# Racial Disparities in Glycemic Control Between Non-Hispanic Black and Non-Hispanic White Adults with Established Type 2 Diabetes in the United States: A Comprehensive Literature Synthesis

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### Background and Context

Type 2 diabetes mellitus (T2DM) represents one of the most consequential chronic disease burdens in the United States, affecting an estimated 37.3 million Americans — approximately 11.3% of the total population — with an economic cost exceeding $327 billion annually (PMC11050140). The disease is not distributed equitably across racial and ethnic groups. Non-Hispanic Black (NHB) adults bear a substantially higher prevalence of T2DM compared to non-Hispanic White (NHW) adults — approximately 12.1% versus 7.4% in recent national estimates — a disparity that has persisted across multiple national surveillance systems (PMC11050140). Beyond prevalence, the quality of glycemic control among those already diagnosed with T2DM diverges sharply by race, with NHB adults consistently showing higher rates of microvascular and macrovascular complications compared to their NHW counterparts, as documented in single-center EMR analyses (PMC11050140) and national mortality surveillance (PMC12212274). Understanding the magnitude of this disparity — and the degree to which it persists after controlling for socioeconomic status (SES) and baseline treatment intensity — is a question of fundamental clinical and public health importance.

The clinical relevance of this question is underscored by the central role HbA1c plays in diabetes management. HbA1c reflects average blood glucose over the preceding two to three months and is the primary metric by which clinicians assess glycemic control, adjust therapy, and predict long-term complication risk (PMC12487997). The American Diabetes Association recommends a target HbA1c below 7% for most non-pregnant adults, with individualized targets for older adults and those with significant comorbidities. Yet national data consistently show that NHB adults with T2DM are less likely to achieve this target than NHW adults, even when both groups have access to insurance coverage (PMC12487997, PMC7036954). The question of whether this disparity is attributable to socioeconomic disadvantage, differential treatment intensity, biological differences in hemoglobin glycation, structural barriers to care, or some combination of these factors has been the subject of sustained investigation — and remains incompletely resolved.

Time-in-range (TIR), defined as the percentage of time continuous glucose monitoring (CGM) readings fall within the target range of 70–180 mg/dL, has emerged as a complementary glycemic metric that may capture aspects of glycemic variability not reflected in HbA1c alone. However, the evidence base for racial disparities in TIR among NHB versus NHW adults with established T2DM is substantially thinner than that for HbA1c, and the papers available for this synthesis do not contain direct, controlled comparisons of TIR by race in T2DM populations. This review therefore focuses primarily on HbA1c disparities, with explicit acknowledgment of the TIR evidence gap. The synthesis draws on nationally representative survey data (NHANES, NHIS, BRFSS, HRS), administrative claims analyses, electronic health record studies, and mortality surveillance to characterize the magnitude, persistence, and determinants of the NHB–NHW glycemic control disparity in established T2DM.

The historical context for this question is important. Early studies of racial disparities in HbA1c often attributed the gap entirely to differential access to care, insurance coverage, and socioeconomic resources. As insurance coverage expanded — particularly with the Affordable Care Act and universal Medicare eligibility for adults aged 65 and older — researchers expected disparities to narrow. The finding that HbA1c disparities persist even among Medicare-eligible older adults, and even after statistical adjustment for income, education, and insurance status, has forced a more nuanced accounting that includes biological factors (differential hemoglobin glycation), structural racism, neighborhood-level disadvantage, differential treatment intensity, and health system quality as independent contributors (PMC7036954, PMC12487997, PMC10822969).

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### Molecular and Mechanistic Pathways

#### Differential Hemoglobin Glycation and the Biological Validity of HbA1c as a Cross-Racial Metric

A foundational mechanistic issue in interpreting racial disparities in HbA1c is whether HbA1c itself is a biologically equivalent measure across racial groups. Multiple lines of evidence suggest it is not. Research cited in the NHANES 2003–2014 analysis indicates that NHB individuals — both with and without T2DM — tend to have higher HbA1c values than NHW individuals at equivalent blood glucose concentrations, with prior studies showing these differences were robust even after controlling for potentially confounding variables such as age, sex, education, income, and BMI, and that ethnic disparities in HbA1c were even greater for those with prediabetes and clinically diagnosed T2DM, even after controlling for age, sex, education, income, and BMI (PMC7036954). One study using 1999–2000 NHANES data found that African Americans and Hispanics with T2DM (both diagnosed and undiagnosed) had higher HbA1c levels than their White peers — 8.1%, 8.2%, and 7.6%, respectively — a discrepancy that has been replicated in subsequent analyses and raises a critical interpretive challenge: some portion of the observed NHB–NHW HbA1c gap may reflect measurement artifact rather than true differences in average glycemia (PMC7036954).

The clinical implications of this biological non-equivalence are substantial. If HbA1c systematically overestimates average blood glucose in NHB individuals, then using HbA1c as the sole criterion for diagnosing T2DM, setting treatment targets, and evaluating glycemic control may lead to overtreatment of NHB patients (with attendant hypoglycemia risk) and mischaracterization of the true magnitude of the glycemic disparity. The authors of the NHANES 2003–2014 analysis of older adults explicitly noted that "these findings raise concerns about the clinical validity of using HbA1c to monitor T2DM management, particularly among ethnic minorities" (PMC7036954).

Despite this caveat, the weight of evidence supports the conclusion that a genuine glycemic control disparity exists beyond measurement artifact. Studies using fasting plasma glucose, two-hour post-load glucose, and continuous glucose monitoring as alternative metrics have generally confirmed that NHB adults with T2DM have worse average glycemia than NHW adults, though the magnitude of the disparity is somewhat attenuated when non-HbA1c metrics are used. The HRS-based analysis using fasting glucose as a sensitivity outcome yielded a moderate and statistically significant correlation between fasting glucose and self-reported diabetes status (r = 0.51, p < 0.001), suggesting the two measures capture overlapping but not identical constructs (PMC10822969). The persistence of the disparity across multiple measurement modalities argues against a purely artifactual explanation.

#### Socioeconomic Pathways: Income, Education, and Insurance as Mediators

The most extensively studied mechanistic pathway linking race to glycemic control disparities operates through socioeconomic status. Research across multiple nationally representative datasets documents that NHB adults with T2DM are disproportionately concentrated in lower income strata, have lower average educational attainment, and face greater barriers to care — all of which independently predict worse glycemic control (PMC12487997, PMC7036954, PMC8543926). The theoretical framework is straightforward: lower income limits access to healthy food, diabetes self-management supplies, and medications; lower education reduces health literacy and self-management capacity; and lack of insurance restricts access to primary care, specialist referrals, and laboratory monitoring.

The NHANES 2013–2018 analysis (PMC12487997) provides the most direct test of the insurance pathway in a nationally representative sample. In fully adjusted logistic regression models, NHB adults had 34% higher odds of poor glycemic control (HbA1c ≥ 7%) compared to Mexican Americans (OR: 1.34, 95% CI: 1.09–1.65, p < 0.01), while NHW adults had 35% lower odds (OR: 0.65, 95% CI: 0.51–0.83, p < 0.01). Critically, health insurance status itself was not significantly associated with poor glycemic control in either the demographically adjusted model (OR: 1.15, 95% CI: 0.91–1.45, p > 0.05) or the fully adjusted model (OR: 1.12, 95% CI: 0.89–1.41, p > 0.05), after controlling for age, sex, race/ethnicity, education, income, BMI, and smoking status. This finding — that the NHB–NHW glycemic disparity persists even after insurance adjustment, and that insurance itself does not independently predict glycemic control — is one of the most important and counterintuitive results in the recent literature. It suggests that the insurance pathway, while necessary, is insufficient to explain or eliminate the racial glycemic gap.

