Diabetes Forecast

Back to Basics: Insulin

Delving into the nitty-gritty of this lifesaving medication

By Tracey Neithercott ,

A model of a human insulin molecule.

When you're new to insulin, there's a bit of a learning curve to using it: practicing how to inject, remembering to rotate your injection sites, and figuring out where to store and how to carry the vials, pens, and needles. That's all important stuff, but the toughest part arguably comes later, out in the real world—where you'll need to put your knowledge of the different types of insulin to use and balance them with your carbohydrate intake.

There are two main forms of insulin: "background" and "mealtime." Background insulin is injected once or twice a day and works to steady blood glucose levels between meals for about 24 hours. Mealtime insulin works fast but lasts only three to six hours. Its job is to make sure the carbs you eat don't spike your blood glucose too high. Figuring out how to balance the carbs you eat, the exercise you do, and the amount of insulin you need is a skill you'll learn from your diabetes educator or doctor and through a lot of practice. To help you navigate the often-confusing task of dosing insulin, we asked diabetes educators to explain the ins and outs of some common (but tricky) situations.

How do you use insulin when you're planning to eat …

… in 30 minutes to an hour?

"It's very individualistic, based on the type of insulin the person is on," says Laura Wise, RN, BSN, CDE, CMSN, a certified diabetes educator at Barnes Jewish Hospital in St. Louis. If you take rapid-acting insulin (Humalog, NovoLog, or Apidra), you'll inject your dose no more than 10 to 15 minutes before you eat. Regular insulin and a mix including regular should be taken 30 minutes before a meal.

… at a restaurant, where you have no clue how long your meal will take to arrive?

Giving yourself a dose of insulin to cover a restaurant meal can get tricky. That's because there's no precise way to predict how long it will take for your server to put in your order, the kitchen to prepare your meal, and the plate to actually make it out to the table. Depending on whether a restaurant likes to set a leisurely pace (as is the case in many high-end spots) and how busy the place is, it could take anywhere from five minutes to an hour for your food to arrive.

"Usually for restaurants we say, 'Dose when you see the food,' as a precaution," says Marilyn Clougherty, RN, MSN, CDE, diabetes program coordinator at Children's Hospital of Pittsburgh of UPMC. If you've already injected, your meal is slow to arrive, and your blood glucose levels are starting to drop, don't be afraid to ask for some bread. In a pinch, check the door. Many restaurants provide mints or other candies for departing customers that can be useful for treating low blood glucose. (But keep in mind it's smart to bring your own source of fast-acting carbs.)

… after morning exercise?

People with diabetes can work out before the first meal of the day, but they need to take morning blood glucose readings into account. If your level is normal or low, "have a small snack of 15 grams of carbs . . . with no insulin coverage," says Clougherty, "ideally, a half hour before." When you're done exercising, and if your blood glucose is normal, dose for breakfast as usual. (Those who tend to go low after exercise should inject a smaller mealtime dose—the reduction varies per person, but a good rule of thumb is to cut it by half the first time and then test to check the result.

How do you use insulin when you're having a snack …

… but plan to exercise shortly after?

"It depends on the blood sugar before you start exercising," says M. Lourdes Pruneda, MSN, RN, the clinical research coordinator at University of Texas Southwestern Medical Center. If your blood glucose is on the lower side of normal and you cover a snack with insulin, you might go even lower or experience hypoglycemia. What if your levels are normal? "If you're going to exercise, you don't need to cover for the snack until you've exercised and checked your blood sugar," she says. That's because exercise can drop blood glucose levels. That snack may prevent you from going low during a workout.

If your blood glucose is high before you eat a snack, cover the high blood glucose with insulin. How much to inject (a full dose or only a portion) depends on the person, the snack, and how a person responds to it, as well as how exercise affects a person's glucose levels. Your doctor or diabetes educator can help you learn the number of units that works best for you.

… but plan to eat again in an hour or so?

