I think it varies with each person and with each drug. For our son, a prebolus does seem to help some, but we don’t have to wait a full hour. In the morning it could be up to 25 minutes but during the day it seems to cause a drop in 10 to 15. Also keep in mind that CGM is measuring interstitial fluid, which is up to 15 minutes behind blood glucose. So once you see the downward arrow on CGM, your blood sugar has likely been dropping for 10 minutes or more.
However, I think it is really important for people to realize that insulin activity actually peaks about an hour into your dose …that means, it’s maximum glucose-lowering effect occurs an hour after you inject. And it takes a full 30 minutes, just based on the graphs shown in the packet inserts for the drugs, for a meaningful level of insulin activity to be present.
Realizing this has led us to do a small pre-bolus about an hour before our son eats (typically about 0.3 units), and then bolus for the rest of his meal (usually 1 to 2 units) upfront. We’ve found that this type of prebolus works better for our son as he’s small and it actually takes him a few hours to digest food. If we prebolus for the entire amount upfront, his food outlasts his insulin and he gets a rise 2 to 3 hours after eating. My hunch is that the sharp spikes we saw before with no prebolus were caused by liver dumping in anticipation of eating, so the prebolus of 0.3 is just enough to suppress that dump, and that the actual metabolism of glucose from the food can be handled by the rest of the bolus given anywhere from 5 to 10 minutes before to 10 minutes after a meal.
The other benefit is that if he decides to throw a tantrum and chooses not eat his meal, we’ve bolused for so little that it’s very easy to stop any impending low with a few jelly beans. By contrasting, making up for a whole prebolused meal that he refuses to eat typically takes a lot of quick thinking and scrambling.
Then again, we are not aiming for very tight control; his endo target is 7.5, I aim for below 6.5. Using this I would say he routinely spikes to between 160 and 180 post-breakfast, 140 to 150 post lunch, and 150 post-dinner. Often it’s higher, of course.