Glycemic Load vs Glycemic Index: Which Number Actually Matters

Watermelon has a glycemic index of 76. White bread has a glycemic index of 75. Almost identical. So they should hit your blood sugar about the same, right?

Wrong. A slice of white bread will spike your blood glucose dramatically. A cup of cubed watermelon — barely. The difference comes down to a number most people have never heard of: glycemic load.

Glycemic load is the more useful metric for almost any practical question about food and blood sugar. Here’s why, and how to use it.

What is the glycemic index?

The glycemic index (GI) is a number between 0 and 100 that measures how quickly a food raises your blood sugar after you eat it, compared to pure glucose (which has a GI of 100). Foods are tested in standardized lab conditions: subjects eat 50g of available carbohydrate from the food, then their blood glucose is measured every 15 minutes for two hours.

The classification is straightforward:

• Low GI: 55 or less
• Medium GI: 56 to 69
• High GI: 70 or higher

This sounds useful, and for some purposes it is. But it has a critical flaw.

The problem with the glycemic index

The GI test always uses 50g of carbohydrate from the food being measured. Not 50g of food — 50g of pure carbohydrate. For some foods, that’s a reasonable serving. For others, it’s absurd.

Take watermelon. To consume 50g of carbohydrate from watermelon, you’d need to eat about 5 cups in a single sitting. Nobody does that. A normal serving is one cup, which contains about 11g of carbohydrate.

Compare to white bread. A single slice contains about 14g of carbohydrate, so you only need to eat a few slices to hit 50g. That’s a normal-sized lunch.

So when GI tells you watermelon and white bread are roughly equivalent, what it’s really saying is: “If you ate a giant industrial portion of each, they’d raise your blood sugar at similar speeds.” That’s not a question anyone is actually asking.

What is the glycemic load?

Glycemic load (GL) corrects this by accounting for the actual amount of carbohydrate in a normal serving. The formula is simple:

GL = (GI × Carbs per serving) ÷ 100

Let’s run the example:

• Watermelon — GI 76, 11g carbs per cup. GL = (76 × 11) ÷ 100 = 8.4
• White bread — GI 75, 14g carbs per slice. GL = (75 × 14) ÷ 100 = 10.5 per slice. A two-slice sandwich is GL 21.

Suddenly the picture is very different. Watermelon at GL 8 has a modest impact. A white bread sandwich at GL 21 hits hard. The GL classification:

• Low GL: 10 or less per serving
• Medium GL: 11 to 19
• High GL: 20 or higher

Why this matters for daily decisions

If you’re trying to manage blood sugar — whether you’re diabetic, prediabetic, doing keto, or just trying to avoid the afternoon energy crash — glycemic load is the number that maps to what you’ll actually feel and measure.

Some practical examples of how GL changes the way foods rank:

Notice the bagel. A single bagel hits your bloodstream like a cup of rice. And notice what happens with low-carb breads — when there’s barely any digestible carbohydrate to begin with, GL stays remarkably low even when GI of the underlying flour isn’t unusually low.

How to use glycemic load in real life

You don’t have to memorize tables. The principles are simple:

1. Pair carbs with protein, fat, and fiber. All three slow absorption and reduce the effective GL of a meal.
2. Mind portions. A small portion of a high-GL food can be a low-GL meal. A large portion of a “low-GI” food can be a high-GL meal.
3. Be aware of liquid carbs. Juice, soda, and sweetened drinks have very high GL because the carbohydrate hits your system unimpeded.
4. Cooking and ripeness matter. Overcooked pasta, ripe bananas, and instant oats all have higher GL than their al dente, less ripe, or steel-cut counterparts.

Glycemic load and keto

On a strict ketogenic diet, glycemic load becomes nearly redundant — if you’re staying under 20g net carbs per day, you’re almost certainly staying under a daily GL of 25 across all your meals combined, which is excellent. The whole point of keto is to keep blood glucose flat, and GL drops naturally when carbs do.

Where GL becomes interesting again is in low-carb plans that aren’t strictly keto — for instance, the Mediterranean-style low-carb approach, or maintenance after weight loss. Here, GL helps you choose between a small portion of berries (low GL, fine) and a large portion of dates (very high GL, rough on glucose) when you’re loosening the reins a bit.

The bottom line

Glycemic index is a chemistry measurement. Glycemic load is a meal-planning tool. They’re related but they answer different questions. If you want to know what a food actually does to your blood sugar in a normal serving, look up its glycemic load. If you can’t find it, calculate it: multiply GI by carbs per serving and divide by 100.

And if you’d rather not calculate at all, the TrueCarbs app shows the GL score for every product it scans, so you can compare two breads, two protein bars, or two breakfast cereals at a glance.

The canonical reference is the 2008 international tables of glycemic index and glycemic load published in Diabetes Care, which is still the dataset most clinicians and researchers cite today.

Frequently asked questions

What’s a good daily glycemic load total?

Most clinicians recommend keeping daily GL under 100 for blood sugar management, and under 80 for active diabetes management. Strict keto dieters typically come in well under 30 just by virtue of how few carbs they eat.

Is glycemic load the same as net carbs?

No. Net carbs measure quantity of digestible carbohydrate. GL multiplies that quantity by how fast it hits your system. They’re complementary numbers.

Where can I find GL values for different foods?

The University of Sydney maintains the most authoritative database of tested foods. The TrueCarbs app pulls calculated GL values for everything you scan.

Does cooking method affect glycemic load?

Yes — significantly. Al dente pasta, parboiled rice, and slightly underripe bananas have lower GL than their fully cooked, soft, or fully ripe counterparts. The texture you can feel maps roughly to the glucose response you’ll get.

Can a low-GI food still be a problem?

Yes, in large enough quantities. A whole sleeve of “low-GI” cookies will still spike your blood sugar. Quantity always wins eventually.

Related reading: What Are Net Carbs? · Low-Carb Diet for Beginners · Hidden Sugars on Food Labels