I asked this question just last week. My DSN referred it to a dietician who came back with the following. Anything but straightforward.
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It is tricky, as you said. In countries where they count fat and protein as well as carbs, they use fat protein units (FPU). 1 FPU is for every 100kcal that comes from fat and protein. They then use a similar ratio to that used for carbs, e.g. if one uses a carb ratio of 1:10g then they would use 1unit:1 FPU.
It has been suggested that in the absence of carbohydrate, up to 50% of protein can be converted to glucose but that this doesn't peak in the blood until 2-4 hours after eating. Likewise, up to 10% of fat can be converted to glucose but this doesn't peak in the blood until 8-10 hours after eating. You can see how this can get tricky as there are likely to be several meals potentially affecting BG at any one time. With the much slower conversion of fat/protein to glucose, it is difficult to suggest how much insulin to take as the rapid acting insulins are designed to work as such, i.e. rapidly!
There may also be implications regarding activity level as glucose disposal rates will be altered according to the amount of exercise and when the exercise is undertaken.
Sorry I can't be more helpful than that. It is a tricky one to answer - probably will come down to a bit of trial and error I'm afraid.
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