Many diabetes educators don't advise injecting insulin for snacks that contain 15 grams of carbs or fewer. "If [people] plan to eat again in about an hour, then they would follow the same guidelines," says Amy Aponick, MPH, RD, LD/N, CDE, a registered dietitian and certified diabetes educator at the Diabetes Center of Excellence at the University of Florida. If you're eating a larger snack, look to your insulin-to-carb ratio (devised by your health care provider) for help. Someone who injects one unit of insulin for every 30 grams of carbs, for instance, wouldn't dose for a snack with fewer than 30 grams of carbs. "However, if that person chose a snack that equaled or exceeded the 30 grams, then insulin would be required," says Aponick.

… right before bed but your blood glucose isn't low?

Dosing mealtime insulin before bed is tricky since it ups the very real risk of nighttime hypoglycemia. If you can avoid eating before bed, do it. "We're leery of giving short-acting insulin before bedtime," says Pruneda. Some people with type 2 diabetes who take only long-acting insulin incorporate a nighttime snack into their meal plan when injecting at night. The goal is to reduce any drop in blood glucose that may occur during the night.

However, people with type 2 who take both long-acting and rapid-acting insulin as well as those with type 1 diabetes need to dose for a snack with more than 15 grams of carbs. But keep in mind: "The nighttime snack is based on a different insulin-to-carb ratio than the rest of the day," says Clougherty, who notes that most people use less insulin at bedtime. So someone who took one unit of insulin to cover 15 grams of carbs during the day might take one unit to cover 20 grams at night.

Understanding Insulin
Knowing how long it takes your insulin to act, when it peaks, and how long it lasts can help you prepare for a dose and avoid "stacking" doses on top of one another. Below is a chart of how insulin works in the body.
Type of Insulin Time to Onset (when it starts working int he body) Peak (when it is at its highest strength) Duration (how long it lasts)
Lispro (Humalog), aspart (NovoLog), glulisine (Apidra)
5 to 15 min. 40 min. to 2 hours 3 to 5 1/2 hours
Humulin R, Novolin R
30 to 60 min. 2 to 5 hours 3 to 6 hours
Humulin N, Novolin N
2 to 4 hours 4 to 10 hours 10 to 16 hours
Glargine (Lantus), detemir (Levemir)
2 hours No real peak  
*These are considered mealtime insulins. **These are considered background insulins.

How do you use insulin when you just ate …

… but forgot to take your insulin beforehand?

Don't beat yourself up if you missed a dose. It happens, especially to people who are new to diabetes and haven't yet established a routine. "If you forgot to take it beforehand, then it would be best to take it at the end of the meal," says Aponick. If it took you an hour or less to realize you missed a dose, administer the dose that you would have taken prior to that meal based on the carb content of your meal and your blood glucose before you ate, if you checked it. Then keep an eye on your blood glucose.

Didn't realize you missed a dose until your next meal came around? Chances are, your premeal blood glucose reading will be higher than normal. That's OK. At the time you would normally take your insulin, dose for the meal plus a correction to cover the higher glucose level.

… but want to have dessert in an hour?

If you happen to test and your blood glucose is high, simply cover the carbs in the dessert—not your high blood glucose. "You wouldn't add on additional insulin using a correction factor or correction scale at that time, as it would be too soon since your last correction," says Aponick. "Correcting elevated blood glucoses too close together can lead to stacking, which increases the chance for hypoglycemia." But you still need to cover the carbs in all meals or snacks regardless of how long it's been since you last ate.

How do you use insulin when you just exercised …

… and your blood glucose is low but you're about to have a big lunch?

Any time your blood glucose is low (whether you've exercised or not), it's important to treat immediately—even if you're about to eat. "The fact that you are about to have a 'big' lunch might sway you to think that you don't really need to treat the low immediately," says Aponick. "However, that 'big' lunch is likely full of carbs and may also likely be full of fat, which can cause a delayed and prolonged rise in your blood glucose because foods that are high in fat content slow gastric emptying and absorption of carbohydrate."

