I’ve only gotten close to perfect results using a fixed ICR with a simple carb meal, which is the way that the ICR was originally defined. The concept doesn’t match real meal response, but is a practical compromise.
For my first decade with diabetes (the 1980s), I had problems with hypo and hyper. A1C wasn’t generally available, insulin and BG tests weren’t covered by insurance - when I could get it. The best I could do was feel good on the average. I could never match insulin in my meals. I had to change my meals to match insulin and how my body digested food. This worked pretty well for me for decades.
That stopped being the case as I developed autonomic neuropathy and gastroparesis. I responsively adjusted meals first and then insulin dosing, but I continued to trend toward wider variation in my BG and my A1C.
After I started using a CGM and could see how my BG fluctuated - when I wasn’t eating - I started to get a better handle on dosing. ICR was an average. Basal rate was an average. A1C was an average. None of these matched how my body responds hour by hour, minute by minute.
The way that insulin is administered- even with a hybrid closed loop pump uses averages. A healthy person’s blood glucose level isn’t regulated by averages. It’s dynamic, adaptive, and the pancreas doesn’t guess what’s coming. It’s reactive, and not just with insulin. A companion hormone tells the other organs to not convert stored fat to glucose. It does it real time, releasing these hormones into the blood.
The CGM showed me that average parameters don’t match my diurnal cycle. So I made changes to simplify things until I had better control. I’m lucky that my lifestyle and my spouse allow me to do controlled experiments like this for months at a time.
We now prepare my meals from fixed menus with carbs between 35 and 42 grams. I know within 2% how many grams of carbs are in every meal, and less precisely, how many grams of fat and protein. I have CGM and BGM data from a hundred identical breakfasts, dozens of identical lunches and dinners.
My 35 gram carb breakfast BG response to the same insulin dose varies from day to day. Not a lot over 5 hours, my pre-lunch BG is within 10 mg/dL of desired, but the height of the peak and the shape vary widely. I initially thought it was from timing the bolus, so I purposely eliminated that variable for a week. The relative peak still varied, within one infusion site and between infusion sites by more than 20 mg/dL. My average ICR is 1:9, my CF is 1:30.
For my lunches and dinners, which have progressively more protein and fat than breakfast, with a “simple” pump a fixed ICR doesn’t work for all three meals. The diffusion rate of the insulin doesn’t match my digestion. The bolus is gone before the carbs after dinner.
A higher fat/protein meal requires both a small ICR and a square wave extended bolus of 3 hours with Basal IQ or Control IQ turned off. Basal IQ will dump the balance of an extended bolus if BG drops enough suddenly at any time, Control IQ doesn’t permit extended boluses longer than 2 hours.
Now I have a basal profile, with an ICR and CF that vary by average meal, and extended boluses to accommodate fat and protein loads, I still need to do small corrections 3-5 hours after most dinners to get my BG before bed to my target.
SInce “upgrading” to Control IQ I still need to watch postprandial peaks. Control IQ doesn’t give a bolus after meals to prevent going over 180. It gives it no sooner than an hour after a meal bolus ends, to blunt the rise. It has never raised my basal rate as I cross past 160 after a meal. It seems that those parts of the algorithm aren’t for persons with carefully managed blood glucose, but for ones who haven’t - between meals.