Are you new to using insulin? Or do you want to refine your current technique? While the basics of injecting are fairly straightforward, the day-to-day ins and outs of balancing insulin, food, and physical activity can be confounding even for a veteran.

Understanding the different types of insulin and how they will affect you is a good first step toward making sure you’re using them properly. First, there’s “background” insulin, which works all day long to steady blood glucose levels. This group includes insulin glargine (Lantus), insulin detemir (Levemir), and NPH (Humulin N and Novolin N).

How Does Insulin Work?
The graphs here show at a glance how long it takes for each type of insulin to begin working, when the dose reaches its peak, and how long it lasts.

On the other hand, “mealtime” insulin starts working fast to make sure a meal doesn’t raise your glucose levels too high. Unlike background insulin, mealtime insulin stays in your system for only three to six hours. The mealtime insulins are insulin lispro (Humalog), insulin aspart (NovoLog), insulin glulisine (Apidra), and regular insulin (Humulin R and Novolin R). In addition, some people use premixed insulins, combinations of mealtime and background insulins.

HOW TO: Cover carbs with insulin
Here’s the most basic thing you need to know: Carbohydrates in food raise your blood glucose. Insulin lowers it. That’s why your doctor will probably tell you to use a certain number of units of insulin to cover the carbohydrates you eat. “That’s individualized,” says Caroline Bohl, MS, RD, CDE, a diabetes educator with the Naomi Berrie Diabetes Center at Columbia University Medical Center. “Each person will know one unit covers a certain number of carbs.”

It’s generally best to inject mealtime insulin up to 15 minutes before you plan to eat if you are using Humalog, NovoLog, or Apidra. For Humulin R and Novolin R, the timing is 30 to 45 minutes prior to the meal. Inject any longer before a meal and you run the risk of hypoglycemia, since the insulin will kick in before there’s any food for it to work on. (If you administer a mealtime dose but aren’t able to eat, you may need to take glucose tablets or other fast-acting carbs to prevent your blood glucose from dropping.) Taken before you eat, on the other hand, the insulin will lower your blood glucose level just as your meal is raising it. The result? Numbers that don’t spike.

Of course, it’s more complicated than that. “We know that higher-fat, higher-protein meals along with higher carbohydrates can slow the absorption [of carbohydrates],” says Amy Hess-Fischl, MS, RD, LDN, BC-ADM, CDE, coordinator of the teen transition program at the University of Chicago Kovler Diabetes Center. When you eat a high-fat food (say, pizza) after taking mealtime insulin, your blood glucose levels may drop shortly after eating, then skyrocket hours later once your body registers the food. In that case you may need to split a dose, taking half of your allotted insulin 15 minutes before the meal and the rest when you’re done eating.

That’s assuming you’re at your target blood glucose level before eating. If your numbers are running high, on the other hand, you’ll have to correct pre-meal. You can give both the correction and mealtime doses at the same time prior to eating.

For the best picture of how foods affect you, use your blood glucose meter to test two hours after a meal. “To see how a bagel works instead of pizza, test often,” says Bohl. You won’t have to do this forever, but it’s a good way to understand how your body reacts to different meals. Discovering how various foods affect your blood glucose—and therefore your insulin dosing—is important because reactions to carbohydrates are individualized. “Alcohol has the potential to lower blood glucose, but there are some people who have a glass of wine and … go high,” says Hess-Fischl.

A lot of the disparity in how different foods affect blood glucose levels is a result of a factor known as the glycemic load. Foods with high glyemic loads raise blood glucose more quickly than those with low ones. That’s why, in part, you might have more of a spike after eating refined carbs than with whole grains. Still, this is more of a general guide than a fixed set of rules. “People really have to know what works for them,” says Sharlene Emerson, CRNP, BC-ADM, CDE, a diabetes educator with the University of Pittsburgh Medical Center. Pay attention, and you’ll soon begin to notice which foods have a particularly bad or good effect on your blood glucose numbers.

Next: How to correct a high