I understand the basic concept that it is a (averaged??) snapshot of prior 90 days of blood glucose levels… but what is the value (% or ppm, etc.) and how is it reached?
With a diabetic, glucose levels aren’t always stable. Let’s say you ranged from 150-350 on a given day…what value is given to that “day” in the 90 day picture?
In: 40
Your blood has proteins called hemoglobin in it. They are responsible for transporting oxygen around your body. These proteins stay in your body for about 90 days before they die are are discarded.
As they run around in your blood delivering oxygen like box trucks they also get all dirty with sugar. The longer they have been in your blood, the more dirty they get. Like box trucks running around on a muddy rainy 3 months vs box trucks running around in beautiful sunny weather.
What the test does, is count how many hemoglobin in your blood are dirty with sugar. It would be like being able to count how many dirty box trucks there was, to determine how many rainy days there was.
But instead, we know how much sugar each hemoglobin collects on a regular basis as it zooms around with it’s oxygen. And we can measure how many, by percentage, of the hemoglobin have become coated with sugar (glaciated).
Then it’s just simple math.
How many proteins are coated divided by how many are collected x 100 = A1C%
And we know from research that each percentage equals an average blood sugar over 90 days.
5 = 97
6 = 126
7 = 154
8 = 183
etc
A1c is measuring glycation byproducts on your red blood cells. Sugar has a nasty habit of “caramalizing” things, it browns your tissue. As glucose comes into contact with your tissues and cells it can glycate them, damaging them. As this damage accumulates over time it can cause issues. In red blood cells the important part we’re looking at is hemoglobin. The iron containing molecule responsible for oxygen transportation around your body. It’s a percentage.
Your cells, no matter how well controlled your blood sugar is will always be attacked by glycation. That’s just the nature of having glucose floating around. As you have higher glucose the glycation byproducts increase. So the less well controlled your blood sugar the more glycation on your red blood cells. Higher blood sugar after a meal you’ll get more glycation, lower blood sugar fasting over night, less glycation during that time. And as you said, red blood cells stick around for *about* 3 months, this results in an average snapshot of your glucose control over that time span.
There’s some issues with a1c though. Certain phenotypes have higher red blood cell turnover than the average. This means that since the cells aren’t as long lived they are not subjected to the same glycation load over time resulting in an A1c that may appear normal even though their blood sugar is poorly controlled. And on the other end of the spectrum there’s now some cases being reported of carnivores having elevated A1c levels despite phenomenal insulin sensitivity and rock solid glucose levels. One explanation might be that due to their extreme metabolic health the red blood cells are simply living longer before the cell dies resulting in more glycation load due to their longer lifespan.
This actually happened to me over my time as a carnivore. My A1c was high as a pre-diabetic with poor blood sugars and high insulin resistance. It went phenomenally low as my insulin resistance improved. Then after a few years started to climb again.
You have hemoglobin in your blood cells that helps deliver oxygen. When blood sugar levels are high some of that hemoglobin gets some sugar/carbs added to it. The higher blood sugar is over a longer period of time the more this happens (and thats what A1c is a measure of).
Red blood cells only circulate for 3-4 months before being replaced, which is why A1c only shows blood sugars over the past few months.
If you were to graph all of someones glucose levels over that period and add up the area under the curve it would correlate to the A1c.
Lots of great responses in what an A1c actually measures in here. Side note: the rise of CGMs (continuous glucose monitors) has also given us a real time “estimated A1c” by actually doing a mathematical average based on measuring a persons blood sugar levels every few minutes.
From a clinical perspective, an A1c gives a general picture of a person’s blood sugar control. For example, a non-diabetic who eats a strict, low carb diet with regular exercise may have a super stable blood sugar that rarely budges out of the 80s. Their A1c will likely be on the super low end. An uncontrolled diabetic with an A1c of 14 has no blood sugar control, and their blood sugars likely range anywhere from 150-400.
There is no value to any given day, as it’s considered just an average over a three month period. You cannot treat a blood sugar based off an A1c, but you can make general treatment decisions based off it, such as needing lifestyle changes or adjusting medication.
There are a few correct answers here, so I will add a bit of nuance.
A1c can be inaccurately low in cases of high red cell turnover, such as sickle cell anemia or bleeding conditions. Average RBC lifespan in a healthy adult is 90 days. If a condition causes RBCs to be lost or recycled more quickly, then there is less time for hemoglobin to be glycated before it gets lost. This will reduce the amount of glycated hemoglobin, I.E. lower the A1C. This is part of the reason why there are multiple clinical definitions of type 2 diabetes, including those that use fasting blood sugar or random blood sugar which are independent of A1C.
This exception doesn’t apply to the vast majority of people but it’s sort of a nifty insight I think.
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