The best time to eat after taking insulin depends on three major factors. First the type of insulin you are taking, the type of food you are eating, and your current blood glucose level.
There are two major types of insulin taken before meals, rapid acting and short acting. Rapid acting insulins include lispro (Humalog), insulin aspart (NovoLog), and insulin glulisine (Apidra). It’s generally best to inject these insulins up to 15 minutes before you plan to eat. For the short acting insulins, Humulin R and Novolin R, the timing is 30 to 45 minutes prior to the meal. If you wait longer than these times, you run the risk of having a low blood glucose level.
The type of food also impacts the timing of insulin. Carbohydrates have the most effect on glucose levels. Meals that are high in fat slow down the absorption of carbohydrates. So eating a pizza, which has a lot of fat, may not affect your glucose level as quickly as eating a bowl of cereal. Protein usually has a small effect on your blood glucose levels.
Thirdly, your blood glucose level prior to eating is important. Having a low blood glucose level and then taking insulin and waiting to eat may cause your blood glucose level to drop even further. If your blood glucose level is high before you eat, you may need to add extra insulin as recommended by your doctor to compensate for the high blood glucose level.
In terms of how much to eat, that depends on your weight, your dietary needs, and your insulin to carbohydrate ratio. This ratio is how many grams of carbohydrates one unit of insulin covers. This is a very individualized number that your doctor will decide when prescribing insulin to you.
The important point is to talk with your doctor, nurse educator, and dietician about the timing of eating and insulin. Keep a diary of the different foods you like to eat so that you can gain insight into what happens when you eat that meal. Be sure to mark down the grams of carbohydrates in the meal so your doctor can evaluate your insulin to carbohydrate ratio. Check your blood glucose level 2 hours after eating to see what effect the meal has on your glucose level. Remember that many other factors affect your reaction to insulin such as exercise, a cold, and stress. Diabetes is a chronic disease and the more you understand about the factors affecting you, the better your glucose control will become.