Retrospective #196: The Diagnostic Power of A1c vs. Fasting Glucose

There have been scads of scientific papers published on this subject. I promise not to cite any of them in this piece. Instead, I will offer the example of two people’s A1c’s and fasting glucoses to illustrate the wisdom of using the A1c rather than the fasting blood glucose in diagnosing incipient or “incident T2DM” (see The Nutrition Debate #182).

Until a few years ago the ADA criterion for diagnosing T2DM (Type 2 diabetes) was 2 consecutive fasting blood glucose tests (on separate office visits) of ≥126mg/dl. The ADA also guided that a fasting value of 100-125mg/dl was to be considered “Pre-diabetic.” A “normal” fasting glucose was then 70-100mg/dl.

The A1c increasingly became the diagnostic tool of choice in 2009, although the fasting glucose test, while seriously flawed, is still widely used. So, what is the A1c test? Technically, the A1c (glycated hemoglobin A1c), is a percentage measurement of the glucose on the surface of red blood cells. Since red blood cells live about 2 to 3 months (before they die and bone marrow replaces them), the test is then a surrogate measurement of the level of glucose circulating in your blood 24/7 for that 2 to 3-month period. It’s like wearing a blood glucose monitor!

The advantage of the A1c over a fasting blood glucose is that the A1c test captures all the postprandial spikes (called “excursions”) that your blood sugar takes after eating. Everyone’s blood sugar surges after eating. As food digests, to the extent that there are carbohydrates in the food, they will break down to glucose, be absorbed in the blood, and be transported, by insulin secreted in the pancreas for the purpose, to the cells. That is how your body gets the “energy” from carbs delivered to, and hopefully into, your cells. In people with a “normal” metabolic function, the glucose moves from the blood into the cells. Then, after a couple of hours when all the circulating glucose has been “taken up,” the level of glucose (and insulin) in the blood lowers to where it began.

However, in people whose metabolism has developed a specific “dysregulation” called Insulin Resistance, the receptor cells on the surface of the destination cells refuse to open, and the level of both insulin and glucose in the blood remains “elevated.” The energy is not taken up. As a consequence, you get hungry and tired and lack energy, literally. So, these people will have a higher A1c, corresponding to the higher blood glucose that is circulating for hours/days, even continuously, if you regularly eat carbs at every meal and with in-between-meal snacks.

Now, the two examples I promised: Person #1 is the email correspondent referred to in Retrospective #195R. He was diagnosed a Type 2 diabetic (A1c 6.5%) about 9 months ago. In response, he changed his diet (“just stopped eating bread, potatoes, pasta and ice cream”). That’s not an “extreme” or “very low carb” diet. The result: nine months later he had “lost 20 pounds” and his “A1c (was) “totally normal” (5.7) and fasting glucose (was) at “100-105 over 3 blood works.” To be clear, he is still a type 2 diabetic. He still has Insulin Resistance, but he has learned how to control his blood glucose, and thus his diabetes, through a moderately low carbohydrate diet. His A1c, at 5.7%, is “borderline.” You could say that his diabetes is almost “in remission,” but it will be so only so long as he stays “moderately low carb.”

Person #2 (lab tests read to me over the phone): fasting glucose 100mg/dl; A1c test = 5.0%. Note: this person has the same blood glucose value (+/-100mg/dl), but a “normal” A1c (for a 70 year old male) of 5.0% This person eats a typical American “balanced diet,” between 40% and 60% carbohydrate, and is not diabetic or even close to being Pre-diabetic, whereas Person #1 definitely is diabetic, even though both have identical fasting blood glucoses.

The A1c test is what differentiates #1 (controlled diabetic) from #2 (non-diabetic). And, the A1c is what made a diagnosis of incipient Type 2 diabetes possible in Person #1, and permitted him to take early action to address it.

Person #1 learned he could control his diabetes through diet alone. That’s what keeps his A1c in the clinically “non-diabetic” range, so long as he stays moderately low carb. I hope this is instructive, for all the Pre-diabetics and newly diagnosed Type 2s out there. If you have Insulin Resistance (as measured by your A1c), watch what you eat!