HBA1C levels are determined not only by average blood glucose concentration but also by the lifespan of red blood cells, with longer red blood cell survival leading to increased glycation of hemoglobin over time.

Study: The influence of shorter red blood cell lifespan on the rate of HbA1c target achieved in type 2 diabetes patients with a HbA1c detection value lower than 7%

The study shows that people with shorter-lived red blood cells have lower HbA1c readings than expected, meaning HbA1c isn't just about blood sugar—it also depends on how long red blood cells live.

Study: Rectify the impact of shorter red blood cell lifespan upon HbA1c detection values in T2DM patients: modeling and internal-external verification

The study shows that how long your red blood cells live affects your HbA1c level, because longer-lived cells have more time to build up sugar. This supports the idea that HbA1c isn’t just about average blood sugar.

Study: Glycated hemoglobin and red blood cell indices in non-diabetic pregnant women

The study shows that HbA1c levels are linked to red blood cell traits, which suggests that how long red blood cells live can affect HbA1c, not just blood sugar levels.

Study: Elevation of erythrocyte creatine post-puberty unrelated to erythrocyte lifespan: Implications from the lack of correlation between HbA1c and erythrocyte creatine.

The study looked at a marker of red blood cell age and found it didn’t always match up with HbA1c, but that’s likely because the marker itself can be misleading after puberty — not because red blood cell lifespan doesn’t affect HbA1c.