What it means
Haemoglobin A1c is the fraction of haemoglobin in red blood cells that has glucose attached. Because red cells live about 120 days and glucose attaches at a rate proportional to its concentration, HbA1c reports the average blood glucose over roughly the previous three months, weighted toward the last month.
Diagnostic ranges: under 5.7% normal, 5.7–6.4% prediabetes, 6.5% or above diabetes. In mmol/mol: under 39, 39–47, 48 and above.
Why it matters for longevity
Glucose exposure over years damages vessels, nerves, kidneys and eyes, and every 1% rise in HbA1c is associated with roughly 20% higher cardiovascular risk even below the diabetes threshold. It is the metabolic marker most linked to hard outcomes. Its limitation is that insulin resistance can run for a decade while HbA1c stays "normal", because rising insulin holds glucose down until the pancreas tires.
What the evidence says
UKPDS and its follow-up established that lowering HbA1c reduces microvascular complications in diabetes and, over the long run, heart attacks. In non-diabetics, the EPIC-Norfolk and ARIC cohorts show a continuous relationship between HbA1c and mortality from about 5.0% upward, with the lowest risk around 5.0–5.4%.
The test misleads in anaemia, haemoglobin variants, after blood loss, in kidney disease and in anyone whose red cells live unusually long or short. Athletes with high red-cell turnover often read low; people with iron deficiency often read high.
How to measure or use it
- A standard blood test, no fasting needed; repeat every 3–6 months if tracking a change.
- Pair it with fasting insulin and HOMA-IR to see insulin resistance before it moves HbA1c, and with a CGM for two weeks if post-meal spikes are the question.
- Levers in rough size: body-fat loss, endurance training, muscle mass (the glucose sink), reduced refined carbohydrate, sleep; then drugs if needed.
Related
- Metabolic health — the full article, with the panel
- Biomarkers of ageing — HbA1c in the core panel
- Baseline labs — what to draw first
Frequently asked questions
What is an optimal HbA1c?
The mortality curve is lowest around 5.0–5.4%. Under 5.7% is "normal"; the longevity target is usually under 5.5%.
Why is my HbA1c normal but my fasting insulin high?
Because insulin is holding glucose down. That pattern is early insulin resistance and is the reason to test fasting insulin alongside HbA1c.
Can HbA1c be wrong?
Yes — anaemia, iron deficiency, haemoglobin variants, kidney disease and altered red-cell lifespan all shift it. A fructosamine test or a CGM resolves the doubt.
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