Glycemic Load: what is it? when does it matter?
You may have heard the buzz out there about Glycemic Load in relation to weight loss and type 2 diabetes management. This article will give you a short and simple description of what glycemic load is… and what it isn’t: Is it the same as Glycemic Index? (No) Is there a practical application for the concept of Glycemic Load? (Well, that depends…)
However, there is a related concept which incorporates both of the above factors (carb content and glycemic index value); that concept is Glycemic Load (GL).
What is “Glycemic Load”?
The Glycemic Load (GL) of a given food refers to how much blood glucose will rise after eating a certain food, PLUS how long it will stay high. You can think of GL as referring to the degree of force, or “load” a given food puts on our bodies in terms of processing and dealing with glucose. From another angle, Glycemic Load can be thought of as the amount of carbs in a food, adjusted for its glycemic potency.
For example, from a table of Glycemic Load values, we find that a potato with 15 g of carbs has a GL of 12, while an apple with an equivalent 15 g of carbs has a GL of 6; the potato has twice the Glycemic Load as the apple. Though both foods have the same carbohydrate content in this case, the apple exerts less “force” (lower Glycemic Load) because of its lower Glycemic Index value.
The scales could be “tipped” in the other direction, however, if the portion of potato eaten was substantially smaller than the portion of apple eaten: one bite of potato will have a lower Glycemic Load than a whole apple, as the substantially larger carb content of the apple will “overpower” the high Glycemic Index of the potato.
In short, it makes intuitive sense that both of the dual concepts of glycemic index and carb content will affect blood glucose, and that the higher a meal is on either one of these components, the greater the effect on blood glucose. Here, we are simply expressing this interplay as “Glycemic Load”.
Big Loads, Small Loads
Like Glycemic Index values, Glycemic Load values can be categorized as Low, Medium and High. However, since Glycemic Load (GL) is related, but not identical, to Glycemic Index (GI), the numbers associated with GL categories are different from those associated with GI categories.
Glycemic Load categories:
Low Glycemic Load =10 or less
Medium Glycemic Load = 11 – 19
High Glycemic Load = 20 or higher
Returning to the example above, the potato with a Glycemic Load (GL) of 12 is considered a Medium Glycemic Load; the apple with a Glycemic Load (GL) of 6 is considered to have a Low Glycemic Load.
(Note: Sometimes Glycemic Load (GL) values are listed along with Glycemic Index (GI) in a table of GI values – if you are interested in exploring the specifics, we have found “The New Glucose Revolution” by Dr. Jennie Brand-Miller (primary author) to be very helpful.)
How is Glycemic Load Calculated?
When calculating the GL of a certain food, the Glycemic Index of a given food is multiplied by the Carbohydrate Content of a given serving size of that food, and the result is divided by the number one hundred. Mathematically, that formula looks like this:
Glycemic Load (GL) = Glycemic Index (GI) x Carb Content
Returning to the example above…
One medium apple (with skin) has 15g of available carbs, and a low GI value of 38, resulting in a Glycemic Load (GL) value of:
38 x 15
= (5.7 rounded to the nearest whole number) = 6
One small baked russet potato (with skin) that has 15g of available carbs, however, has a high GI value of 77, resulting in a Glycemic Load (GL) value of:
77 x 15
= (11.6 rounded to the nearest whole number) = 12
So two foods, both with the same carb content (15g) can have two very different Glycemic Load values.
Does Glycemic Load Matter?
Glycemic Load (GL) been a useful concept in scientific research, and, on a practical level, can be useful for individuals with type 2 diabetes (who do not use insulin), as they are more reliant on modifying the foods they eat in order to control post-meal blood glucose spikes.
However, for individuals with type 1 diabetes (who, therefore, take insulin), the concept of Glycemic Index (GI) is more helpful in a direct way in terms of matching food to insulin action, and thereby controlling post-meal blood glucose spikes. The concept of Glycemic Load is helpful indirectly in that taking it into account can help us understand when more or less insulin may be needed to deal with post-meal spikes; glycemic load is the reason that many people with T1D report that, compared to a regular-sized meal, when they eat the same meal as a very large portion (say, twice as large) they need to give relatively much more insulin (more than the twice as much insulin that would be expected for this doubled portion-size).
Similarly, you may find that if your child eats a moderate amount of a given food (let’s use 1 cup of pasta as an example), the usual insulin-to-carb ratio (I:C) seems to work just fine; however, if he is ravenous one evening and eats much more than the usual amount (let’s say 3 cups of pasta), it may be that the usual I:C doesn’t provide enough insulin to avoid a high blood sugar following the meal. Assuming that the I:C is set correctly, this phenomenon may be explained by Glycemic Load: the glycemic index in both portions remains the same (it’s the same pasta in both scenarios), but the carb content of the larger portion is significantly higher, resulting in a higher “load” on the digestive system. Thus, proportionally more insulin may be required for a very large portion than for a smaller portion. (This doesn’t happen with everyone with type 1 and may be more pronounced in small children - a valuable reminder of the complexities involved with diabetes: two plus two rarely equals four when you’re living with the diabetes dragon!)
In short, if we already understand that both glycemic index and carb content affect blood glucose, then it's only a small step to knowing that the higher a meal is on either one of these components (that is, the higher the glycemic load is) the greater the effect will be on blood glucose. Being aware of this fact can help us adjust our approach to insulin delivery and (hopefully!) reduce post-meal blood glucose spikes.
The above information was reviewed for content accuracy by clinical staff of the Alberta Children’s Hospital Diabetes Clinic.
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