It is especially important to inject before breakfast when the need for insulin is usually higher than at other times of the day. At daytime, for example, when having lunch away from home, a person with diabetes is often forced to compromise and take the rapid-acting insulin only after the meal portion has been put together.
Rapid-acting insulin begins to act roughly within 10–20 minutes. If your blood sugar level is optimal before the meal, it is beneficial to administer the bolus insulin 10–20 minutes before you start to eat. If your blood sugar level is higher than optimal before the meal, it may be necessary to inject 20–30 minutes before eating or first correct the high blood sugar.
The mealtime insulin dose and the timing of the injection have been correctly estimated if the blood sugar is no more than 2-3 mmol/l higher about two hours after the meal than before the meal. If mealtime insulin is administered only after eating, the blood sugar can easily rise significantly 1-2 hours post meal and may even drop too low before the next meal.
For foods rich in protein or fat or both, it's advisable to administer rapid-acting insulin at the beginning of eating and possibly in two doses.