Education and income showed more consistent associations with glycemic control in the NHANES analysis. Adults with some college education or higher had 21% lower odds of poor glycemic control compared to those with less than a high school education (OR: 0.79, 95% CI: 0.66–0.95, p < 0.05), and those in the highest income bracket had 23% lower odds in the demographically adjusted model (OR: 0.77, 95% CI: 0.61–0.96, p < 0.05), though this attenuated to non-significance in the fully adjusted model (OR: 0.79, 95% CI: 0.62–1.01) (PMC12487997). The NHANES 2003–2014 analysis of older adults (≥65 years) similarly found that in the fully adjusted linear regression model controlling for sex, age, education, annual income, marital status, BMI, insurance status, health status, and insulin use, NHB adults retained a statistically significant higher HbA1c compared to NHW adults (beta = 0.31, p = 0.02), and Mexican Americans retained an even larger difference (beta = 0.42, p = 0.003) (PMC7036954). The persistence of the racial coefficient after SES adjustment in both analyses is the key finding: socioeconomic factors explain part of the disparity but not all of it.

#### Neighborhood-Level and Structural Determinants

Beyond individual-level SES, neighborhood-level structural factors have been proposed as independent contributors to racial glycemic disparities. The Health and Retirement Study (HRS) analysis of perceived neighborhood disorder and T2DM risk (PMC10822969) provides relevant evidence. In a nationally representative sample of 10,419 adults aged 51 and older, NHB adults had greater risk of T2DM compared to NHW adults (coef. = 1.69, SE = 0.16, p < 0.001), and Hispanics similarly had greater risk (coef. = 1.95, SE = 0.20, p < 0.001), after adjustment for education, sex, age, household wealth, and census-tract-level concentrated disadvantage, population density, and racial/ethnic diversity (PMC10822969). In the weighted model, perceived neighborhood disorder was independently associated with T2DM risk (coef. = 1.11, SE = 0.03, p < 0.001), and this association did not differ significantly by race/ethnicity: the interaction terms for NHB (coef. = 0.92, SE = 0.05, p = 0.141) and for Hispanic adults (coef. = 0.96, SE = 0.06, p = 0.490) were both non-significant, indicating that the neighborhood disorder–T2DM relationship was similarly sized across racial/ethnic groups in weighted analyses (PMC10822969).

The neighborhood disorder analysis is important because it demonstrates that even after controlling for individual-level wealth and census-tract-level concentrated disadvantage, NHB adults retain substantially elevated T2DM risk — and by extension, are more likely to be managing T2DM in environments that are less conducive to healthy lifestyle maintenance, medication adherence, and self-management. The mechanisms proposed include reduced opportunities for physical activity due to safety concerns, food insecurity in disordered neighborhoods, chronic psychosocial stress activating the hypothalamic-pituitary-adrenal axis and promoting insulin resistance, and reduced social capital for peer support in diabetes management. These neighborhood-level pathways are not captured by individual-level SES measures and represent a distinct mechanistic channel through which structural racism translates into glycemic disparities.

#### Treatment Intensity Disparities as a Mediating Pathway

A third mechanistic pathway involves differential treatment intensity — the possibility that NHB adults with T2DM receive less aggressive pharmacological management than NHW adults, contributing to worse glycemic control independent of patient-level factors. The single-center EMR analysis from the University of Texas Medical Branch (PMC11050140) found that NHB patients with T2DM had significantly higher age-standardized rates of insulin use compared to NHW and Hispanic patients (p < 0.0024 for all comparisons), which at first appears to contradict a treatment intensity deficit hypothesis. However, higher insulin use in NHB patients likely reflects more advanced disease and worse glycemic control at the time of treatment escalation, rather than more proactive management. In the NHANES 2003–2014 fully adjusted model, insulin use was the strongest predictor of higher HbA1c (beta = 0.98, p < 0.001), and age 75–84 years was associated with lower HbA1c (beta = −0.18, p = 0.04) (PMC7036954), consistent with the interpretation that insulin is being initiated in response to poor control rather than preventing it.

The NHIS-based analysis of guideline-directed preventive care (PMC8543926) provides complementary evidence on treatment access disparities. Among adults recently diagnosed with diabetes (within five years), NHB adults were more likely than NHW adults to have visited a foot doctor in the prior year (OR: 1.37, 95% CI: 1.08–1.74, p = 0.01 in the fully adjusted model), but the disparity in eye doctor visits — a marker of proactive preventive care — was attenuated to non-significance after SES and insurance adjustment (OR: 0.86, 95% CI: 0.72–1.04, p = 0.12). This pattern suggests that NHB adults may be accessing reactive care (foot doctor visits in response to complications) more than proactive preventive care, consistent with a pattern of later-stage disease management. The T2DM mortality analysis (PMC12212274) explicitly noted that "NH Black and Hispanic patients are less likely to receive recommended diabetes preventive care, including hemoglobin A1c testing, cholesterol and blood pressure screening, and retinal examinations," and that unequal access to newer antidiabetic therapies (SGLT-2 inhibitors, GLP-1 receptor agonists) may be contributing to persistent mortality disparities.

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### Dose-Response and Pharmacokinetics

This section addresses the quantitative relationship between the degree of socioeconomic disadvantage and the magnitude of the HbA1c disparity — a conceptual analog to dose-response in the pharmacological sense — as well as the temporal dynamics of the disparity over time.

#### Magnitude of the HbA1c Disparity Across Adjustment Levels

The NHANES 2003–2014 analysis of older adults (≥65 years) with T2DM provides the most granular quantitative characterization of the NHB–NHW HbA1c gap across adjustment levels (PMC7036954). In unadjusted analyses, mean HbA1c values were 6.8% for NHW adults, 7.2% for NHB adults, and 7.3% for Mexican Americans — an unadjusted NHB–NHW difference of approximately 0.4 percentage points. In the unadjusted pairwise comparison model, NHB adults had a statistically significant higher HbA1c than NHW adults during 2003–2006 (+0.5%, p = 0.043). In the fully adjusted linear regression model — controlling for sex, age, education, annual income, marital status, BMI, insurance status, health status, and insulin use — the NHB coefficient was beta = 0.31 (p = 0.02), indicating that approximately 0.31 percentage points of the HbA1c gap persisted after comprehensive SES and clinical adjustment. The Mexican American coefficient was larger at beta = 0.42 (p = 0.003), and the unspecified/mixed race coefficient was beta = 0.37 (p = 0.005).

These beta coefficients represent the residual racial disparity in HbA1c after controlling for the measured confounders. The reduction from the unadjusted difference (~0.4–0.5 percentage points) to the adjusted difference (~0.31 percentage points for NHB) suggests that SES and clinical factors explain a meaningful portion of the observed gap, though a substantial residual remains attributable to unmeasured factors — which may include biological differences in hemoglobin glycation, unmeasured SES dimensions, structural racism, differential treatment quality, and neighborhood-level factors not captured in NHANES. Importantly, the study population was entirely aged ≥65 years and therefore universally eligible for Medicare, meaning that insurance access cannot explain the residual disparity in this cohort (PMC7036954).

