Protocol

Glycemic Index vs Glycemic Load: What the Numbers Mean on Your Plate

Two foods can share a glycemic index and still land very differently on a plate. The difference is one multiplication, and it is worth doing once by hand.

Every guide to the glycemic index eventually says “glycemic load is more useful”, and then moves on without showing why. This page shows why with arithmetic you can check yourself. If you only remember one thing: glycemic index (GI) is a property of a carbohydrate food, glycemic load (GL) is a property of a portion of it. GI does not know how much you ate. GL does.

This is general information, not medical advice. If you have diabetes, or take medication that lowers glucose, take any change in how you eat to your clinical team first.

What glycemic index measures

The University of Sydney’s GI database (glycemicindex.com) describes GI as a ranking of carbohydrates on a scale from 1 to 100 according to how much they raise blood glucose after eating. It classifies a food as low GI at 55 or under and high GI at 70 or over. Foods that are digested and absorbed quickly sit at the high end; those that produce smaller swings in glucose and insulin sit at the low end.

The detail that matters for this page is how the ranking is built. GI is tested by feeding people a portion of the food that contains a fixed 50 g of available carbohydrate (carbohydrate that is digested and absorbed, so total carbohydrate minus fibre), then comparing the blood-glucose response with the response to a reference food. It is a per-50-grams-of-carbohydrate comparison, not a per-serving one. Nobody eats “50 g of carbohydrate of watermelon”; at the Sydney entry’s 8.8 g per 120 g that would be roughly 680 g of fruit, a pound and a half.

What glycemic load adds

GL corrects for that. The formula, as given by Oregon State University’s Linus Pauling Institute and by the Sydney database:

GL = GI x grams of available carbohydrate in the serving / 100

The result is a number for the serving. The same Linus Pauling Institute page sorts GL per serving into three bands: low is 10 or under, medium is 11 to 19, high is 20 or over. For GI the same page uses low at 55 or under, moderate at 56 to 69, and high at 70 or over.

The watermelon example, worked

Watermelon is the standard teaching case because it has a high-looking GI and very little carbohydrate in a normal portion. Here is the arithmetic from the Sydney database’s own entry for watermelon:

• GI: 50 (average of four varieties, 2021 tables)
• Serving: two thin slices, 120 g
• Available carbohydrate in that serving: 8.8 g
• GL = 50 x 8.8 / 100 = 4.4, which is low

Harvard Health uses the same food to make the same point with a different GI. In its article on the lowdown on glycemic index and glycemic load, watermelon “has a high glycemic index (80)” but “a serving of watermelon has so little carbohydrate that its glycemic load is only 5.” Harvard does not state the serving size in that example, so we have not tried to reproduce its 5 from the inputs.

The two sources disagree on the GI (50 versus 80) and agree on the conclusion. The GL is low either way. That is the practical lesson: a GI that looks alarming does not mean a portion that behaves alarmingly.

Same GI, different load

The Linus Pauling Institute publishes a table with GI, serving size, available carbohydrate and GL side by side. Four rows from it show the spread:

Check the first two: 76 x 11 / 100 = 8.4, and 76 x 23 / 100 = 17.5. The table rounds down to 8 and 17. Watermelon and the doughnut have an identical GI of 76 in this table, but the doughnut carries about twice the carbohydrate per serving, so its load is about twice as high. GI alone would have told you they behave the same. GL tells you they do not.

The table also shows the reverse case. Boiled carrots have a low GI and a very small load. The baked potato has a GI above 100 in this table. A medium potato has 30 g of carbohydrate, giving a GL of 33, well into the high band.

A plate is a sum, not a single food

Load is useful on a plate because it adds up. The GL of a meal is the sum of the GL of each item, which makes it easy to see where the bulk of a meal’s load is coming from. GI cannot be added that way; you cannot average the GIs of a potato and a carrot and get a meaningful number for the plate.

Even so, the Linus Pauling Institute is clear that the GI of a mixed meal calculated with the formula is “unlikely to accurately predict the postprandial glucose response”, because fat, protein, fibre and the way the food is combined all change how quickly carbohydrate is absorbed. The sum is a guide to where the load sits, not a forecast of your glucose curve.

Why the same food has different GI values

You will have noticed watermelon at 50, 76 and 80 in the sources above. That is normal. GI is measured in small groups of volunteers, on a specific variety, at a specific ripeness, prepared a specific way. Processing, cooking and ripeness all move the number, as does what a food is eaten with. Two credible sources disagreeing by 20 or 30 points on a fruit is a reminder to treat GI as a band (low, medium, high), not a decimal.

GL inherits that uncertainty, because it is built from GI. A low GL on a food whose published GI ranges widely is still a low GL if every published GI keeps it under 10, as with the small watermelon portion above. Where the published values straddle a boundary, treat the answer as “somewhere near the line”.

Where neither number is the right tool

GI and GL are averages. The Linus Pauling Institute notes that GI “does not take into account individuals’ characteristics like ethnicity, metabolic status, or eating habits.” Your own response to a doughnut or a bowl of rice may differ from the table’s, and the only way to see it is to measure it, with a glucose meter or continuous monitor, ideally with your clinical team.

Neither number tells you how many grams of carbohydrate to count. If you dose insulin against carbohydrate, the gram count is the working number and GL is background about the shape of the curve. Our pages on carb counting basics and net carbs versus total carbs cover the gram side, and the earlier clinical note on GL and GI covers how the two fit into insulin workflows. For the general picture of glucose control before a diabetes diagnosis, see prediabetes and glycemic control.

GI and GL also say nothing about a food’s other qualities, such as its fat, salt or nutrient content, so a low number is not by itself a verdict on whether a food is a good choice.

What the health agencies say

MedlinePlus, the National Institutes of Health’s patient-information service, defines GI as “a measure of how quickly a food can make your blood sugar (glucose) rise.” It notes that for many people with diabetes, carbohydrate counting helps limit carbohydrates to a healthy amount and may be sufficient, and that if you have trouble regulating your blood sugar or want tighter control you should talk with your health care provider about using the glycemic index as part of your action plan.

The American Diabetes Association’s carbohydrate-counting guidance does not use GI or GL as the method. It says that to work out how many carbohydrates you should be eating you should schedule an appointment with a registered dietitian nutritionist or a certified diabetes care and education specialist, and that you should discuss your intake and blood sugar patterns with your diabetes care team.

A five-minute way to use this

1. Pick a food you eat regularly and find its GI in the University of Sydney database at glycemicindex.com.
2. Find the grams of total carbohydrate and fibre on the label or in a nutrition database, and subtract to get available carbohydrate for your usual portion.
3. Multiply GI by that figure and divide by 100.
4. Compare the result with the 10 and 20 cut-offs.
5. If the answer surprises you, note it and bring the question, with your glucose readings if you have them, to your clinician or dietitian.

Sources

• University of Sydney, Glycemic Index Foundation: What is GI; Watermelon; Glycemic load and diabetes.
• Harvard Health Publishing: The lowdown on glycemic index and glycemic load.
• Linus Pauling Institute, Oregon State University: Glycemic index and glycemic load.
• MedlinePlus, US National Library of Medicine (NIH): Glycemic index and diabetes.
• American Diabetes Association: How to count carbs.