I'm a type 1 diabetic and data scientist. Estimating the causal effect of a unit of insulin or food on blood sugar is an absolute crap shoot. Consider that there's a +/-20% margin of error on the reported carbohydrates on nutrition facts. We might consider this irreducible error that just cannot be modelled (Maybe you could get a calorimeter, estimate the distribution of errors, and reduce that error somewhat). Therefore, even if we created a model that explained all explainable variance, we still have a 20% margin of error. If a meal has enough carbohydrates, a 20% overestimate of insulin requirements would lead to an insulin overdose that would kill you if the resulting low blood sugar is not dealt with. In other words, the irreducible variance is so large that a "perfect" model would regularly suggest lethal insulin doses.
My "solution" is to eat low-carb/keto as a "variance reduction" strategy. Still, removing carbs also introduces gluconeogenesis (the production of glucose from protein) as a factor to consider. The synthesis of protein to glucose also occurs on a much time different time horizon than the consumption of carbs themselves which has implications for insulin dosing and insulin type.
I could go on! But long story short, modelling blood glucose is bloody hard.
And there can be large errors in the CGM data as well!
Not really. The majority of diabetics get used to those things quickly (of course there are some of course that deal with a major major needle phobia that can make it even harder). The hard part is that it never ends. Almost every moment of every day, your brain has a background process running that's evaluating every decision in context of your diabetes. There are no breaks. Your prefrontal cortex now has to take the place of a previously complex and automatic bodily process. It's the last thing you think about when you go to bed and it's the first thing you think about when you wake up. It's what you think about when you want to go on a walk, are about to enter a meeting, go into an interview, get on a plane, take a shower.
It's usually little things: "okay, where am I at now? which direction is it going? when did I last eat? do I have snacks ready? do I have enough insulin for the day? what if I start to go low during this meeting? should I pop some carbs and run high for this interview, so I don't risk a hypo partway through? why am I going low right now when I took the same dose I took yesterday for the same meal? why am I now skyrocketing for no discernible reason, I didn't even eat anything? shoot, I'm starting to hypo out of nowhere in the middle of this great conversation, which I now have to interrupt to eat a snack and recover for 15 minutes. I fell asleep with a perfect BG, but now I'm awake at 2AM half delirious because my BG fell all the way down to 50, and I'm in the kitchen shoving cookies down my throat because hypoglycemia activates a survival instinct to EAT EVERYTHING that's extremely hard to control, and I know that I'm gonna shoot all the way up to 250 shortly, which I'll have to treat with insulin, and I'm basically not going to get any sleep tonight".
And then the math often doesn't make any sense. There are so many factors that effect it. One day the same number of carbs + insulin may make you go high, and the next low, because of other environmental factors. (See the "42 factors that effect blood glucose" chart in the post.) You're constantly having to adjust.
I'm literally crying while writing this post, because it's so exhausting and it never ends.