HbA1c

HbA1c (glycated hemoglobin) measures the percentage of hemoglobin molecules in your red blood cells that have glucose permanently attached to them. Because red blood cells live approximately 90–120 days, HbA1c provides a weighted average of your blood sugar over the past 2–3 months, with more recent weeks contributing more heavily. This makes it far more informative than a single fasting glucose reading, which only captures a snapshot in time.

Clinically, HbA1c is the primary tool for diagnosing and monitoring diabetes. An HbA1c of 6.5% or above indicates diabetes, while 5.7–6.4% defines the prediabetic range — a critical window where lifestyle intervention can reverse the trajectory. However, longevity-focused clinicians argue that even 'normal' HbA1c levels above 5.4% are associated with increased risk of cardiovascular disease, cancer, and all-cause mortality, pushing the optimal target well below the conventional threshold.

It is important to note that HbA1c can be misleading in certain conditions. Anything that affects red blood cell lifespan will alter HbA1c independently of actual glucose levels: iron deficiency anemia, hemolytic anemias, recent blood loss, and hemoglobin variants (common in people of African, Mediterranean, or Southeast Asian descent) can all cause falsely high or low readings. In these situations, fructosamine or continuous glucose monitoring (CGM) may provide a more accurate assessment.