Today’s the big day! After months of working hard to achieve tight blood glucose control and actually succeeding most of the time, you’re about to get the result of your latest glycosylated hemoglobin (HbA1c) test, the blood test that gives you an indication of your overall blood glucose control over the previous 2–3 months.
“It’s just got to be lower than last time,” you think as you wait in your doctor’s exam room. “I’ve been good. Really good. My average on my meter is lower than it’s been in years. He’s going to be impressed for sure. No lectures about the risks associated with a high A1c this time! I’ll be out of here in 10 minutes, easy.”
In walks your doctor, with the results of your latest lab work in hand. Thirty minutes later, after a series of careful examinations and a lengthy lecture on the need to get your HbA1c down, you walk out shaking your head.
“I don’t get it. How can my A1c still be so high? My mealtime readings are almost always where they should be. It just doesn’t make sense. Maybe the lab made a mistake. Maybe my blood is different from everyone else’s.”
Or maybe you’ve been the victim of postprandial hyperglycemia.
Postprandial hyperglycemia refers to high blood glucose levels that occur soon after eating meals or snacks. For anyone with diabetes, it is normal for blood glucose to rise somewhat after eating. But if your after-meal rises are dramatic and occur consistently, they can result in higher HbA1c test results than your premeal blood glucose readings would otherwise indicate, and the higher your HbA1c, the higher your risk of serious diabetes complications.
The HbA1c reflects an average of all blood glucose levels at all times of day — before eating, after eating, during the night, etc. — over 2–3 months. (To see how HbA1c levels compare to average blood glucose levels, see the table “Blood Glucose and HbA1c Correlations.”) However, research presented in 2004 suggests that a person’s HbA1c correlates closely to his average blood glucose levels during the three hours after — not before — eating. So even though after-meal high blood glucose levels are temporary (often resembling a spike when plotted on a graph), frequent between-meal rises can cause your HbA1c to go up.
Research on the effects of postprandial hyperglycemia has shown an increase in the risk of death from heart disease in those with Type 2 diabetes and earlier onset of kidney disease in those with Type 1. But postprandial hyperglycemia can have more immediate effects as well. Just as a big turkey dinner can turn the peppiest person into a slumbering slug, a rapid rise in blood glucose levels after meals has been shown to affect the ability to concentrate, stay alert, and perform athletically and intellectually.
How high is too high?
In most cases, blood glucose levels peak about an hour after finishing a meal or snack. Ideally, the blood glucose level at the peak should be below 180 mg/dl, or less than 80 mg/dl higher than it was before the meal. With children, after-meal peaks can be a bit more liberal. Teenagers should try to keep peaks below 200 mg/dl, school-age children below 225 mg/dl, and pre-schoolers and toddlers below 250 mg/dl.
In practice, these ideals may not be achieved by many. Research conducted at Yale University on children with Type 1 diabetes indicated that after-meal blood glucose peaks are generally much higher than the recommended levels. After breakfast, the average peak blood glucose level was 293 mg/dl; after lunch it was 291 mg/dl; after dinner, 280 mg/dl. In fact, nearly half (46%) of meals were followed by a blood glucose peak of over 300 mg/dl!