Timing is potentially a significant issue, primarily for quick acting insulin. Although insulins like Novorapid, Humalog and Apidra are ‘rapid acting’, they still take around 30 minutes to be absorbed. This means that carbohydrate based meals (sugars, bread, potatoes, pasta) can result in a big peak of glucose before the insulin has had an opportunity to start working. On the other hand, meals which contain a lot of fat can delay glucose being absorbed from the gut and often respond better to slightly later administration of insulin (i.e. after eating).
Timing can be an issue with background insulin, particularly if it looks like a once daily dose is not lasting 24 hours (where splitting the dose in two is advisable) or in people who get a ‘Dawn Phenomenon’ where taking background insulin dose as close to bedtime as possible can sometimes help prevent such a large rise between 3 and 7 am.
Working out the best timing
Looking at glucose results 2 hours after eating, and again prior to the next meal, provides good evidence of whether your quick acting timing is working effectively. It is worth thinking about this for different meal types. In the example above, taking insulin after a carb heavy breakfast has caused a big early rise in glucose but taking insulin before a fatty meal (with slowly absorbed carbohydrate – e.g. Mexican food) at lunch time has failed to match the later peak in glucose.
You can see, in the example above, that simply checking a glucose before lunch (which was quite low) may result in reducing the dose of breakfast insulin which would result in an even bigger glucose ‘spike’ after breakfast. Less insulin may be required but getting the timing right is the critical thing here.
Similarly, just looking at the pre- evening meal glucose level (which was high) may cause you to increase future doses of insulin at lunch time, which would increase the risk of hypoglycaemia in the early afternoon with this type of food. Again, timing and not the amount of insulin is perhaps a bigger issue in this case.