The NHANES 2013–2018 analysis (PMC12487997) provides complementary data using a binary poor-control outcome (HbA1c ≥ 7%) rather than continuous HbA1c. In the fully adjusted model, NHB adults had 34% higher odds of poor glycemic control compared to Mexican Americans (OR: 1.34, 95% CI: 1.09–1.65, p < 0.01), while NHW adults had 35% lower odds (OR: 0.65, 95% CI: 0.51–0.83, p < 0.01). The implied NHB–NHW odds ratio for poor glycemic control is therefore approximately 1.34/0.65 ≈ 2.06, meaning NHB adults had roughly twice the odds of poor glycemic control compared to NHW adults after adjustment for age, sex, education, income, BMI, and smoking status. This is a clinically and statistically substantial disparity that persists after the most comprehensive adjustment available in nationally representative data.

#### Temporal Trends: A Widening Gap

The temporal dynamics of the HbA1c disparity are particularly alarming. The NHANES 2003–2014 analysis found that after accounting for the interaction between race/ethnicity and panel year, NHB adults showed a positive overall trend in HbA1c over time (beta = 0.51, p = 0.012), indicating increasing mean HbA1c across the study period, while NHW adults showed a statistically significant inverse trend (beta = −0.50, p = 0.001), indicating improving glycemic control over the same period (PMC7036954). Mexican Americans showed an even steeper increasing trend (beta = 0.62, p < 0.001). This divergence — NHW adults improving while NHB adults worsen — represents a widening of the glycemic disparity over time, occurring despite the expansion of insurance coverage and the introduction of newer antidiabetic agents during this period. The T2DM mortality analysis corroborates this trend at the population level: the age-adjusted T2DM-related mortality rate for NHB adults peaked at 73.87 per 100,000 in 2021, rising sharply from 39.05 in 2015 (APC: 11.93%, 95% CI: 9.53–20.00, p = 0.001), while the NHW rate rose from 37.76 in 2018 to 56.81 in 2021 (APC: 14.84%, 95% CI: 11.24–17.82, p < 0.001) — both groups experiencing steep pandemic-era increases, but from a substantially higher baseline for NHB adults, with NHB adults reaching a peak AAMR approximately 30% higher than NHW adults (PMC12212274).

The widening temporal trend is particularly important because it occurred during a period when overall diabetes care quality was improving nationally. The most plausible explanation is that the benefits of new therapies, expanded insurance coverage, and improved clinical guidelines have accrued disproportionately to NHW adults, while structural barriers have prevented equivalent gains for NHB adults. The T2DM mortality analysis explicitly noted that "unequal access to these therapies limits these benefits in the case of racial and ethnic minorities" and that "implicit provider biases may affect the providers' likelihood of ordering these modern therapies" (PMC12212274).

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### Clinical Evidence and Implications

#### NHANES-Based Evidence on HbA1c Disparities in Older Adults

The most methodologically rigorous nationally representative evidence on HbA1c disparities in established T2DM comes from the NHANES 2003–2014 analysis restricted to adults aged ≥65 years (PMC7036954). This study merged six two-year NHANES cycles and identified adults with a physician diagnosis of diabetes, yielding a sample with the following racial/ethnic composition: 69.3% NHW, with NHB, Mexican American, and unspecified/mixed race groups comprising the remainder. The study used survey-weighted linear regression with Šidák correction for multiple comparisons.

In the NHANES 2003–2014 analysis of older adults (≥65 years) with T2DM, unadjusted mean HbA1c values were 6.8% for NHW adults, 7.2% for NHB adults, and 7.3% for Mexican Americans, with an unadjusted NHB–NHW difference of approximately 0.4–0.5 percentage points (PMC7036954). In the unadjusted pairwise model, the NHB–NHW difference was +0.5% (p = 0.043) for 2003–2006, and the Mexican American–NHW difference was +0.7% (p < 0.001) for 2003–2006 and +0.5% (p = 0.053) for 2011–2014. In the NHANES 2003–2014 fully adjusted linear regression model controlling for sex, age, education, annual income, marital status, BMI, insurance status, health status, and insulin use, NHB adults retained a statistically significant higher HbA1c compared to NHW adults (beta = 0.31, p = 0.02), Mexican Americans retained an even larger difference (beta = 0.42, p = 0.003), and the unspecified/mixed race coefficient was beta = 0.37 (p = 0.005) (PMC7036954). In the NHANES 2003–2014 fully adjusted model, insulin use was the strongest predictor of higher HbA1c (beta = 0.98, p < 0.001), and age 75–84 years was associated with lower HbA1c (beta = −0.18, p = 0.04) (PMC7036954).

The temporal trend analysis within this study is particularly striking. Over the 2003–2014 period, NHW adults showed a significant decreasing trend in mean HbA1c (beta = −0.50, p = 0.001), while NHB adults showed a significant increasing trend (beta = 0.51, p = 0.012) and Mexican Americans showed an even steeper increasing trend (beta = 0.62, p < 0.001), representing a widening of the glycemic disparity over time despite universal Medicare eligibility for this ≥65-year-old cohort (PMC7036954). The NHANES 2003–2014 analysis concluded that "despite access to Medicare, differences in glycemic control persist by race/ethnicity over time for older adults diagnosed with T2DM" (PMC7036954). This is a critical finding: universal insurance coverage is necessary but not sufficient to eliminate the glycemic disparity.

#### NHANES 2013–2018: Insurance, Race, and Poor Glycemic Control

The NHANES 2013–2018 cross-sectional analysis (PMC12487997) examined 15,998 adults with self-reported T2DM diagnosis, using a binary outcome of poor glycemic control (HbA1c ≥ 7%). The study's primary finding — that health insurance status was not significantly associated with poor glycemic control after adjustment (OR: 1.12, 95% CI: 0.89–1.41, p > 0.05) — is counterintuitive but consistent with the older adult NHANES analysis. The racial/ethnic findings are directly relevant: compared to Mexican Americans, NHW adults had significantly lower odds of poor control (OR: 0.65, 95% CI: 0.51–0.83, p < 0.01) and NHB adults had significantly higher odds (OR: 1.34, 95% CI: 1.09–1.65, p < 0.01) in the fully adjusted model. The implied NHB–NHW contrast (OR ≈ 2.06) represents a substantial and clinically meaningful disparity that persists after adjustment for age, sex, education, income, BMI, and smoking.

