Can You Have Insulin Resistance with Normal A1C and Fasting Glucose?
Yes, you can absolutely have insulin resistance despite having normal A1C and fasting glucose levels—this represents an early metabolic abnormality that precedes the development of prediabetes and type 2 diabetes.
Understanding the Disconnect
Insulin resistance with compensatory hyperinsulinemia represents one of the earliest metabolic abnormalities in the progression toward type 2 diabetes, often occurring years before glucose levels become abnormal 1. The American Diabetes Association recognizes that type 2 diabetes is characterized by both relative insulin deficiency and peripheral insulin resistance, but these abnormalities exist on a continuum 1.
Why Standard Tests Miss Early Insulin Resistance
A1C has limited sensitivity for early metabolic dysfunction: Research demonstrates that A1C between 5.7-6.4% detects only 23.6% of all at-risk individuals, with particularly poor sensitivity in non-Hispanic whites (9.9%) compared to African Americans (31.4%) and Hispanics (35.2%) 2.
Fasting glucose correlates more strongly with insulin resistance markers than A1C: Studies show fasting plasma glucose has stronger correlations with fasting insulin (r=0.38 vs 0.26), acute insulin response (r=-0.20 vs -0.09), and waist circumference (r=0.43 vs 0.25) compared to A1C 2.
Normal fasting glucose can mask underlying insulin resistance: Even within the normal fasting glucose range (<6.1 mmol/L), higher values are associated with reduced insulin sensitivity and augmented baseline insulin secretion, indicating compensatory hyperinsulinemia 3.
The Metabolic Progression
The pathophysiology follows a predictable sequence:
Initial insulin resistance develops with normal glucose homeostasis maintained through compensatory hyperinsulinemia 1
Beta-cell function begins to decline as the pancreas cannot sustain the increased insulin output required 3
Postprandial glucose rises first, detectable only with oral glucose tolerance testing, not fasting measurements 2, 4
Fasting glucose eventually rises as hepatic glucose production can no longer be adequately suppressed 3
A1C increases last as average glucose levels become chronically elevated 1
Clinical Detection Strategies
When to Suspect Hidden Insulin Resistance
Look for these clinical markers even with normal A1C and fasting glucose 1:
- Body mass index ≥25 kg/m² (≥23 kg/m² in Asian Americans)
- Waist circumference elevation (strong correlation with insulin resistance independent of glucose levels) 2
- Family history of type 2 diabetes in first-degree relatives
- History of gestational diabetes
- Polycystic ovary syndrome
- Acanthosis nigricans or other signs of insulin resistance
- Hypertension (≥140/90 mmHg or on antihypertensive therapy)
- HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL
- Physical inactivity
- High-risk ethnicity (African American, Latino, Native American, Asian American, Pacific Islander)
Diagnostic Approach
For routine clinical practice, the American Diabetes Association does not recommend fasting insulin measurement as a diagnostic test 5. However, the following approach can identify insulin resistance with normal standard tests:
Oral glucose tolerance test (OGTT) is superior to A1C or fasting glucose for detecting early metabolic dysfunction, particularly identifying individuals with isolated postprandial hyperglycemia 2, 4.
2-hour post-load glucose correlates more strongly with insulin sensitivity (r=-0.40) than A1C (r=-0.27) 2.
In women with previous gestational diabetes, OGTT identifies a higher-risk metabolic phenotype with worse beta-cell function and insulin sensitivity compared to those diagnosed by A1C alone, even when both groups have "dysglycemia" 4.
Triglyceride-glucose index can serve as a surrogate marker, with increased values associating with lower-than-expected HbA1c in people with prediabetes, suggesting non-glycemic effects of hyperinsulinemia 6.
Important Clinical Caveats
The A1C Paradox
Emerging evidence suggests that hyperinsulinemia may actually lower A1C through non-glycemic mechanisms 6. Mendelian randomization studies indicate that increased fasting insulin increases erythrocytosis (hemoglobin, red cell count, reticulocytes), which can reduce HbA1c independent of glucose levels 6. This means insulin-resistant individuals may have falsely reassuring A1C values.
Risk Stratification Despite Normal Tests
Even with A1C between 5.5-6.0% (still below the prediabetes threshold of 5.7%), individuals have substantially increased diabetes risk with 5-year incidence of 9-25% 1. Those with A1C 5.7-6.4% have 25-50% 5-year diabetes risk 1. This continuous risk relationship extends below the lower diagnostic limits 1.
Management Implications
When insulin resistance is suspected despite normal glucose tests:
Intensive lifestyle intervention is indicated for individuals who are overweight/obese with risk factors, even without meeting glucose-based diagnostic criteria for prediabetes 1.
Behavioral counseling interventions have demonstrated moderate benefits in reducing cardiovascular risk, lowering blood pressure, improving glucose and lipid levels, reducing obesity, and increasing physical activity 1.
Lifestyle interventions are more effective than metformin for preventing progression to diabetes in at-risk individuals 1.
Cardiovascular risk factor management should be systematically assessed, as insulin resistance increases risk for atherosclerotic cardiovascular disease independent of glucose levels 1.
The bottom line: Normal A1C and fasting glucose do not exclude insulin resistance. Clinical suspicion based on risk factors should prompt consideration of OGTT and aggressive lifestyle intervention, as waiting for glucose abnormalities to develop means missing a critical window for prevention.