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Learn How Food Affects You: Test Blood Sugar Twice

I usually recommend that my patients with diabetes test blood sugar in pairs. Two numbers are much more informative than just one. To learn how a meal affects your blood sugar level, measure it right before eating and two hours after. The difference between the two numbers is largely the effect of what you just ate.

Man recording number after testing blood sugar

Remembering to test blood glucose before eating and 2 hours after the first bite helps assess the effects of a meal. You may need to set a phone timer to help you remember to test blood sugar after eating.

A patient with diabetes told me that his blood sugar jumped to over 200 from a single lick of ice cream. I’m skeptical that a tiny bit of ice cream could make his blood sugar skyrocket. Of course, I don’t know that it did or didn’t. And neither does he because he measured his blood sugar only after tasting his son’s ice cream. I suspect that his blood sugar was already close to or above 200 mg/dl before he sampled the dessert.

Carbohydrate – whether it’s naturally-occurring sugar in milk or blueberries, added sugar in your coffee, or starch in brown rice or white rice – has the greatest impact on your blood sugar. Your body digests the carbohydrate-containing foods so the sugar (aka glucose) can enter your bloodstream. When you eat a little carbohydrate, your blood sugar will likely go up just a little. But if you eat a lot of carbohydrate, your blood sugar will rise more. How quickly glucose enters your bloodstream will depend on several factors, including the form of the food (like an apple versus apple juice), what else you ate such as foods with fat or protein, if you consumed alcohol with the food, and of course, your diabetes medications.

Need a refresher or an intro to diabetes and blood sugar? Take a look at Blood Sugar Basics.

Blueberries in a dish

Blueberries are health-boosting foods. They have about 15 grams of carbohydrate in a 3/4-cup serving.

When To Test Blood Sugar

It’s smart to work with a certified diabetes educator (CDE) to create a personalized blood glucose monitoring schedule. A CDE will help you develop a routine that helps to assess the adequacy of your medications, the safety and benefits of your exercise routine, and the effects of various foods, and even the same foods at different times or under different circumstances. Here are some opportunities to gain meaningful information from blood sugar checks.

  • Right before and 2 hours after a meal
  • Around 2 a.m. or 3 a.m., especially if you take insulin, suspect that you have nighttime hypoglycemia (low blood sugar), or if you have unexplained hyperglycemia (high blood sugar) first thing in the morning
  • When you have symptoms of hypoglycemia (measure your blood sugar frequently if have trouble recognizing the symptoms of hypoglycemia)
  • Before, during and after exercise
  • When you’ve consumed more alcohol than usual (alcohol can cause hypoglycemia)

Additionally, measure your blood sugar more frequently than usual

  • when you change diabetes medications, dosages of your diabetes medication, or change any medication that can affect blood sugar levels such as prednisone.
  • when you change your usual diet or exercise routine.
  • if you’ve gained or lost weight.

Sample Blood Sugar Testing Schedule for Meal Effects

Here is a common monitoring schedule to learn about the effects of food. Be sure to keep a record of your food intake too. Write down what you eat, especially the amount of carbohydrate. For carb counting tips, check out How to Count Carbs for Diabetes Management and 7 Tips and Tricks to Carb Count Like a Pro.

Fasting blood sugar checks, by the way, are meaningful, but they tell us very little about the effects of food. Instead they are important to monitor the effectiveness of your medications and to tell us if your liver is cranking out too much blood sugar during the early morning.

By using this sample schedule, you’ll gather data for each meal two to three times each week. You should work with a CDE and/or a registered dietitian nutritionist (RDN) to interpret the numbers and make changes to your meals.

 Fasting2 Hours After BreakfastBefore Lunch2 Hours After LunchBefore Dinner2 Hours After Dinner
Sunday
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday

Many people prefer a slightly different way. Instead of measuring the pre-meal and post-meal blood sugar levels of a different meal each day, they focus on a single meal for a week. For example, they may measure pre-breakfast and post-breakfast blood sugar levels every day for a week, share that information with their CDE or RDN, make changes as necessary, and move on to lunch and then to dinner. There is no ideal way. Choose the schedule that suits you.

You may also be wondering what your blood sugar targets are. This is also individualized. Some people will benefit from lower numbers than others. This depends on many factors, including age and health status. Please check in with your healthcare provider for guidance. Just as an FYI, the American Diabetes Association has these general target goals, but they recommend that you discuss your targets with a member of your diabetes team.

Fasting and pre-meal: 80 – 130 mg/dl

Peak post-meal: < 180 mg/dl

Measuring blood sugar levels before and after meals is quite enlightening. Look for future posts in which I’ll suggest other opportunities to measure in pairs. Let me know what you learn about various foods and meals after monitoring your blood sugar like this.

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Jill Weisenberger

Jill Weisenberger, MS, RDN, CDE, CHWC, FAND, is a Nutrition, Culinary & Diabetes Expert, Wellcoach®-certified health and wellness coach, Freelance Writer, Registered Dietitian Nutritionist and Certified Diabetes Educator. She's also the author of four books, including a best-seller. She's a nationally-recognized media expert in high demand for print and online interviews, as well as corporate and one-on-one nutritional counseling. Jill's philosophy is that nutrition science should be understandable, realistic and oh so delicious.

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