Glycemic Index vs. Glycemic Load: Why the Distinction Matters

By Kristin Neusel, MS, RD, LD, CDCES

These statements are entirely thoughts of the author and not claims made by the company. The Author was compensated to create this article.

Being a Registered Dietitian, I get tons of questions all the time! One of the most common questions I get from clients trying to manage their blood sugar is whether a food is “high glycemic” or “low glycemic”. The glycemic index eating plan has been popular for decades, and comes and goes in popularity. But one thing remains consistent about this eating plan: confusion! The glycemic index doesn’t tell the whole story, yet there are way more books on glycemic index than the other key piece of the puzzle: the glycemic load. To truly understand how a food will impact and affect your blood sugars, you need to look at the glycemic load too. How do you do that? Let’s get into it…

First, let’s chat about what the glycemic index exactly is. It’s a scale from 0 to 100 (1) that ranks how quickly a carbohydrate food raises blood sugar compared to pure glucose (which is ranked at 100). Foods that quickly break down and cause a rapid spike in your blood sugar, like candy for example, will have a high score on the GI scale. On the contrary, foods that digest more slowly and cause a gradual rise in your blood sugar will have a lower scale on the GI scale. It’s a useful concept, but there’s a catch that a lot of folks don’t realize…

So what’s the catch, exactly? PORTION SIZE! When they came up with the glycemic index, they didn’t take into account the portion size of the food. Portion size isn’t part of the equation unfortunately. GI is measured using a fixed 50 gram serving of available carbohydrate from a single food and it’s tested in isolation (2). What this means, exactly, is that the number assigned to a food doesn’t account for how much of it you’d realistically eat in one sitting. Furthermore, it doesn’t account for what else you’re eating alongside it. This is where things start to get confusing and misleading. I’ll share an example after I explain what exactly Glycemic Load (GL) is.

Glycemic Load was developed to fill in this gap left by the glycemic index. It combines the quality of a carbohydrate (read: its GI), with the quantity of carbohydrate in an actual serving (2). The formula looks like this if you really want to nerd out:

• GL = (GI x grams of carbohydrate per serving)/100. To sum up, GI tells you how fast a food raises blood sugar and GL tells you how fast and how much, which gives you a far more realistic picture of what happens after you eat.

Now let’s talk about an example so you can see what I’ve just taught you above. Let’s use watermelon for an example- it’s a classic example dietitians use to explain this, and you’ll see why! If you were only using the GI, you’d see watermelon has a GI of around 72-80, which on paper looks similar to a donut or traditional white bread (3,4). However, you wouldn’t likely sit down and eat 50 grams of watermelon! A typical one cup serving of watermelon only contains around 11 to 12 grams of available carbohydrate, since it’s mostly water (3). If you run that through the GL formula mentioned above, watermelon lands in the single digits, classifying it as a “Low-GL food”. This means that a normal serving has a mild real-world effect on blood sugar despite having a high GI ranking (3,4)! It’s a helpful reminder that GI only tells part of the story, and you shouldn’t judge a food by GI alone. Portion size matters for sure!

Something that lowers both GI & GL is: what else are you eating with that specific food? Certain characteristics of a food can help bring both the GI & GL number down, which is super useful info when you’re building your plate. Things like fiber, resistant starch, protein, and fat all slow the rate at which carbs are broken down & absorbed, which then subsequently blunts the speed and size of a blood sugar rise (5).

This is one of the many reasons I like to recommend Hero Bread to clients who are trying to be more mindful of their blood sugar without giving up bread entirely. Hero Bread baked goods are made with insoluble and soluble fiber and resistant wheat starch, which acts differently in the body than the refined starch found in traditional bread. Swapping a standard slice of bread for a fiber-and resistant starch-rich option like Hero Bread is a super easy way to support a more gradual post-meal glucose response and not have to give up your favorite foods that include bread!

So, which should you use- GI or GL? They aren’t in competition in my opinion. They work best together. Think of GI as a learning tool to help get you up to speed to use GL on the regular (if you want to use one regularly!). GL is more practical and a real-life measure of how a typical serving will actually affect you. For day to day food choices, GL is generally the more useful number but it is helpful to understand GI. As always, it’s individualized so chat with your Registered Dietitian about which approach makes the most sense for your goals and health history.

The bottom line:

A high GI doesn’t automatically mean a food is off limits (remember the watermelon example!). And a low GI doesn’t automatically mean a food is “safe”. Glycemic Index helps show the full picture and factors in portion sizes- super helpful! Making sure you choose carbs that are higher in things like fiber and resistant starch like Hero Bread is a super simple way to ‘have your cake and eat it too’!. What I mean by that is, enjoying bread and not having to cut it out of your eating plan because it’s made with healthier ingredients.

Want to learn more on this topic? We have an established Practitioner Program for Registered Dietitians, and conducted a webinar solely on this topic! Bonus? This is an evidence-based session awarding 1.0 CPEUs in accordance with the Commission on Dietetic Registration’s CPEU Prior Approval Program.

Click Here to Watch!

Related Posts:

• How Protein & Fiber Keep You Feeling Full
• Everything You Want to Know About Fiber, but Were Too Scared to Ask
• Bread, Fiber, & Blood Sugar: What Dietitians Want You to Know
• The Right Way to Increase Your Fiber Intake
• Myth Busters: Fiber

References:

1. Atkinson FS, Foster-Powell K, Brand-Miller JC. International tables of glycemic index and glycemic load values: 2008. Diabetes Care. 2008;31(12):2281-2283.
2. Linus Pauling Institute, Oregon State University. Glycemic Index and Glycemic Load. Micronutrient Information Center.
3. Harvard T.H. Chan School of Public Health / Harvard Health Publishing. The lowdown on glycemic index and glycemic load.
4. Foster-Powell K, Holt SH, Brand-Miller JC. International table of glycemic index and glycemic load values. Am J Clin Nutr. 2002;76(1):5-56.
5. Reynolds A, Mann J, Cummings J, et al. Carbohydrate quality and human health: A series of systematic reviews and meta-analyses. The Lancet. 2019;393(10170):434-445.