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When deciding the quantity and timing of an insulin bolus, another important question to ask yourself is: "What's in my meal?" Along with your current blood sugar reading, making adjustments for the protein, fat, carbohydrate, and fiber content of your meal will help you decide on an accurate insulin bolus.
For example, certain meals, such as cheese enchiladas, one of my personal favorites, contains a high percentage of fat. Although the corn tortillas contain carbohydrates, I cannot bolus enough insulin to cover the carbs all at once, or I'd likely end up low before the sugar enters my bloodstream. Since fat, protein, and fiber all slow the absorption of carbohydrates, any meal containing high amounts of these nutrients will delay the rise in blood sugar you'd expect from a certain quantity of carbohydrates.
On the other hand, certain food choices contain refined or simple carbohydrates that enter your system and spike your blood sugar very quickly. Typical breakfast cereals, white rice, and candies are common examples. When you eat these food items, your regular insulin bolus, delivered right before you eat, won't be active quickly enough to keep your blood sugar from spiking. When I enjoy the rare movie night out, I love to snack on red licorice! I know that I must bolus early so the insulin can start to lower my blood sugar before I enjoy that first taste of licorice. I've found that eating highly refined carbs takes a little extra planning ahead of time.
However, another note about the timing of an insulin bolus: I've noticed that when I indulge in a particularly large or high carbohydrate meal, I have to be careful not to deliver the entire insulin bolus all at once. Even if the meal doesn't contain a lot of protein, fat, or fiber, the sheer quantity of insulin can lead to a low blood sugar. When eating those high carb meals, it's best to deliver a combination, or square-wave bolus so that the insulin is delivered over a certain period of time, say 30 to 60 minutes. That way the insulin won't bombard your system lowering your blood sugar too quickly.
To me, that's one of the most annoying aspects of diabetes, when you eat a meal and then have to eat more to correct a low blood sugar, only to require more insulin later, once my blood sugar climbs back up!
While timing your insulin is half the battle, you also have to be mindful about the timing of your meals. For example, if my blood sugar is elevated at mealtime, it's best if I can delay the meal while a correction bolus lowers my blood sugar to a more normal range. Eating a full meal (even if I've bolused for it) while my blood sugar is above 200 mg/dl will almost always yield a high postprandial blood glucose value. Simply put, your body needs time to utilize the insulin to drop your blood sugar. If you add food to the mix, you'll delay the decrease in your blood glucose.