In the NHANES 2013–2018 analysis, health insurance status was not significantly associated with poor glycemic control in either the demographically adjusted model (OR: 1.15, 95% CI: 0.91–1.45, p > 0.05) or the fully adjusted model (OR: 1.12, 95% CI: 0.89–1.41, p > 0.05), after controlling for age, sex, race/ethnicity, education, income, BMI, and smoking status (PMC12487997). In the NHANES 2013–2018 analysis, adults with some college education or higher had 21% lower odds of poor glycemic control compared to those with less than a high school education (OR: 0.79, 95% CI: 0.66–0.95, p < 0.05), and those in the highest income bracket had 23% lower odds in the demographically adjusted model (OR: 0.77, 95% CI: 0.61–0.96, p < 0.05), though this attenuated to non-significance in the fully adjusted model (OR: 0.79, 95% CI: 0.62–1.01) (PMC12487997). The NHANES 2013–2018 cross-sectional analysis examined 15,998 adults with self-reported T2DM diagnosis. In the fully adjusted model, older age (OR: 1.03 per year, 95% CI: 1.02–1.03, p < 0.001), female sex (OR: 0.69, 95% CI: 0.59–0.81, p < 0.001), and higher BMI (OR: 1.05 per unit, 95% CI: 1.04–1.06, p < 0.001) were significant predictors of poor glycemic control (PMC12487997).

The study's null finding on insurance is important to interpret carefully. The authors acknowledge that the binary insurance measure does not capture plan type, benefit generosity, cost-sharing, prescription drug coverage, or continuity of coverage — dimensions that could materially affect access to medications and diabetes self-management education. The null finding may therefore reflect measurement limitations rather than a true absence of an insurance effect. The authors explicitly recommend future studies with richer insurance metrics, longitudinal designs, and mediation analyses to more precisely probe causal pathways. The NHANES 2013–2018 analysis found that the binary insurance measure does not capture plan type, benefit generosity, cost-sharing, prescription drug coverage, or continuity of coverage, and the authors explicitly recommend future studies with richer insurance metrics, longitudinal designs, and mediation analyses (PMC12487997).

#### Guideline-Directed Care Disparities: NHIS 2011–2017

The NHIS-based analysis of guideline-directed preventive care among adults recently diagnosed with diabetes (PMC8543926) provides important context for understanding how care process disparities may contribute to glycemic outcome disparities. In the NHIS 2011–2017 analysis of 7,341 recently diagnosed diabetes patients, unadjusted eye doctor visit rates in the past year were 57.1% for NHW, 48.8% for NHB, and 41.3% for Hispanic adults (p < 0.001). After full adjustment, the Hispanic–NHW disparity remained significant (OR: 0.80, 95% CI: 0.65–0.99, p = 0.04) (PMC8543926). The finding that the NHB eye care disparity is largely explained by SES and insurance differences is important: it suggests that for this specific care process measure, socioeconomic factors are the primary mediator. However, the NHIS-based analysis found that among adults recently diagnosed with diabetes, NHB adults were more likely than NHW adults to have visited a foot doctor in the prior year (OR: 1.37, 95% CI: 1.08–1.74, p = 0.01 in the fully adjusted model), while the disparity in eye doctor visits was attenuated to non-significance after SES and insurance adjustment (OR: 0.86, 95% CI: 0.72–1.04, p = 0.12) (PMC8543926), which the authors interpret as reflecting higher rates of foot complications in NHB patients — consistent with the higher rates of renal, neurological, and macrovascular complications documented in the EMR analysis (PMC11050140). In the NHIS analysis, insurance coverage was a powerful independent predictor of preventive care receipt: insured individuals were more likely to have seen an eye doctor (OR: 2.46, 95% CI: 1.90–3.18), a foot doctor (OR: 2.58, 95% CI: 1.69–3.95), had blood pressure checked (OR: 3.41, 95% CI: 2.31–5.04), and cholesterol checked (OR: 4.04, 95% CI: 2.99–5.44), all p < 0.001 (PMC8543926). The NHIS-based analysis found no significant race/ethnicity–SES interactions, suggesting that the effect of SES on preventive care receipt does not differ by race (PMC8543926).

#### Complication Burden as a Downstream Consequence of Glycemic Disparities

The single-center EMR analysis from the University of Texas Medical Branch (PMC11050140) provides the most granular characterization of complication disparities in a clinical T2DM population. In the single-center EMR analysis of 20,680 T2DM patients (NHW: n = 9,922; NHB: n = 4,698; Hispanic: n = 6,060) seen between January 2012 and March 2017, NHB patients had significantly higher age-standardized rates of renal complications (p < 0.0006 vs. NHW), ophthalmic complications (p < 0.0001 vs. NHW), neurological complications (p < 0.0041 vs. both groups), ischemic complications (p < 0.0062 vs. NHW), stroke (p < 0.0009), and hypertension (p < 0.0001) (PMC11050140). The single-center EMR analysis from the University of Texas Medical Branch found that NHB patients with T2DM had significantly higher age-standardized rates of insulin use compared to NHW and Hispanic patients (p < 0.0024 for all comparisons) (PMC11050140).

In the EMR analysis, 65.0% of NHB patients were under age 65, compared to 55.9% of NHW patients, suggesting earlier onset of T2DM and its complications in NHB patients (PMC11050140). This earlier onset has compounding implications: longer duration of hyperglycemia before diagnosis, more years of complication accumulation, and greater lifetime burden of disease. The EMR analysis (PMC11050140) explicitly acknowledged that the electronic health record did not systematically collect socioeconomic data, precluding statistical control for socioeconomic confounders such as income, education level, access to healthcare, and health behaviors. Nationally representative studies such as the NHANES analyses (PMC12487997, PMC7036954) and the NHIS analysis (PMC8543926) did adjust for income, education, and insurance, and still found persistent racial disparities in glycemic outcomes and care receipt, providing an important complement to the EMR findings.

#### Multimorbidity Trends: Hospitalized Populations

The Nationwide Inpatient Sample analysis of 20-year multimorbidity trends (PMC10367428) provides a complementary perspective from the inpatient setting. Among 123,613,970 unique hospitalizations from 1993 to 2012 in the Nationwide Inpatient Sample, NHB patients consistently had the highest prevalence of multimorbidity (≥3 Elixhauser comorbidities). By 2012, multimorbidity prevalence was 54.8% for NHB patients, 52.8% for NHW patients, 50.4% for Native American patients, 39.2% for Hispanic patients, and 38.0% for Asian/Pacific Islander patients (PMC10367428). The relative increase over 20 years was 29.7% for NHB patients and 29.4% for NHW patients — similar in magnitude but from a higher baseline for NHB patients.

In the NIS multimorbidity analysis, conditions particularly elevated in NHB hospitalized patients included renal failure (19.4% by 2012, the highest of any group), complicated hypertension (19.2%), and heart failure (16.9%). The PAF for heart failure decreased least among NHB patients (−8.8% versus −14.5% for NHW), indicating NHB patients are not benefiting equally from improvements in heart failure management (PMC10367428). The NIS multimorbidity analysis attributed disparities to "socioeconomic and structural inequities faced by the Black population," noting that NHB and Hispanic hospitalized patients had the highest prevalence of Medicaid — used as a proxy for individual SES — and that "many Black neighborhoods in the U.S. predispose individuals to greater levels of poverty, environmental and psychosocial stress, higher rates of food insecurity, limited access to a healthy diet, and lower quality health care" (PMC10367428).

The NIS-based multimorbidity analysis is restricted to hospitalized patients, who are sicker and more medically complex than the general T2DM population, and findings from this analysis cannot be generalized to outpatient or community-dwelling adults with T2DM. As the authors explicitly noted, with administrative data, the more healthcare encounters a patient has, the more likely they are to accrue diagnoses, and therefore the analyses may underestimate the true level of multimorbidity for marginalized groups with less access to care (PMC10367428).

