HbA1c is glycated hemoglobin — a measurement of what percentage of your hemoglobin has been chemically bound to glucose over the last three months. It matters for skin because the same reaction that glycates hemoglobin is happening, more slowly, to your collagen.
The process is called the Maillard reaction. When circulating glucose is chronically elevated, glucose molecules attach non-enzymatically to long-lived proteins — hemoglobin (3-month half-life), collagen (15-year half-life). Over time, these cross-linked structures accumulate as advanced glycation end products (AGEs), which stiffen tissue and reduce its ability to remodel (Monnier, 1990; Verzijl, 2000).
Clinically, HbA1c below 5.7% is normal. Above 6.5% is diabetes. What matters for skin longevity is the sub-clinical range — 5.5 to 5.9 — where clinicians say fine but the glycation biology is already accelerating. Selvin's landmark 2010 NEJM paper established elevated risk beginning well below the pre-diabetic threshold (Selvin, 2010).
Interventions that move HbA1c: reducing refined carbohydrates, eating protein and fiber before starch, post-meal walking (Reynolds, 2016), resistance training, and consistent sleep. In a well-designed skin panel, HbA1c is a leading indicator you can act on.
HbA1c is one of the nine biomarkers on the JenSkin panel.