HbA1c (Hemoglobin A1c): What This Blood Sugar Marker Means

Written by Dr. Ryan Kneessi, ND

Quick answer: HbA1c measures the share of hemoglobin in red blood cells that has glucose attached, which reflects average blood sugar over roughly the past three months. A higher result means blood sugar has run higher on average, and in a large study it was associated with future diabetes and cardiovascular risk. Conditions that change red blood cells, such as blood loss or very low iron, can skew the number.

Have you ever wondered why your bloodwork includes HbA1c when it already shows a glucose number? A fasting glucose result is a snapshot of one morning, while HbA1c looks back over roughly the past three months. It measures how much of the hemoglobin in your red blood cells has picked up glucose, so it reflects your average blood sugar over the life of those cells. That longer view is what makes it useful, and it is also why anything that changes the life span of red blood cells can shift the result. The habits most often discussed around steady blood sugar are daily movement, especially after meals, building and keeping muscle, consistent sleep, meals built around protein, fiber and whole foods, and managing ongoing stress.

This article is educational. It explains what the marker reflects and how researchers connect it to the deeper mechanisms of aging, so you can have a more informed conversation with your own provider.

What does an HbA1c result actually measure?

Glucose in the bloodstream slowly attaches to hemoglobin, the protein that carries oxygen inside red blood cells. The more glucose there is over time, the more hemoglobin becomes glycated, and because red blood cells circulate for a few months, the percentage gives a rolling average. According to the National Institute of Diabetes and Digestive and Kidney Diseases, the A1C test reflects average blood glucose over the past three months, and within the range that agency describes as 5.7 to 6.4 percent, a higher A1C goes along with a greater risk of developing type 2 diabetes.

The same agency notes that the result can be thrown off by anything that changes red blood cells or hemoglobin, including recent blood loss, sickle cell disease, transfusion and dialysis. Very low iron can produce a falsely high result, and kidney or liver disease can also skew it. Certain inherited hemoglobin variants can make the test unreliable as well, which is one reason HbA1c is best read alongside fasting glucose and fasting insulin rather than on its own.

How is HbA1c connected to aging and root causes?

HbA1c is itself a glycation marker, meaning it captures sugar sticking to a protein. That same basic chemistry, carried further over time, produces advanced glycation end products that build up in long-lived tissues such as collagen and blood vessel walls. Researchers who study aging see this slow sugar-and-protein damage as part of how tissues stiffen and lose function, so HbA1c offers a practical, everyday window into a process that matters well beyond blood sugar.

It also connects to nutrient sensing, one of the core mechanisms researchers describe in aging. When blood sugar runs higher over months, the body leans on insulin and related growth signals more heavily, and those pathways sit close to the center of how cells decide whether to grow or to repair. Higher average glucose is also associated with more oxidative stress and chronic low-grade inflammation, and with strain on mitochondria, which have to handle the extra fuel. Some of this evidence comes from laboratory and animal work, so it is fair to say the human picture is still being filled in.

From a root-cause perspective, the useful question is why the average has drifted upward. Short or irregular sleep makes the body less responsive to insulin, and ongoing stress raises cortisol, which pushes glucose higher. Long stretches of sitting, loss of muscle with age, and a diet heavy in refined starches, sweetened drinks and ultra-processed foods all make it harder to clear glucose after meals. Gut health and body composition play a part, and genetics and family history influence how early these patterns appear. Because the result reflects months rather than days, it tends to respond to steady changes in daily habits rather than to any single meal. Looking at it next to fasting insulin can also show whether the body is working harder behind the scenes to keep that average where it is, which is often the earlier part of the story.

What does the research show about HbA1c?

The ARIC study measured HbA1c in 11,092 adults without diabetes or heart disease. Compared with an HbA1c of 5.0 to 5.4%, an HbA1c of 6.0 to 6.4% was linked with 4.48 times the risk of later diabetes and 1.78 times the risk of coronary heart disease. HbA1c was measured once, and conditions that change red blood cell lifespan can shift the result. (Selvin et al., 2010)

The EPIC-Norfolk study followed 4,662 men aged 45 to 79 for about 4 years. Each 1-point increase in HbA1c was linked with a 28% higher risk of death independent of age, blood pressure, cholesterol, weight and smoking, and the lowest death rates were in men with HbA1c below 5%. The study included only men and had short follow-up. (Khaw et al., 2001)

These studies show that HbA1c carries information even below the diabetes range.

Related reading

If you would like to explore related topics, my articles on fasting insulin and HOMA-IR, glycemic control and aging and insulin sensitivity go deeper. You can also see the specialty tests I commonly recommend on my functional labs page.

Common questions about HbA1c (Hemoglobin A1c)

What does a high HbA1c mean?

A higher HbA1c means blood sugar has averaged higher over roughly the past three months. Within the range the NIH describes as 5.7 to 6.4 percent, a higher value goes along with a greater risk of developing type 2 diabetes.

Can HbA1c be wrong?

Yes, it can be skewed. Recent blood loss, transfusion, dialysis, sickle cell disease, some inherited hemoglobin variants, very low iron, and kidney or liver disease can all shift the result.

How is HbA1c different from fasting glucose?

Fasting glucose is a snapshot of one morning, while HbA1c reflects an average over about three months. In a large study, HbA1c was more strongly associated with cardiovascular risk than fasting glucose.

Sources

This article is for educational purposes only and is not medical advice. It does not replace an individual evaluation. Talk with your healthcare provider before making changes to your diet, exercise, supplements, or medications, especially if you are pregnant, nursing, or managing a health condition.