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Home » Medicine Usage Guides » A User’s Guide to Insulin

A User’s Guide to Insulin

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.

How to Correct a High

Your blood glucose level is soaring. You need more insulin. But how much? The amount one unit of insulin will lower your blood glucose is called your insulin sensitivity factor, or ISF. You’ll work with your doctor or diabetes educator to determine the number of units of insulin that safely lowers your levels without risking hypoglycemia. And since the ISF is highly individual (one unit of insulin may lower an adult’s blood glucose by, say, 30 mg/dl, while a child’s level may drop by 100 or 150), it requires a bit of testing. Check your blood glucose two hours after injecting, says Hess-Fischl. If it’s within 50 mg/dl of your goal, your correction factor is spot-on. If your level is higher, try correcting with more insulin next time you’re high. Too low? Use less insulin to correct a high.

If you just ate and your blood glucose is high, hold off on a correction dose until your mealtime insulin is out of your system (about three to four hours). Likewise, if your blood glucose is still high after a correction dose, give it time. Injecting another dose too soon after a previous dose can result in “stacking” of the insulin doses, and can lead to dangerously low blood glucose. “If somebody’s correcting high blood sugar without [having eaten], I usually tell people [their blood glucose is] going to take 45 minutes to start coming down,” says Bohl. “Usually give the insulin two to three hours to work. Be a little cautious to not overcorrect. Sometimes patients keep doing it, and they just plummet.” You’ll want to work with your care provider to figure out what’s best for you. It may take time to get it right.

How to Exercise Without Going Low

Exercise lowers blood glucose levels, so hypoglycemia during or after a workout is a real worry. But, according to Bohl, it’s manageable. “I tell people they need to have a gauge of how much their exercise makes their blood sugar drop,” she says. Sounds easy, right? While finding this number will take a bit of trial and error, it should be safe to start exercising when your blood glucose is at 150 mg/dl. Test before, during, and after your activity to see how it affects your level. Don’t forget to test later in the day, too; some people have a drop in blood glucose hours after a workout.

Another tip: “Inject in an area that’s not affected by the exercise,” says Emerson. If you’re going for a run, inject insulin into your abdomen, not your leg. Since you’ll be working your leg muscles during the exercise, insulin injected there will be absorbed more quickly than the same amount injected elsewhere in the body.

Once you know how your body reacts to physical activity, you can determine a blood glucose level that’s safe for exercising. Some people will work out when their blood glucose is a little above their target range, knowing it will drop. (Avoid exercising when blood glucose is above 250 mg/dl, however.) Others will eat a carbohydrate-containing snack or a glucose tablet to raise their glucose and prevent a low. Athletes who are on a regular schedule might adjust their insulin to make up for a workout. One of Hess-Fischl’s patients always exercises after lunch, so she cuts her pre-lunch insulin dose by half. If and how you’ll modify your insulin dosages is a matter to discuss with your doctor or diabetes educator.

There’s a lot to learn, and that can seem daunting. But as long as you keep testing to understand how insulin affects your body, you can achieve good glucose control. In time, you’ll be a wiz.

Popular Questions

Q: If my blood glucose is 70 to 80 mg/dl and stable before mealtime, should I still take the same amount of insulin I would normally take to cover the carbs?
A: If your blood glucose is on the lower side of normal, you can reduce your mealtime insulin dose by a small percentage. How much less you should take is tricky since there’s no general rule; you’ll need to work that out with your doctor. And keep in mind that if you’re experiencing hypoglycemia before a meal, you should treat it with fast-acting carbs first. Then, when you’re certain your blood glucose isn’t falling, dose insulin for a meal and eat right away.

Q: Should I take insulin if I’m only having a small snack with very few carbs?
A: Probably not. People with type 1 diabetes, and those with type 2 who use mealtime insulin to cover carbs, need to cover most food with insulin, but not if it is a snack of 15 grams of carbohydrates or less.

Q: Why can’t I just eat anything I want (say, a tub of ice cream) and cover it with mealtime insulin—even if that means a lot of insulin?
A: It’s important to pay attention to good nutrition whether you have diabetes or not. Indulging in anything and everything you want will only set you up for weight gain, high LDL (“bad”) cholesterol, and high blood pressure. And while taking large doses of insulin isn’t necessarily a bad thing if your body requires it, loading up in order to cover a jumbo meal puts you at risk for hypoglycemia. Since your body can only digest so much food at once (and fat in foods slows digestion even more), a supersized dose of insulin may hit the blood before your entire meal is digested, causing low blood glucose. Then, after your meal is fully digested, your level can spike.

Q: If I take background insulin before I go to sleep at night, will it affect me differently than if I take it in the morning?
A: It depends on the type of background insulin and how long it lasts. In some people, the effect may start to wane before the next dose. So if you inject in the morning, you may experience a rise in blood glucose the following morning as the insulin wears off. There’s a greater difference with NPH, which lasts a shorter amount of time. Regardless of the type of background insulin you use, you should inject at the same time each day or night to keep blood glucose levels stable.

Q: Should I still take the same mealtime insulin before lunch if I just worked out and tend to go low a couple of hours after exercise?
A: Yes, but take a smaller dose. You’ll want to take about half as much insulin as you normally would for this meal. The drop you experience from working out will bring you to your goal.

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