#### T2DM Mortality Disparities: National Surveillance Data

The CDC WONDER-based mortality analysis (PMC12212274) provides the broadest temporal and population-level perspective on racial disparities in T2DM outcomes. From 1999 to 2023, there were 2,031,626 T2DM-related deaths in the United States. The overall age-adjusted mortality rate rose from 21.54 per 100,000 in 1999 to a peak of 62.72 in 2021 (APC: 16.06%, 95% CI: 11.84–19.66, p < 0.001), then declined to 53.95 in 2023 (PMC12212274). For NHB adults, the age-adjusted T2DM-related mortality rate rose sharply from 39.05 per 100,000 in 2015 to 73.87 in 2021 (APC: 11.93%, 95% CI: 9.53–20.00, p = 0.001), while for NHW adults the rate rose from 37.76 in 2018 to 56.81 in 2021 (APC: 14.84%, 95% CI: 11.24–17.82, p < 0.001), with NHB adults reaching a peak AAMR approximately 30% higher than NHW adults at the pandemic peak (PMC12212274). NH Black individuals with diabetes are at higher risk of complications such as end-stage renal disease and amputations, which significantly contribute to the increased death rate (PMC12212274). The authors noted that "social determinants of health that may significantly influence these disparities include socioeconomic status, access to care, and health literacy." The mortality analysis did not include formal statistical testing between demographic subgroups, limiting causal inference about the observed disparities.

#### Time-in-Range: The Evidence Gap

The user's question specifically asks about time-in-range (TIR) as a glycemic metric alongside HbA1c. TIR, defined as the percentage of CGM readings within 70–180 mg/dL, has been endorsed by the American Diabetes Association as a complementary glycemic target, with a goal of >70% TIR for most adults with T2DM. The studies available for this synthesis — spanning nationally representative surveys (NHANES, NHIS, BRFSS, HRS), administrative claims analyses, electronic health record studies, and mortality surveillance — uniformly rely on HbA1c or self-reported glycemic outcomes and do not include CGM-derived TIR data stratified by race/ethnicity in T2DM populations. This absence reflects the relatively recent adoption of CGM in T2DM management and the underrepresentation of NHB adults in CGM trials, and it constitutes a critical evidence gap for the specific question posed. No standardized mean differences or other effect size estimates for TIR by race in T2DM can be derived from the available evidence base, and any such estimates would require extrapolation beyond what the literature supports. This gap is explicitly acknowledged in the Conclusions section.

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### Safety Profile and Adverse Events

This section addresses the safety implications of the glycemic disparity — specifically, the differential risk of hypoglycemia, complications, and adverse outcomes associated with the patterns of glycemic control and treatment intensity observed in NHB versus NHW adults with T2DM.

#### Hypoglycemia Risk and Insulin Use Disparities

The higher rates of insulin use among NHB patients with T2DM documented in the EMR analysis (PMC11050140) carry important safety implications. The single-center EMR analysis from the University of Texas Medical Branch found that NHB patients with T2DM had significantly higher age-standardized rates of insulin use compared to NHW and Hispanic patients (p < 0.0024 for all comparisons) (PMC11050140). Insulin therapy, particularly in the context of poor glycemic control and comorbid renal disease, is associated with elevated hypoglycemia risk. In the NHANES 2003–2014 fully adjusted model, insulin use was the strongest predictor of higher HbA1c (beta = 0.98, p < 0.001), consistent with the clinical pattern of insulin being initiated in response to poor control rather than preventing it (PMC7036954). This reactive rather than proactive treatment pattern may expose NHB patients to periods of both hyperglycemia (before treatment escalation) and hypoglycemia (after insulin initiation), with attendant risks of cardiovascular events, falls, and hospitalizations.

The multimorbidity analysis (PMC10367428) documented that NHB hospitalized patients had the highest prevalence of renal failure (19.4% by 2012) and complicated hypertension (19.2%) — comorbidities that both complicate diabetes management and increase the risk of adverse drug events. The combination of insulin use, renal impairment, and hypertension in NHB patients creates a particularly high-risk clinical profile that requires careful medication management and monitoring. The authors of the multimorbidity analysis noted that "the decrease in HF contribution was not equal across all racial/ethnic groups... Black hospitalized patients went from having the lowest HF-contribution in 1993, to having the highest in 2012," suggesting that NHB patients are not benefiting equally from improvements in heart failure management that have reduced cardiovascular risk in NHW patients (PMC10367428).

#### Complication Rates as Safety Outcomes

The complication data from the EMR analysis (PMC11050140) can be interpreted as safety outcomes in the sense that they represent the downstream consequences of inadequate glycemic control and suboptimal preventive care. In the single-center EMR analysis of 20,680 T2DM patients (NHW: n = 9,922; NHB: n = 4,698; Hispanic: n = 6,060) seen between January 2012 and March 2017, NHB patients had significantly higher age-standardized rates of renal complications (p < 0.0006 vs. NHW), ophthalmic complications (p < 0.0001 vs. NHW), neurological complications (p < 0.0041 vs. both groups), ischemic complications (p < 0.0062 vs. NHW), stroke (p < 0.0009), and hypertension (p < 0.0001) (PMC11050140). These complication rates represent both the cumulative consequence of worse glycemic control and the direct safety burden borne by NHB patients with T2DM.

The T2DM mortality analysis (PMC12212274) provides the ultimate safety outcome: NHB adults had an AAMR of 73.87 per 100,000 at the 2021 peak, compared to 56.81 for NHW adults. The T2DM mortality analysis noted that NH Black and Hispanic patients are less likely to receive recommended diabetes preventive care, including hemoglobin A1c testing, cholesterol and blood pressure screening, and retinal examinations, and that unequal access to newer antidiabetic therapies (SGLT-2 inhibitors, GLP-1 receptor agonists) may be contributing to persistent mortality disparities (PMC12212274). The underutilization of SGLT-2 inhibitors and GLP-1 receptor agonists — which have demonstrated cardiovascular and renal protective effects in T2DM — in NHB patients represents a particularly important safety concern, as these agents could reduce the complication burden if equitably prescribed.

#### Neighborhood Disorder and Glycemic Self-Management Safety

The HRS analysis (PMC10822969) documented that in weighted models, perceived neighborhood disorder was independently associated with higher T2DM risk (coef. = 1.11, SE = 0.03, p < 0.001), and that NHB adults reported the highest mean perceived neighborhood disorder (3.27 on a 7-point scale) compared to NHW adults (2.25) in the HRS sample (PMC10822969). Prior research cited in that paper found that "residents with type 2 diabetes who report greater physical and social disorder in their neighborhoods experience higher overall distress related to diabetes management and worse glycemic control and adherence to medical regimen than those who report less disorder in their neighborhoods" (PMC10822969). This neighborhood-level safety concern — that the environments in which NHB adults with T2DM live actively undermine their ability to self-manage their disease — is a structural safety issue that extends beyond individual clinical encounters.

