Bolus Timing & Glycemic Index

Bolus Timing when Considering Glycemic Index

Are all carbs created equal? For example, does 30 grams of carb from a serving of tortilla chips have the same effect as 30 grams of carb from a glass of prune juice or 30 grams of carbs from wild rice? 

The answer is no. There are different kinds of carbs: some carbs are digested quickly, some slowly, and some are not digested at all! The rate at which the carbs are digested alters their effect on blood sugar. Carbs that are digested more slowly raise blood sugar over a longer period of time. This leads to less extreme after-meal blood glucose levels than carbs that are digested quickly. Slowly digested carbs also tend to contribute less to weight gain while leaving you feeling full longer.

To place a number on how quickly various foods are moved into the bloodstream after digestion, a measurement called the “glycemic index” (GI) is used. The GI of a food rates how much a person’s blood sugar will increase over 2 hours compared to a reference, which is usually pure glucose. On the glycemic index scale, foods rank between 0 and 100 (pure glucose has a glycemic index of 100). Foods are labeled as low-GI when they have a GI value of less than 55.

Foods with low GI (below 55) tend to produce a slow, gradual blood sugar rise. The blood sugar peak is usually modest and may take several hours to appear.  Examples include whole grain pasta, whole fat yogurt, beans, chocolate. The same slow, gradual BG rise occurs when eating meals or snacks over an extended period of time (like a Thanksgiving dinner), when you have very large portions (typically meals >60g of carb) or when meals are exceptionally high in fat (pizza, Alfredo sauce, etc.). In these instances, bolusing before the meal tends to cause a low BG after the meal, followed by a rise in BG a few hours later as the bolus wears off and food starts to take its effect. Try to use a combination or dual-wave bolus with 25–50% of the bolus delivered immediately and the rest delivered gradually over 2 hours.

Foods with moderate GI (between 56 and 69) digest a bit faster than low GI, resulting in a more modest BG peak about 60–90 minutes after eating.  Examples include Quick oats, brown rice and popcorn. Bolusing too soon before eating these types of foods could produce a low BG soon after eating. Try to bolus about 15 minutes prior to moderate GI foods.

Foods with high GI (greater than 70) tend to raise BG fastest, with a significant rise occurring about 30–40 minutes after the meal. Examples include bread, pretzels, crackers, potatoes, breakfast cereal, white rice and sugary candy like jelly beans.  For these types of foods, you should bolus 20–30 minutes before taking your first bite of food. Doing so will allow the insulin peak to coincide with the peak in BG. Bolusing for high GI foods just before, during or after eating is not ideal as the food will raise the BG long before the insulin kicks in—this usually causes a significant after meal spike in BG followed by a marked drop a bit later.

Bolus Timing Based on GI and Pre-meal BG Value

These recommendations provide a place to start and it can be helpful to establish a list of foods you prefer, where they fall in the glycemic index scale and then how your BG responds to the food and the type of bolus you deliver.

It’s great if you have, and can use, a continuous glucose monitor (CGM) to follow the post meal/bolus trend—it will provide good information to show exactly when the rise in BG started after the first bite of food and when the bolus was delivered.

An insulin pump can be very helpful for bolusing appropriately and allowing a bit of extended delivery based on what you are eating. Using technology to your advantage with foods that are common in your meals can help you navigate better when you are eating foods out of the norm.

Reference

©Gary Scheiner MS, CDCES – Integrated Diabetes Services. May be reproduced and used for patient education, but not sold.