HbA1c

Why HbA1c Is One of the Most Powerful Longevity Biomarkers

Blood glucose is a fundamental driver of biological aging. When glucose is chronically elevated, it reacts with proteins throughout your body in a process called glycation — permanently altering their structure and impairing their function. The resulting compounds, called advanced glycation end-products (AGEs), accumulate in blood vessel walls, joints, the lens of the eye, the kidneys, and brain tissue over decades. This damage is largely irreversible.

HbA1c captures this exposure better than any single blood glucose test because it reflects a three-month average rather than a snapshot. A fasting glucose reading taken after an unusually light evening meal, or after a stressful morning, tells you little. HbA1c tells you what your glucose environment has actually looked like over the past 90 days — the timeframe that matters for cumulative tissue damage.

A landmark 2010 study published in the New England Journal of Medicine by Selvin et al. demonstrated that HbA1c was a strong independent predictor of cardiovascular disease, coronary heart disease, and all-cause mortality even in people without diagnosed diabetes — a finding that challenged the prevailing view that HbA1c was only relevant once someone crossed the clinical threshold into prediabetes or diabetes. ¹

The implication for longevity: waiting until HbA1c reaches 5.7% to pay attention is waiting until significant metabolic damage has already been accumulating for years.

Standard Reference Ranges vs. Longevity-Optimal Ranges

Standard clinical cutoffs for HbA1c are designed to identify and manage diabetes and prediabetes. They are not calibrated to minimize the risk of aging-related disease over a lifespan. The longevity-optimal range is considerably tighter.

Research published in The Lancet by Currie et al. identified a J-shaped relationship between HbA1c and all-cause mortality in people with type 2 diabetes — but importantly, the nadir of the curve (lowest mortality) was at approximately 7.5% in that treated diabetic population, with risk rising above and below this value. ² For non-diabetic, healthy adults, population data consistently point to optimal longevity in the 5.0–5.2% range.

The practical takeaway: a reading of 5.6% is technically "normal" by any clinical standard, but it represents a glucose environment that is meaningfully less healthy than 5.1% — and that gap compounds over years and decades into measurable differences in arterial health, brain function, and biological age.

How Elevated Blood Glucose Accelerates Aging

Understanding why HbA1c matters requires understanding what chronically elevated glucose actually does to the body at a molecular level. The primary mechanism is glycation.

When glucose molecules encounter proteins in the bloodstream or in tissues, they can bind to them through a non-enzymatic chemical reaction. This forms compounds called AGEs (advanced glycation end-products). Unlike most chemical reactions in the body, glycation is irreversible — the damaged proteins cannot be repaired. They accumulate.

The effects are systemic:

• Blood vessels: AGEs stiffen arterial walls, increase permeability, and accelerate atherosclerosis — independent of LDL cholesterol levels. This is one reason elevated HbA1c raises cardiovascular risk even in people with "normal" cholesterol.
• The brain: Neurons are highly vulnerable to glucose toxicity. Chronic hyperglycemia impairs insulin signaling in the brain (sometimes called "type 3 diabetes"), reduces BDNF (brain-derived neurotrophic factor), and is increasingly understood as a major contributor to Alzheimer's disease risk.
• Kidneys: The glomeruli — the filtration units of the kidney — are damaged by sustained elevated glucose, which is why diabetic nephropathy is one of the most common causes of kidney failure.
• Mitochondria: Excess glucose drives oxidative stress inside mitochondria, impairing cellular energy production and accelerating cellular senescence.
• Inflammation: AGEs bind to receptors called RAGEs (receptor for AGEs), triggering an inflammatory cascade that elevates markers like hsCRP and IL-6 throughout the body.

None of these effects require a diabetes diagnosis to begin. They operate on a continuum, starting well below clinical thresholds and accumulating silently over years.

What Drives HbA1c Up — and Down

HbA1c is among the most responsive longevity markers to lifestyle intervention. Understanding the key levers gives you a clear optimization roadmap.

Factors that raise HbA1c

• High intake of refined carbohydrates and added sugars — the most direct driver of postprandial glucose spikes
• Sedentary lifestyle — muscle is the primary tissue for glucose disposal; inactivity reduces this capacity significantly
• Excess visceral adiposity — abdominal fat secretes hormones and inflammatory signals that impair insulin signaling
• Chronic poor sleep — even a single week of poor sleep meaningfully impairs glucose tolerance in healthy adults
• Chronic psychological stress — cortisol elevates blood glucose directly, and chronically elevated cortisol contributes to insulin resistance
• Certain medications — glucocorticoids (prednisone, cortisone) and some antipsychotics can raise HbA1c
• Conditions affecting red blood cell turnover — iron deficiency anemia, hemolytic anemia, and certain hemoglobin variants can artificially alter HbA1c readings independent of actual glucose levels

