of insulin secretion from peripheral C-peptide concentrations alone. Polymorphisms in several genes involved in β-cell function have been associated with T2D in AA but not EA^32,33. It would be of interest to determine if these polymorphisms are associated with phenotypes in β-cell function.

Independent of ethnicity, “age” was uniquely associated with several aspects of glucose metabolism. After accounting for total and intra-abdominal adiposity, insulin sensitivity increased with age. Although previous studies have shown either no effect of age on insulin sensitivity, or lower insulin sensitivity with older age ^34–36, these studies did not take into account body composition and fat distribution. In studies where total and/or intra-abdominal adiposity were accounted for, the age-related decline in insulin sensitivity was attributed to adiposity ^{37, 38}. The current finding of greater insulin sensitivity with older age after adjustment for adiposity is consistent with results from a study of premenopausal women that also used a robust measure of insulin sensitivity ³⁹. Taken together, current and previous results suggest that older age per se does not impair insulin sensitivity, and observations of an age-associated decrease in insulin sensitivity can be attributed to increased adiposity. However, given that all of the studies cited above used healthy, glucose tolerant women, it is possible that an age-associated decrease in insulin sensitivity may occur among individuals predisposed to T2D.

Older age also was associated with greater PhiB and Phi2. The difference in PhiB was attributed to greater adiposity. Other studies have noted that obesity is associated with greater basal insulin secretion⁴⁰. Taken together, previous and present results suggest that adiposity increases the sensitivity of the β-cell to glucose during the basal state. Further, our data suggest that no age-related change in basal β-cell responsiveness occurs in the absence of a change in adiposity. Greater Phi2 among older subjects was attributable to neither adiposity nor insulin sensitivity, suggesting an independent effect of age, or of an unidentified age-associated factor. It is generally thought that aging is associated with a decrease in overall insulin secretion (for review see ⁴). Our results here are not incompatible with this concept, as they refer only to the sensitivity of the β-cell to glucose during the second phase. It is possible that second phase β-cell responsivity increases in compensation for an age-related decline in glucose tolerance ^{4, 38}. Alternatively, given that this study was confined to healthy glucose tolerant women, we could hypothesize that the ability to increase Phi2 with age may be a mechanism by which some women defend against T2D.

The results of this study complement our previous work with this population regarding associations of adiposity with insulin sensitivity and β-cell function¹⁶. We previously showed that adiposity was more closely associated with insulin sensitivity among EA than AA. We also noted that adiposity was inversely associated with Phi1 among older subjects, particularly older AA women. Now in the current study, we’ve characterized unique effects of both ethnicity and age on insulin sensitivity and β-cell function. Further, we have teased apart the extent to which these main effects of ethnicity and age are, or are not, explained by adiposity or fat distribution. Taken together, results from these studies suggest that while on a population level, ethnic and age-associated differences in insulin sensitivity and β-cell function occur independently of adiposity, within a given age or ethnic group, adiposity appears to have unique associations with these processes.

Given that this series of studies was limited to a population of women and girls, it remains to be seen whether these results would generalize to men. We would anticipate however, that ethnic differences in insulin sensitivity and β-cell function shown in this study would also be found among AA men because they too are at greater risk for T2D as compared to EA men¹. Conversely, the effect of age on insulin sensitivity and β-cell function may be limited to women, given the more extensive changes in hormonal milieu that occur in aging women. Finally, while the contribution of obesity toward impaired insulin sensitivity and β-cell function may be gender-neutral and apply to every ethnic group, the greater prevalence of obesity among AA women may contribute to their increased T2D risk. Consequently, the combined effect of AA ethnicity, greater prevalence of obesity, and female status (particularly during post-menopausal years) may present AA women with a cumulative deleterious effect on insulin sensitivity and β-cell function that renders this population more vulnerable to T2D.

The strengths of this study included the inclusion of girls and women across a wide age range, the use of robust measures of insulin sensitivity and β-cell responsivity, and the use of DXA and CT to determine body composition and intra-abdominal fat. Limitations included the small size within each ethnic-age subgroup, and the use of only healthy, normal glucose tolerant participants, which may limit our ability to draw conclusions regarding inherent ethnic differences in the progression toward T2D. In addition, no data regarding dietary intake or physical activity were available for this cohort.

To conclude, in this sample of healthy girls and women, AA showed lower insulin sensitivity but greater intravenous glucose tolerance, compared to EA. Furthermore, differences in the pattern of insulin secretion were noted, with greater X0 and Phi1, but reduced Phi2 after adjustment for S_I, among AA. These differences were not attributable to adiposity, suggesting that inherent ethnic differences in insulin sensitivity and β-cell physiology may exist. Further study is needed to determine if these differences play any role in the etiology T2D among AA. In this group of healthy subjects, older age was associated with increased insulin sensitivity after accounting for total and central adiposity, and with greater Phi2. These differences were similar in both ethnic groups, suggesting that aging does not exert a disproportionate effect on elements of glucose metabolism within AA. It remains to be determined whether greater S_I and Phi2 are inherent characteristics of women who are destined to remain glucose tolerant following menopause.