Here's a discovery that's been slowly crystallizing for a long time. I'm curious if anyone else has reproduced something like this:
If a meal is over 45g carbs, then only the first 45g absorbs in the bloodstream fast enough for the insulin to counteract it.
Bolusing over 45g all at once usually results in a low at about 2 hours. The bigger the meal, the more horrible the low. Two things are happening here - there's a larger surplus of insulin for a larger meal meaning a lower low, but also, the low is also harder to recover from, since glucose tabs are competing for digestion with the large meal.
3.5 hours later, the rest of the food appears to digest all at once, causing a monster spike. The bigger the meal over 45g, the bigger the spike is. (If the meal was a high-fat meal, the spike will be very difficult to bring down, but that's not the topic here.)
Combo boluses or square waves don't seem to work here. A square wave might still deliver too much up front. A combo bolus will frequently not give enough on the back end.
Correcting the spike at the 3.5 hour mark requires the insulin to be proportional to the food, not the blood sugar at that time. If the meal was large, then the correction factor is not enough to overwhelm the glucose entering the bloodstream. But the pump doesn't know this. It will try use the ISF to correct the BG, which won't work since ISFs assume the BG is not rising. Worse, it might still think you have some IOB and reduce the correction even further.
The food type doesn't seem to matter. 100g of pasta act similar to 100g of ice-cream, or 100g or chicken nuggets.
So here's what we do nowadays - when we are expecting a large meal, we bolus 45g carbs sight unseen. We'll even prebolus this at a restaurant after the order is in. (If food is very late, I know I can replace with juice or tabs or bread.) Then, when the meal comes in, we carb count everything, subtract the 45g, and write it down. For this example, let's say the whole meal was 100g. We eat in peace.
Then we watch the CGM. Like clockwork, at 3.5 hours the spike starts. Then, we bolus the remaining 55 carbs with no calcs or IOB. Bolus those carbs straight up. It's not perfect yet, but more than a few times we've avoided lows, and only stayed in the low 200's. Compare that to being stuck at 40 for 45 minutes and then 400 all night!
Do you notice this pattern? If so, do you find a different limit than 45g? Based on age or weight?