What Is Your A1C Actually Telling You?

Most people who have had blood work done have probably heard of A1C. If you have prediabetes or type 2 diabetes, you may know your A1C better than you know your cholesterol numbers. You’re told it was 5.7, 6.5, 7.0 or maybe higher, and usually you’re told whether that number is good, concerning or something that needs attention.

But here’s the question I think we should be asking: Do you actually know what your A1C represents, how your body creates that number and why what happened during the last few weeks can influence it differently than what happened several months ago? Because once you understand the biology behind A1C, that little percentage on your laboratory report starts telling a much more interesting story.

First, What Exactly Is A1C?

To understand A1C, we first need to understand a little biology, which happens to be one of my specialties, but I promise to keep it simple and break it down in a way everyone can understand.

Your blood contains red blood cells, and inside those cells is a protein called hemoglobin. Hemoglobin’s primary job is to carry oxygen throughout your body. As glucose circulates through your bloodstream, some of that glucose naturally attaches to hemoglobin through a process called glycation. Generally speaking, the greater your glucose exposure over time, the greater the percentage of hemoglobin that becomes glycated. Your A1C test measures that percentage.

Think of glucose as being a little sticky. Your red blood cells are traveling through your bloodstream all day, every day, and their hemoglobin is continually being exposed to glucose. Some of that glucose attaches to the hemoglobin and remains there for the life of that red blood cell.

This is an important distinction because A1C is not directly measuring how much sugar you ate. It is providing an indirect measure of your body’s glucose exposure over time. Blood glucose can certainly come from the carbohydrates you eat, but your body can also produce glucose. Your liver, for example, can make glucose through a process called gluconeogenesis, something I discuss extensively in my second book, Stop Dieting & Start Eating. Break that word apart and it becomes much easier to understand: gluco refers to glucose, neo means new and genesis means creation or formation. In plain English, gluconeogenesis means creating new glucose.

Why Does A1C Look Back About Three Months?

Red blood cells generally circulate for about 120 days before being replaced. Because glucose that attaches to hemoglobin remains associated with that red blood cell during its lifespan, measuring the percentage of glycated hemoglobin gives us information about glucose exposure over the preceding two to three months.

Here’s an easy way to picture it. A blood glucose reading is like taking a photograph. It tells you what your glucose looks like at that particular moment. An A1C is more like watching a movie. It gives us information about what has been happening over a much longer period of time. The photograph is useful, but it cannot tell you the entire story. The movie gives you a much broader view.

But Those Three Months Are Not Equal

This is where A1C becomes much more interesting. A1C is often described as your “three-month average,” but that description can be misleading if we take it too literally. Your A1C is not calculated by adding up 90 days of glucose readings and dividing by 90, and each part of those three months does not contribute equally to the result.

The reason has to do with the age of your red blood cells. The cells circulating through your body today aren’t all the same age. Some are relatively new, some have been circulating for weeks, and others are approaching the end of their lifespan. Your body is constantly removing older red blood cells and replacing them with new ones.

Because of this continuous turnover, more recent glucose exposure has a greater influence on your A1C than glucose exposure farther back in time. This is why a meaningful improvement or deterioration in blood glucose can begin moving A1C sooner than many people realize. You don’t necessarily have to wait three full months before significant changes in glucose exposure begin influencing the number.

What Do the A1C Numbers Actually Mean?

For people without known diabetes, an A1C below 5.7% is generally considered below the prediabetes range. An A1C from 5.7% through 6.4% falls within the prediabetes range, while an A1C of 6.5% or greater falls within the diabetes diagnostic range.

There is an important distinction here. An A1C in the diabetes range is an important diagnostic finding, but in the absence of unequivocal hyperglycemia, the diagnosis generally needs to be confirmed with another abnormal result. That can be a repeat of the same test or another diagnostic glucose test. This is one reason laboratory numbers should always be interpreted within the context of the individual rather than in isolation.

But percentages such as 5.7%, 6.5% or 7.0% can feel pretty abstract when the numbers you’re accustomed to seeing on a glucose meter are 100, 125 or 150 mg/dL. That’s where another number called eAG can help.

What Is eAG?

eAG stands for estimated Average Glucose. It essentially translates your A1C percentage into an estimated glucose value expressed in mg/dL, the same measurement most people recognize from a glucose meter or laboratory glucose result.

