Interestingly, research has shown that for people with an A1C below 7.5%, post-meal blood glucose readings have a greater influence on A1C than pre-meal readings. In other words, lowering your pre-meal readings will only get you so far. If you want your A1C level to be as close to normal as possible, you need to pay attention to your after-meal numbers as well.
The long-term effects of high blood glucose levels after meals have been studied extensively. For people with Type 1 diabetes, significant after-meal rises have been shown to produce earlier onset of kidney disease and to accelerate the progression of existing retinopathy, the eye problem most commonly associated with diabetes. In people with Type 2 diabetes, high blood glucose after meals is a risk factor for cardiovascular problems.
But the problems are not limited to long-term complications. Any time blood glucose levels rise particularly high, even temporarily, quality of life suffers. Energy decreases, brain function falters, physical and athletic abilities become diminished, and moods become altered. An Australian study of young people with Type 1 diabetes indicated that short-term high blood glucose negatively affects thinking performance, coordination, and emotions and moods. A study conducted on people with Type 2 diabetes showed that sharp rises in blood glucose level slowed mental performance, lowered attention, reduced energy levels, and led to feelings of sadness and tension.
Additionally, the effects of a bout of post-meal high blood glucose do not go away immediately when blood glucose returns to normal. Each episode of high blood glucose can alter the way certain genes function, resulting in the production of harmful chemicals called free radicals, which cause inflammation and damage to the linings of the blood vessels for hours, if not days. So clearly, post-meal spikes represent a challenge worthy of attention.
The exact timing of blood glucose spikes can vary from person to person and meal to meal. However, on average, after-meal peaks tend to occur about one hour and 15 minutes after the start of a meal. So checking your blood glucose (using a blood sample from a fingerstick) about an hour after finishing a meal should provide a good indication of how much of a spike is taking place. Check before and after breakfast, lunch, and dinner several times to determine how much of a spike is taking place after each of these meals. It is most common to see significant spikes after breakfast, but check after each meal at least a few times just to see what is happening.
When interpreting your numbers, take your pre-meal readings into account. For example, a pre-meal blood glucose of 210 mg/dl followed by a 1-hour post-meal reading of 240 mg/dl shows just a 30-point rise, whereas a 110 mg/dl followed by a 240 mg/dl shows a 130-point rise.
So exactly how high is too high after a meal? There is no universal consensus on this issue. The American Diabetes Association recommends keeping blood glucose below 180 mg/dl one to two hours after the start of a meal. The European Diabetes Policy Group recommends keeping it below 165 mg/dl at the peak, and the American Association of Clinical Endocrinologists and International Diabetes Federation suggest keeping it below 140 mg/dl after eating. However, no specific guidelines are provided by any of these groups for Type 1 versus Type 2 diabetes, insulin users versus non-users, or children versus adults.