Reflects average blood glucose over the prior ~90 days — the gold-standard measure of chronic glycemic control.
HbA1c (also written A1C) measures the percentage of hemoglobin molecules in red blood cells that have become glycated — non-enzymatically bound to glucose. Because red blood cells have a lifespan of ~120 days, HbA1c reflects average blood glucose over the preceding ~3 months. It is less sensitive to acute fluctuations (a single stressful day, a recent meal) than fasting glucose, making it the preferred marker for chronic glycemic assessment.
HbA1c maps to estimated average glucose (eAG) roughly as: eAG = (28.7 × A1C) − 46.7. So an A1C of 5.5% corresponds to about 111 mg/dL average glucose. Optimization-focused clinicians often target sub-5.4% rather than just "not diabetic" (under 6.5%), because emerging evidence suggests cardiovascular and cognitive risks begin accumulating well below the diabetic threshold.
HbA1c is influenced by: baseline glucose regulation, insulin sensitivity, body composition, dietary carbohydrate load and timing, physical activity, sleep (sleep debt raises A1C), chronic inflammation, and pharmacologic agents — GLP-1 agonists (semaglutide, tirzepatide, retatrutide), metformin, berberine, SGLT2 inhibitors, and insulin. It can also be affected by conditions that alter red blood cell lifespan (iron deficiency, recent blood loss, hemolytic conditions), which can falsely lower or elevate values. Growth hormone peptides and HGH can transiently increase A1C via induced insulin resistance.
For GLP-1 users, A1C reductions of 0.5–2.0 percentage points are typical over 3–6 months — this is the single most reliable biomarker move in the class. For growth hormone protocols (tesamorelin, HGH, MK-677), A1C should be monitored quarterly because these compounds can reduce insulin sensitivity. The best cadence for most users is: baseline, 3 months, 6 months, then annually if stable. Fasting glucose + fasting insulin in parallel gives a more complete metabolic picture than A1C alone.
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Start tracking →Informational only — not medical advice. Reference ranges vary by lab and individual context. Work with a licensed provider to interpret your specific results.