For example, an A1C of 5.4% corresponds to an estimated average glucose of approximately 108 mg/dL. At 6.0%, the estimate is approximately 126 mg/dL. At 6.5%, it’s approximately 140 mg/dL, and at 7.0%, approximately 154 mg/dL. These are estimates rather than a reconstruction of every glucose reading you experienced, but they make an A1C percentage much easier to relate to the glucose numbers many people see every day.

There is also an important misconception worth clearing up here. You do not need to fast specifically for an A1C test. Your A1C reflects glucose exposure over an extended period, so skipping breakfast that morning isn’t going to erase what has been happening during the previous several months. If your healthcare provider asks you to fast, it may be because another test being performed at the same blood draw requires fasting, such as a fasting glucose. The A1C itself does not require fasting.

Prediabetes Is a Warning, Not a Waiting Room

This is where I think we need to change the way we look at prediabetes. An A1C of 5.7% shouldn’t simply become, “Well, at least I don’t have diabetes.” Prediabetes does not mean progression to type 2 diabetes is inevitable, but it is an important warning that glucose regulation has moved in the wrong direction and deserves attention.

This is a very good time to take an honest look at nutrition, physical activity, body composition, sleep and other lifestyle and medical factors that can influence metabolic health. Think about the check-engine light in your car. You don’t wait until smoke is pouring out from under the hood before deciding something deserves your attention. The warning light gives you an opportunity to investigate what’s happening and address the problem before it potentially becomes much larger. Prediabetes should encourage the same kind of thinking.

An Average Can Hide a Lot

This may be one of the most important things to understand about A1C. Two people can have the same A1C and still experience very different glucose patterns. One person might spend most of the day relatively stable, while another repeatedly moves between lower and much higher glucose levels. Their averages could ultimately look similar even though their daily glucose experiences were very different.

Think about two students who both finish a class with an average test score of 80. One consistently scores around 78, 80 and 82, while the other has some very low scores and some very high scores that ultimately average out to approximately the same number. Both students finish with an 80 average, but they certainly didn’t get there the same way.

A1C works in much the same way. It provides extremely valuable information about overall glucose exposure, but by itself it cannot show every spike, drop, fasting pattern, overnight change or period of glucose variability. That’s why A1C should be viewed as an important piece of the metabolic puzzle rather than the entire puzzle. Glucose readings and trends, medications, nutrition, physical activity, body composition and other relevant metabolic information can provide context that one A1C number simply cannot provide by itself.

There Is One More Thing You Should Know About A1C

Because A1C depends on red blood cells and hemoglobin, anything that significantly changes red blood cell lifespan, turnover or hemoglobin can potentially influence the result. Certain types of anemia, iron deficiency, significant blood loss, blood transfusions, some hemoglobin variants, kidney disease, pregnancy and other medical circumstances can sometimes make A1C less representative of a person’s actual glucose exposure.

This doesn’t make A1C a bad test. Far from it. A1C is an extremely useful tool, but like virtually every laboratory measurement, it needs to be interpreted within the context of the individual rather than looked at in isolation. If someone’s glucose readings and A1C consistently don’t seem to match, that discrepancy deserves a closer look.

Bottom Line

Your A1C isn’t just some mysterious percentage printed on your laboratory report. It’s a valuable window into your glucose exposure over time, but it’s a weighted window, and it cannot tell us exactly how you arrived at that number. Two people can have the same A1C and very different glucose patterns, and certain medical circumstances can sometimes make that number less representative of what is actually happening with blood glucose.

That’s why understanding the WHY matters. Once you understand what A1C measures, how the number develops, why recent glucose exposure can influence it more heavily and what an average can potentially hide, that percentage on your laboratory report becomes considerably more meaningful.

Don’t simply collect health numbers and wait for someone to tell you whether they’re good or bad. Learn what those numbers are actually telling you. Because when you understand the biology behind the number, you can begin asking much better questions about what’s happening inside your body.

About the Author
Coach Tony is a Board-Certified Nutrition Specialist and Master Personal Trainer with over 40 years of experience in the health and fitness industry. He specializes in metabolic health, fat loss, and body composition, helping clients restore their metabolism through structured nutrition and resistance training.