Factors that lower HbA1c

• Reducing refined carbohydrates — particularly ultra-processed foods, sugar-sweetened beverages, and white starches; this is the single most impactful dietary change
• Post-meal walks — even a 10–15 minute walk after eating blunts the postprandial glucose spike by directing glucose into working muscles before insulin is required
• Resistance training — increases muscle mass, the body's primary glucose disposal organ; each pound of added muscle permanently improves glucose clearance
• Weight loss — especially loss of visceral fat; a 5–10% reduction in body weight can lower HbA1c by 0.5–1.0% in people with metabolic dysregulation
• Consistent aerobic exercise — improves insulin sensitivity in muscle and liver tissue through multiple pathways including GLUT4 upregulation
• Adequate sleep (7–9 hours) — restoring normal sleep in chronically sleep-deprived individuals can improve glucose tolerance within weeks
• Metformin — the most prescribed diabetes drug globally; also commonly used off-label in longevity medicine for its metabolic and potential aging-related benefits
• Berberine — a plant-derived compound with demonstrated glucose-lowering effects comparable to metformin in several studies

HbA1c vs. Fasting Glucose vs. Fasting Insulin

These three metabolic markers each reveal a different dimension of glucose health, and all three are necessary for a complete picture. Relying on any one alone can create dangerous blind spots.

Fasting glucose is a snapshot of blood glucose after an overnight fast. It is the most commonly ordered metabolic marker and the most prone to misinterpretation. It can be temporarily elevated by stress, poor sleep, or a late-night meal — and it can look normal even when someone has significant postprandial glucose dysregulation, because the body recovers between meals.

HbA1c smooths out this noise by reflecting a 90-day average. It is more informative than fasting glucose as a chronic exposure marker. However, HbA1c can be artificially lowered in people with conditions that shorten red blood cell lifespan (such as hemolytic anemia or recent blood loss) and artificially elevated in people with iron deficiency anemia. If your HbA1c seems inconsistent with other metabolic markers, consider continuous glucose monitoring (CGM) for a more direct measure.

Fasting insulin is the marker that makes the others interpretable. It measures how much insulin the pancreas is producing to maintain normal glucose — and it rises years before glucose does. A fasting insulin above 8–10 µIU/mL in the context of a normal HbA1c is a strong signal of insulin resistance: the pancreas is working overtime to compensate, glucose looks fine, but the underlying metabolic dysfunction is already in motion. This is the window in which intervention is most effective.

For complete metabolic health assessment, always test all three: HbA1c, fasting glucose, and fasting insulin.

How to Test HbA1c

HbA1c is a standard blood test available at any clinical lab. Unlike fasting glucose, it does not require fasting before the draw — you can have it done at any time of day, which makes it one of the more convenient biomarkers to add to a panel.

Through a longevity testing service: Function Health, InsideTracker, and Marek Health all include HbA1c alongside fasting glucose, fasting insulin, and a full metabolic and lipid panel. For anyone building a longevity biomarker baseline for the first time, this is the highest-value approach — you get HbA1c in context with the other markers you need to interpret it correctly.

À la carte through Ulta Lab Tests: If you specifically need to retest HbA1c to track progress, Ulta Lab Tests offers it without a doctor's visit for $15–25. Results are available in 24–48 hours at most US locations.

Through your physician: Any primary care physician can order HbA1c. It is often included in annual metabolic panels, particularly for patients over 40. However, a standard panel may not include fasting insulin — request that separately if your physician does not routinely order it.

How Often Should You Test?

Because HbA1c reflects a 90-day average, testing more frequently than every three months provides little additional signal. If you are actively intervening through dietary changes, exercise, or medication, test at the 90-day mark to measure the response. This is the minimum time needed to see the full effect of any lifestyle change on your HbA1c.

Once you've achieved and maintained a level below 5.3% for two consecutive tests, testing every 6–12 months is sufficient. Always pair HbA1c with fasting insulin and fasting glucose to get the complete metabolic picture — and consider adding triglycerides and hsCRP for a full cardiovascular and inflammation assessment.

Sources

1. Selvin E, et al. "Glycated Hemoglobin, Diabetes, and Cardiovascular Risk in Nondiabetic Adults." New England Journal of Medicine, 2010. PubMed →
2. Currie CJ, et al. "Survival as a Function of HbA1c in People with Type 2 Diabetes: a Retrospective Cohort Study." The Lancet, 2010. PubMed →
3. Stratton IM, et al. "Association of Glycaemia with Macrovascular and Microvascular Complications of Type 2 Diabetes (UKPDS 35)." BMJ, 2000. PubMed →