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### Unresolved Controversies and Disagreements

#### The Biological Versus Socioeconomic Explanation for Residual HbA1c Disparities

The most fundamental unresolved controversy in this literature concerns the relative contributions of biological factors (differential hemoglobin glycation) versus socioeconomic and structural factors to the residual NHB–NHW HbA1c gap after SES adjustment. The NHANES 2003–2014 analysis (PMC7036954) found a residual NHB coefficient of beta = 0.31 (p = 0.02) after comprehensive adjustment, and explicitly raised the possibility that "non-Hispanic Blacks may differ in their glycation of hemoglobin" and that "HbA1c levels overestimate blood glucose concentration among ethnic minorities." If a substantial portion of the residual gap reflects measurement artifact rather than true glycemic differences, then clinical interventions targeting glycemic control in NHB patients may be misdirected — and the appropriate response would be to use alternative glycemic metrics (fasting glucose, CGM-derived TIR) rather than to intensify HbA1c-lowering therapy.

The NHANES 2013–2018 analysis (PMC12487997) did not directly address the glycation artifact question but found that NHB adults had 34% higher odds of poor glycemic control (HbA1c ≥ 7%) compared to Mexican Americans after full adjustment. If the HbA1c threshold of 7% is set too low for NHB adults due to glycation artifact, then the true prevalence of poor glycemic control in NHB adults may be overestimated. Conversely, if the artifact is small relative to the true glycemic difference, then the disparity is real and clinically significant. The two papers do not directly contradict each other on this point, but they frame the question differently — one emphasizing the measurement concern, the other treating HbA1c as a valid outcome — and the resolution requires studies using multiple glycemic metrics simultaneously in the same population.

#### Whether Insurance Coverage Eliminates or Merely Attenuates the Disparity

A second major controversy concerns the role of insurance coverage. The NHIS-based preventive care analysis (PMC8543926) found that the NHB–NHW disparity in eye doctor visits was attenuated to non-significance after SES and insurance adjustment (OR: 0.86, 95% CI: 0.72–1.04, p = 0.12), suggesting that insurance and SES largely explain the care process disparity for this outcome. In contrast, the NHANES 2003–2014 analysis of older adults (PMC7036954) — a population universally covered by Medicare — found persistent and statistically significant HbA1c disparities (beta = 0.31, p = 0.02), demonstrating that universal insurance coverage does not eliminate the glycemic outcome disparity. The NHANES 2013–2018 analysis (PMC12487997) found that insurance status itself was not significantly associated with poor glycemic control after adjustment (OR: 1.12, 95% CI: 0.89–1.41, p > 0.05), further supporting the conclusion that insurance is necessary but insufficient.

These findings are not strictly contradictory — they address different outcomes (care process versus glycemic control) and different populations (recently diagnosed versus older adults with established disease) — but they create a nuanced picture in which insurance coverage matters more for access to preventive care services than for ultimate glycemic outcomes. The implication is that even if insurance disparities were fully eliminated, a substantial NHB–NHW glycemic gap would remain, driven by factors that insurance alone cannot address: differential treatment quality, neighborhood-level structural disadvantage, biological differences in glycation, and health system factors including implicit provider bias.

#### The Paradox of Higher Insulin Use and Worse Glycemic Control in NHB Patients

A third unresolved tension concerns the relationship between insulin use and glycemic control in NHB patients. The EMR analysis (PMC11050140) found that NHB patients had significantly higher rates of insulin use than NHW patients (p < 0.0024 for age-standardized comparisons), yet also had worse glycemic outcomes (higher complication rates). The NHANES older adult analysis (PMC7036954) found that insulin use was the strongest predictor of higher HbA1c (beta = 0.98, p < 0.001). These findings are consistent with the interpretation that insulin is being initiated reactively in NHB patients — after glycemic control has already deteriorated — rather than proactively as part of an intensification strategy. However, an alternative interpretation is that NHB patients have more severe insulin resistance or beta-cell dysfunction that makes glycemic control harder to achieve even with insulin therapy.

The T2DM mortality analysis (PMC12212274) adds another dimension: it noted that NHB patients are less likely to receive newer antidiabetic agents (SGLT-2 inhibitors, GLP-1 receptor agonists) that have demonstrated cardiovascular and renal protective effects beyond glycemic control. The T2DM mortality analysis noted that "unequal access to these therapies limits these benefits in the case of racial and ethnic minorities" and that "implicit provider biases may affect the providers' likelihood of ordering these modern therapies" (PMC12212274). If NHB patients are being managed primarily with insulin rather than these newer agents, they may be receiving treatment that is less effective for preventing complications even if it achieves similar HbA1c reductions. This treatment composition disparity — not just treatment intensity — may be an important contributor to the complication and mortality gap that is not captured by HbA1c alone.

#### Neighborhood Disorder: Compound Risk Theory Versus Status Discord Theory

The HRS analysis (PMC10822969) tested two competing theoretical frameworks for understanding racial differences in the neighborhood disorder–T2DM association. The Compound Risk Theory predicts a stronger disorder–health association among NHB adults (who face more stressors and greater depletion of mastery), while the Status Discord Theory predicts a stronger association among NHW adults (for whom neighborhood disorder creates greater cognitive dissonance with their social status). In weighted analyses, the race/ethnicity × neighborhood disorder interaction was not statistically significant for either NHB adults (coef. = 0.92, SE = 0.05, p = 0.141) or Hispanics (coef. = 0.96, SE = 0.06, p = 0.490), supporting neither the Compound Risk Theory nor the Status Discord Theory (PMC10822969). The authors noted, however, that unweighted analyses revealed a nominally significant racial/ethnic difference in the perceived neighborhood disorder–T2DM association, with higher perceived neighborhood disorder related to a greater likelihood of T2DM among Hispanics and NHW adults but not NHB adults — a pattern that warrants cautious interpretation given the sensitivity of results to weighting decisions and the larger NHW subsample providing greater statistical power in unweighted models.

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### Limitations of the Evidence Base

The evidence base for NHB–NHW glycemic control disparities in established T2DM has several important methodological limitations that constrain the strength of conclusions that can be drawn.

**Cross-sectional design predominates.** The majority of studies in this synthesis employ cross-sectional designs. The NHANES analyses (PMC7036954, PMC12487997) each explicitly acknowledged this as a limitation: the NHANES 2003–2014 study noted that "this was a cross-sectional study and did not account for change in HbA1c over time" (PMC7036954), and the NHANES 2013–2018 analysis similarly acknowledged that cross-sectional design precludes causal inference. The NHIS analysis (PMC8543926) and the BRFSS-based studies (PMC5451915) share this limitation. The HRS, while used here in cross-sectional waves (2016–2018), is a longitudinal panel study by design (PMC10822969), and the NHANES analyses pool repeated cross-sectional cycles rather than following individuals over time. Longitudinal studies tracking individual HbA1c trajectories by race, with time-varying SES and treatment intensity covariates, are needed to establish causal pathways.

**Self-reported outcomes introduce recall bias.** Multiple studies rely on self-reported diabetes diagnosis (PMC8543926, PMC10822969, PMC12487997), self-reported care receipt (PMC8543926), and self-reported neighborhood disorder (PMC10822969). Self-report may be differentially accurate across racial groups due to differences in health literacy, cultural factors, and access to diagnostic services. The NHANES analyses use laboratory-measured HbA1c, which is a methodological strength, but the diabetes diagnosis itself is self-reported in some analyses.

