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@EllieM i find my insulin resistance increases when my levels are high (for me, high is double figures - over 10mmol/l). This has two affects - I need more insulin to reduce my levels and insulin takes longer to start working.
Therefore, my pre-bolus time depends upon my BG.
If my BG is over 9mmol/l, I pre bolus at least 30 minutes (up to an hour if convenient), before eating.
If my BG is in the 4s or low 5s, I bolus
after eating.
if my BG is between 6 and 9, I bolus before eating.
I avoid bolusing and eating immediately if my bg is over 10 because I know this will cause my BG to spike much higher.
Do you know why your BG is over 10mmol/l before your meal? Have you done a basal test to check your basal is enough or whether you did not take enough bolus with your previous meal? Or was the rise due to some stress or exercise?
Our insulin dose can vary from day to day depending upon things like stress, exercise (exercise can affect BG for 48 hours), illness (including fight a bug which never has symptoms beyond a high BG), time of the month, weather, … So it should not be surprising (although it still surprises me) that we need to tweak our dose.
My advice would be to focus on that 10mmol/l before your meal. if you can reduce this, the post meal spike will come down.