After you treat the low and once your blood glucose has returned to normal, dose enough insulin for the carbs in your meal. (Don't count the carbs you just ate to treat hypoglycemia, however.) Aponick says it's safe to inject insulin before the meal as you normally would, but if the hypoglycemia has made you nervous, it's OK to treat when you're done eating. Just don't forget.

After you eat, keep a close eye on your blood glucose for the rest of the day. "Exercise makes your blood sugar go down for the next six hours, not just then," says Pruneda. "People get tired of those little finger sticks, but it's really a lifesaver."

… but your blood glucose is high and you plan to eat in an hour?

While exercise generally lowers blood glucose, sometimes it temporarily raises it, and intense spurts of exercise—say, an all-out sprint—can release hormones like adrenaline and cortisol that cause spikes. "It doesn't last long," says Clougherty. "We don't dose for a high immediately after exercise." The same is true if you're about to eat. And while it might seem smart to correct your high when you inject your mealtime dose, don't. That rise in blood glucose will soon subside—and the exercise may then cause it to drop—so correcting it with insulin may make you go too low.

How do you use insulin when you aren't eating …

… but your blood glucose is high?

If you only take background (long-acting) insulin, you won't do anything. Note each blood glucose result so you and your doctor can review the numbers. If your glucose levels are consistently high, you may need a higher background insulin dose.

Those who take rapid-acting insulin will use a correction dose to lower high blood glucose levels outside of mealtime. Your doctor or diabetes educator will help you determine the right "correction scale" (also called "sliding scale") so you'll know how many units of insulin to inject in order to bring your glucose down a certain number of points. "You have to be real careful about insulin stacking," says Wise. If you correct high blood glucose too soon after a previous correction, you could have too much insulin in the body, increasing the risk for hypoglycemia.

Finally, just because you have a correction scale doesn't mean your numbers will be perfect. "Just because we figured out what Mr. A needs for a normal day doesn't mean that's how they'll be," says Wise. "Things change in our lives." Keep checking your blood glucose because factors like stress and anxiety can affect your glucose levels.

… but your blood glucose is high and you want to exercise?

Exercise is a great way to lower your blood glucose naturally, but you want to make sure it's safe. If your blood glucose is at 250 mg/dl or above, especially if you have type 1 diabetes, test for ketones in your urine. If you have ketones, avoid exercise and give yourself a correction dose to lower your blood glucose. As long as ketones are negative, says Clougherty, you should be OK to exercise. Just don't inject insulin beforehand.

How do you use insulin when you are sick …

… and eating nothing but clear liquids?

Handling an illness is hard work. That's why you and your doctor should come up with a sick-day plan: a list of dos and don'ts for when you're under the weather. Blood glucose tends to be higher when you're sick, so you'll be advised to keep taking insulin—or even increase the dose—if you're not well.

But what if you're barely eating? Whatever you do, don't assume you can skip your dose. That can put you at risk for diabetic ketoacidosis (DKA), which can lead to coma or death. If you can't get carbs from food, sip on a sugary (not diet) soda. Test your blood glucose levels often, and use correction doses for high blood glucose. If your readings continue to be too high, or if you have lows, call your doctor.

… and vomiting?

The first step is to determine whether you're vomiting from a virus or whether you're experiencing DKA. Ketones can cause vomiting, so the first step is to do a test to check for them. If your blood glucose is high, you have ketones, and you're vomiting, inject insulin using your correction dose schedule. Keep testing and correcting with insulin until you're free of ketones. If you keep vomiting for two hours, call your doctor or seek emergency care.

Vomiting that's not caused by ketones may be from a virus (like the flu) or bacteria (as in food poisoning). In all instances, monitor your blood glucose and ketones often and stay hydrated to prevent DKA. Following your sick-day plan is the best thing you can do to stay safe when you're ill.



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