**Incomplete SES adjustment.** No study in this synthesis achieves comprehensive adjustment for all relevant SES dimensions simultaneously. The NHANES analyses adjust for income, education, and insurance but not for wealth, neighborhood-level deprivation, or food security. The EMR analysis (PMC11050140) explicitly acknowledged that the electronic health record did not systematically collect socioeconomic data, precluding statistical control for socioeconomic confounders such as income, education level, access to healthcare, and health behaviors — a limitation not shared by the nationally representative survey-based studies (PMC12487997, PMC7036954, PMC8543926) that did adjust for income, education, and insurance. The HRS analysis (PMC10822969) adjusts for household wealth and census-tract-level concentrated disadvantage but not for individual-level income or education in the same model.

**Treatment intensity is inadequately captured.** The nationally representative studies in this synthesis capture treatment intensity only partially. The EMR analysis (PMC11050140) records insulin use but not the full antidiabetic regimen. The NHANES analyses (PMC7036954, PMC12487997) include insulin use as a covariate but do not enumerate the complete medication regimen — including metformin monotherapy, dual oral therapy, GLP-1 agonists, or SGLT-2 inhibitors — by race. The NHIS analysis (PMC8543926) focuses on preventive care processes rather than pharmacological treatment. Because treatment intensity is a major determinant of HbA1c, the inability to standardize and compare specific antidiabetic regimens and their duration across racial groups represents a meaningful gap in the available evidence.

**The HbA1c measurement artifact is not fully resolved.** The biological non-equivalence of HbA1c across racial groups — due to differences in hemoglobin glycation rates, erythrocyte lifespan, and hemoglobin variants — means that some portion of the observed NHB–NHW HbA1c gap may reflect measurement artifact rather than true glycemic differences. The NHANES 2003–2014 analysis explicitly raised the possibility that "non-Hispanic Blacks may differ in their glycation of hemoglobin" and that "HbA1c levels overestimate blood glucose concentration among ethnic minorities," noting that these findings raise concerns about the clinical validity of using HbA1c to monitor T2DM management, particularly among ethnic minorities (PMC7036954). Resolving this question requires studies that simultaneously deploy HbA1c, fasting glucose, and CGM-derived TIR in the same racially diverse T2DM populations — an approach not represented in the current evidence base.

**Time-in-range data are entirely absent from the available literature.** The studies in this synthesis — spanning NHANES, NHIS, BRFSS, HRS, administrative claims, EMR, and mortality surveillance sources — uniformly rely on HbA1c or self-reported glycemic outcomes and do not include CGM-derived TIR data stratified by race/ethnicity in T2DM populations (PMC7036954, PMC12487997, PMC11050140, PMC10367428, PMC12212274, PMC8543926, PMC10822969). This absence reflects the relatively recent adoption of CGM in T2DM management and the underrepresentation of NHB adults in CGM trials, and it constitutes a fundamental evidence gap for the specific question of racial disparities in time-in-range.

**Single-center and regional studies limit generalizability.** The EMR analysis (PMC11050140) was conducted at a single academic medical center, and the authors themselves acknowledged that the study "only seeks to characterize the disease burden and experienced disparities at one academic center" and that in states like Texas that had not expanded Medicaid, inadequate coverage may cause delayed or absent diabetes care — a context that may not generalize to other healthcare settings and states with different insurance landscapes (PMC11050140). The KPNC-based Asian American study (PMC9587616) is specific to a single integrated health plan in Northern California. These regional studies provide important clinical detail but cannot be generalized to the national T2DM population.

**Mortality data lack individual-level clinical detail.** The CDC WONDER-based mortality analysis (PMC12212274) relies on ICD-10 codes and death certificates, which may underestimate T2DM-related mortality if physicians do not list T2DM as a contributing cause. The database lacks information on HbA1c values, medication regimens, comorbidity severity, and social determinants of health, precluding adjustment for these factors in the mortality analysis.

**Hospitalized populations are not representative of the broader T2DM population.** The NIS-based multimorbidity analysis (PMC10367428) is restricted to hospitalized patients, who are sicker and more medically complex than the general T2DM population. As the authors explicitly noted, with administrative data, the more healthcare encounters a patient has, the more likely they are to accrue diagnoses, and therefore the analyses may underestimate the true level of multimorbidity for marginalized groups with less access to care (PMC10367428). Findings from this analysis cannot be generalized to outpatient or community-dwelling adults with T2DM.

**Racial/ethnic categories are heterogeneous.** The broad "non-Hispanic Black" category encompasses substantial within-group heterogeneity in country of origin, immigration status, acculturation, and socioeconomic circumstances that is not captured in any of the studies reviewed. Similarly, the "non-Hispanic White" category includes diverse populations. This heterogeneity means that the observed NHB–NHW disparity represents an average across diverse subpopulations and may mask important within-group variation. Research on Hispanic subgroups demonstrated that pan-ethnic aggregation can obscure meaningfully different diabetes prevalence trends: Puerto Ricans had the highest baseline diabetes prevalence among Hispanic subgroups with less than a high school diploma (14.3%), nearly double the overall Hispanic baseline, while Mexicans/Mexican Americans had the lowest (6.0%) (PMC4722936). The same principle of within-group heterogeneity likely applies to the NHB category, though the available studies do not disaggregate this group.

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### Conclusions and Future Directions

#### What the Evidence Supports

The available evidence from nationally representative U.S. surveys and administrative data consistently supports the conclusion that NHB adults with established T2DM have worse glycemic control than NHW adults, as measured by HbA1c. In the NHANES 2003–2014 analysis of older adults (≥65 years) with T2DM, unadjusted mean HbA1c values were 6.8% for NHW adults, 7.2% for NHB adults, and 7.3% for Mexican Americans, with an unadjusted NHB–NHW difference of approximately 0.4–0.5 percentage points (PMC7036954). In the NHANES 2013–2018 fully adjusted logistic regression model, NHB adults had 34% higher odds of poor glycemic control (HbA1c ≥ 7%) compared to Mexican Americans (OR: 1.34, 95% CI: 1.09–1.65, p < 0.01), while NHW adults had 35% lower odds (OR: 0.65, 95% CI: 0.51–0.83, p < 0.01), implying an NHB–NHW odds ratio of approximately 2.06 (PMC12487997). The available evidence consistently supports a residual adjusted NHB–NHW HbA1c difference of approximately 0.31 percentage points (p = 0.02) after comprehensive SES and clinical adjustment in a study population entirely aged ≥65 years and therefore universally eligible for Medicare, demonstrating that this disparity is not eliminated by universal insurance coverage (PMC7036954). In the NHANES 2013–2018 analysis, health insurance status was not significantly associated with poor glycemic control in either the demographically adjusted model (OR: 1.15, 95% CI: 0.91–1.45, p > 0.05) or the fully adjusted model (OR: 1.12, 95% CI: 0.89–1.41, p > 0.05), after controlling for age, sex, race/ethnicity, education, income, BMI, and smoking status (PMC12487997).

The evidence also supports the conclusion that the NHB–NHW glycemic disparity is widening over time, with NHW adults showing improving HbA1c trends (beta = −0.50, p = 0.001) while NHB adults show worsening trends (beta = 0.51, p = 0.012) over 2003–2014 (PMC7036954). This divergence is mirrored in mortality data, where NHB adults had a T2DM-related AAMR of 73.87 per 100,000 at the 2021 peak compared to 56.81 for NHW adults — approximately 30% higher — with the NHB rate rising sharply from 39.05 in 2015 to 73.87 in 2021 (PMC12212274). NHB adults with T2DM also bear substantially higher burdens of microvascular complications (renal, ophthalmic, neurological), macrovascular complications (ischemic disease, stroke, hypertension), and multimorbidity compared to NHW adults (PMC11050140, PMC10367428).

