As is true with everything involving diabetes the answer is not simple due to variations in individual blood sugar responses.
The reason we test one hour after a meals is to learn how high our blood sugar goes in response to the specific meal. So we want to be testing at the moment when our blood sugar is at its peak.
Studies tell us something about the average time it takes for the carbohydrate in our food to turn into blood sugar (carbohydrates are the main nutrient that causes elevated blood sugars). Such studies suggest that most Americans who eat our meals fairly quickly will see a peak somewhere between one hour and seventy-five minutes after we start eating. But because studies only come up with averages, they don't take into account individual variations--and you are, of course, an individual.
And when we move from group averages to individual response we learn that when the blood sugar peak occurs depends on a multitude of factors that include how fast we eat our meals, how much we eat at each meal, how tightly bound the glucose is in the carbohydrates we eat, and how efficient our digestive system is at digesting the carbohydrate bound in our food.
That explains why the same meal consumed at the same time by two different people may peak at different times--and why I can't tell you exactly when to test.
That's why you might try varying the time at which you test a carefully chosen test meal to see if your personal peak is later than average. Choose a simple meal that contains a known quantity of carbohydrate--a single measured portion of something rather than a meal where you have to guess what you are actually eating. Test it on different days at a slightly different interval each time to see if your blood sugar might be peaking a bit later than average.
FACTORS THAT CAN AFFECT WHEN A MEAL CAUSES A BLOOD SUGAR PEAK
There are a couple things to keep in mind when you are testing your meals.
1. Hard Pasta (the kind that comes in dry form in boxes) takes several hours to digest and often will cause no rise in blood sugars at one or two hours. If you eat pasta you should test it at three and four hours to find a blood sugar peak. So-called "fresh pasta", however, digests at the same speed as bread. As discussed HERE Dreamfields Pasta produces the identical blood sugar curves as much cheaper normal pasta.
2. Pizza eaten with crust may also produce a delayed blood sugar peak due to the high fat content and the food volume involved.
3. Most people are more insulin resistant at breakfast than they are later in the day so you may see a higher reading after eating a food at breakfast than you will see if you eat the same food at lunch, even if you start the meal with the same blood sugar level.
4. If your blood sugar is very slow to come down after eating you may have elevated blood sugars left from a previous meal when you start eating your next. If you see much higher readings after dinner than you did eating the same food at lunch it is possible this is happening. In that case, you might want to test before you start eating, and if you see a high reading then, you would be well-advised to avoid eating any starch or sugar with your next meal. Eating only meat, green veggies, cheese and eggs won't raise your blood sugar significantly when it is already elevated.
5. If you have a condition called "gastroparesis" in which the stomach holds on to food for many hours at a time, delaying digestion, it may be impossible to predict when your blood sugar will be at its highest. This is not a common condition. It can be a diabetic complication, but usually it is a very late complication that affects people with Type 1 diabetes who have had very high blood sugars for decades. However gastroparesis can also be caused by other things that have no relationship to diabetes, for example, a viral infection.
6. Januvia, Onglyza, and Byetta all cause delayed stomach emptying (usually accompanied by constipation or cramping). If you are on these drugs, you may be experiencing blood sugar peaks later than usual and should experiment with later testing times.
7. Pure glucose--the sugar used in glucose tolerance tests--is absorbed directly from the stomach without having to be digested by enzymes. Other sugars have to be digested which takes longer. There is a theory beloved by nutritionists that says that whole grains and "complex sugars and starches" digest much more slowly than other carbohydrates, too. My experiences and that of many people with diabetes is that there is a difference in the speed with which glucose (and things like maple syrup or honey which contain pure glucose) hit the blood stream but that starch is pretty much starch (except for pasta) and whole grains digest at the same speed as potatoes or white bread. A study you can read about here confirms this.
Once you establish the interval after eating at which your blood sugar is likely to be at its highest, you need only test at that time to assess the impact of a new food (or meal) on your blood sugar.
DON'T OBSESS ABOUT TESTING TIMES--OR EXACT READINGS
But don't make yourself crazy worrying about the timing of the test. Because of the shameful inaccuracy of our overpriced blood sugar strips the readings these strips produce can easily be off by 10-15 mg/dl (approx .5 mmol/L) when blood sugar is in the range around 100 mg/ld (5.5 mmol/L) and by a lot more if it is over 200 mg/dl (11 mmol/L).
So when you test, look for ranges rather than exact data points. Your goal when you start testing is to find foods that keep your blood sugar in the low 100s (6s) and to avoid those that push it up into the 200s (11s) at any time. Very small differences in your readings--120 mg/dl compared to 128--are not significant because they fall within the range of meter error.
What you don't want to see is readings that are consistently in the range of 160 mg/dl and higher (8.9 mmol/L) at one hour after eating or readings that don't drop below 140 mg/dl (7.7 mmol/L) at two hours after eating. And consistency is key here. Readings that are high day after day are what we have to avoid. But if you see one or two high readings occasionally, don't panic.
That's because diabetic complications come from prolonged exposure to high blood sugars: from many hours spent above 140 mg/dl every day for months and years. An occasional brief high won't make you go blind. It's the overall pattern that matters here.
WHICH IS BETTER, TESTING AT ONE HOUR OR TWO?
Both one and two hour readings are informative. The one hour reading tells us how high a food pushes our blood sugar. If we know how much carbohydrate there is in the meals that keep our blood sugar in the safe zone (learn what that is HERE) we can predict what other foods are safe to eat simply by looking up how much carbohydrate they contain. (You can find software and apps online that makes it easy to do this.)
The two hour reading tells us how efficiently our body was at getting rid of that temporary high. If it is elevated (over 140 mg/dl (7.7 mmol/L) it gives you warning that your blood sugar peak may be lasting long enough to start damaging your organs. If that is the case, you will need to cut back more on your carbohydrate intake or talk to your doctor about the safe medications that can help you achieve healthier blood sugar levels.
But it usually is a waste of time to talk to your doctor about your post-meal values until your readings are consistently over 140 mg/dl at two hours after eating. And even then they may dismiss your concern. That 140 mg/dl is the level that the American Association of Clinical Endocrinologists has set as the target that all patients with diabetes should be encouraged to reach. But most family doctors have been trained (by the American Diabetes Association) to believe that any two hour reading under 180 mg/dl (10 mmol/L) is "good control." This is, of course, why all the patients these doctors treat eventually develop diabetic complications, but it can be very hard to get treatment from a doctor who has absorbed this toxic ADA teaching.
WHAT ABOUT HIGH HALF HOUR READINGS?
There is one last, related issue that people write to me about which needs to be disposed of and it is this: If you test at half an hour after eating you are likely to see a high even if your blood sugar is completely flat at one hour. A surprising number of people write to me in a panic because they saw blood sugars over 140 mg/dl (7.7 mmol/L) half an hour or even fifteen minutes after eating.
A high shortly after eating is normal--as long as the reading has come down from that peak by an hour after eating. That is because, the way normal blood sugar control works, it may take a sharp brief spike in blood sugar to stimulate first phase insulin release.
But what makes a blood sugar response normal is that, in a normal person, as soon as this spike occurs, a burst of insulin is secreted which takes care of it completely. Blood sugar response is only considered abnormal if, after that initial blood sugar surge caused by glucose from a meal hitting the blood stream, the blood sugar level doesn't start heading back down.
You can see a graph of what happens to normal peoples' blood sugars when they eat a high carb breakfast HERE.