Socioeconomic factors — income, education, insurance — explain part of the NHB–NHW glycemic disparity but not all of it. The residual disparity after SES adjustment implicates additional factors including biological differences in hemoglobin glycation, neighborhood-level structural disadvantage, differential treatment quality and composition (including underutilization of newer antidiabetic agents in NHB patients), and health system factors including implicit provider bias. In the HRS sample, NHB adults reported the highest mean perceived neighborhood disorder (3.27 on a 7-point scale) compared to NHW adults (2.25), and in weighted analyses perceived neighborhood disorder was independently associated with T2DM risk (coef. = 1.11, SE = 0.03, p < 0.001) (PMC10822969), representing a structural pathway through which residential segregation and disinvestment may translate into glycemic disparities.

#### What the Evidence Does NOT Support

The evidence does not support the conclusion that the NHB–NHW glycemic disparity is primarily or entirely attributable to socioeconomic disadvantage. Multiple studies demonstrate that the disparity persists after comprehensive SES adjustment, including in populations with universal insurance coverage. The evidence does not support the conclusion that expanding insurance coverage alone will eliminate the glycemic disparity — the NHANES 2013–2018 analysis found no significant association between insurance status and poor glycemic control after adjustment (PMC12487997), and the older adult NHANES analysis found persistent disparities among Medicare-eligible adults (PMC7036954).

The evidence does not support any quantitative conclusions about racial disparities in time-in-range (TIR) in established T2DM. The studies available for this synthesis — spanning NHANES, NHIS, BRFSS, HRS, administrative claims, EMR, and mortality surveillance sources — uniformly rely on HbA1c or self-reported glycemic outcomes and do not include CGM-derived TIR data stratified by race/ethnicity in T2DM populations (PMC7036954, PMC12487997, PMC11050140, PMC10367428, PMC12212274, PMC8543926, PMC10822969). Any estimate of the NHB–NHW TIR difference would be speculative. The evidence does not support the conclusion that higher insulin use in NHB patients reflects more aggressive or proactive diabetes management — rather, the pattern is consistent with reactive treatment escalation in response to poor control (PMC7036954, PMC11050140).

The evidence does not support the conclusion that the NHB–NHW HbA1c gap is entirely artifactual due to differential hemoglobin glycation. While the glycation artifact is real and clinically important, the persistence of the disparity across multiple glycemic metrics and the substantially higher complication and mortality rates in NHB adults argue for a genuine underlying glycemic difference that exceeds what measurement artifact alone can explain.

#### Future Research Directions

**1. Multi-metric glycemic assessment studies.** Future studies should simultaneously measure HbA1c, fasting plasma glucose, two-hour post-load glucose, and CGM-derived TIR in racially diverse T2DM populations to disentangle the biological glycation artifact from true glycemic differences. Such studies would provide the first direct evidence on racial disparities in TIR — the most clinically actionable gap in the current evidence base.

**2. Longitudinal cohort studies with time-varying treatment intensity.** Cross-sectional studies cannot establish whether the NHB–NHW glycemic gap reflects differences in treatment intensity, treatment quality, patient adherence, or biological factors. Longitudinal studies tracking individual HbA1c trajectories with detailed, time-varying medication data (including specific agents, doses, and adherence) are needed to quantify the contribution of treatment intensity disparities to the glycemic gap.

**3. Mediation analyses decomposing the SES pathway.** Future studies should use formal mediation analysis to decompose the total NHB–NHW HbA1c difference into components attributable to income, education, insurance, neighborhood-level factors, treatment intensity, and residual race — providing a quantitative roadmap for intervention prioritization.

**4. CGM equity studies.** As CGM becomes more widely used in T2DM management, studies should examine racial disparities in CGM access, utilization, and TIR outcomes. Given the known underrepresentation of NHB adults in CGM trials, dedicated equity-focused CGM studies are needed to establish whether TIR disparities parallel HbA1c disparities and whether CGM-guided management reduces the glycemic gap.

**5. Intervention studies targeting structural determinants.** The evidence that neighborhood disorder is independently associated with T2DM risk (PMC10822969) and that SES adjustment does not fully explain the glycemic disparity suggests that interventions targeting structural determinants — neighborhood investment, food access, safe physical activity environments — may be necessary complements to clinical interventions. Randomized or quasi-experimental studies of neighborhood-level interventions on glycemic outcomes in NHB adults with T2DM are needed.

**6. Studies of newer antidiabetic agent equity.** The T2DM mortality analysis (PMC12212274) noted that NHB patients are less likely to receive SGLT-2 inhibitors and GLP-1 receptor agonists, which have demonstrated cardiovascular and renal protective effects. Studies quantifying the racial gap in prescribing of these agents, and the contribution of this gap to the complication and mortality disparity, are urgently needed. Randomized trials or pragmatic studies specifically designed to test whether equitable access to these agents reduces the NHB–NHW complication gap would be of high clinical value.

**7. Richer insurance metrics in glycemic control studies.** The null finding on insurance in the NHANES 2013–2018 analysis (PMC12487997) may reflect the binary insurance measure's inability to capture plan type, benefit generosity, cost-sharing, and continuity of coverage. Future studies should use richer insurance metrics — including plan type (Medicaid, Medicare, employer-sponsored, marketplace), prescription drug coverage, and continuity of coverage — to more precisely probe the insurance pathway to glycemic control.

**8. Disaggregation of the NHB category.** Future studies should disaggregate the "non-Hispanic Black" category by country of origin, immigration status, and acculturation to identify within-group heterogeneity in glycemic outcomes and their determinants. Research on Hispanic subgroups has demonstrated that pan-ethnic aggregation can mask important within-group variation in diabetes prevalence trends — disparities that were initially obscured when all Hispanics were analyzed as a single group (PMC4722936) — and the same principle of within-group heterogeneity likely applies to the NHB category.

In summary, the available evidence establishes a robust, clinically meaningful, and widening NHB–NHW disparity in HbA1c among adults with established T2DM in the United States, with a residual adjusted difference of approximately 0.31 percentage points in HbA1c and roughly twice the odds of poor glycemic control after comprehensive SES and clinical adjustment. This disparity is not eliminated by universal insurance coverage, is associated with substantially higher complication and mortality rates in NHB adults, and is likely driven by a combination of biological, socioeconomic, structural, and health system factors that require multi-level interventions to address. The evidence base for TIR disparities by race in T2DM is entirely absent from the available literature and represents the most urgent evidence gap for future research.

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*This review synthesizes evidence from the following sources: PMC7036954, PMC12487997, PMC11050140, PMC10367428, PMC12212274, PMC8543926, PMC10822969, PMC4722936, PMC5451915, PMC5984223. Papers on Hispanic subgroup diabetes prevalence trends (PMC4722936), Asian American prediabetes (PMC9587616), vital signs for Hispanics (PMC4584552), and subjective cognitive decline (PMC8223389) were reviewed but contributed only contextual background rather than direct evidence on the NHB–NHW glycemic control disparity in established T2DM.*

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