Glycemic Load vs Glycemic Index: Why GI Alone Can Be Misleading

Glycemic index was a breakthrough idea when David Jenkins and colleagues introduced it in 1981. Before that, nutrition advice treated all carbohydrates as basically equivalent in how they affected blood sugar. GI showed they weren’t. Brown rice and white bread behave differently. Lentils and cornflakes behave very differently.

But GI has a design flaw that took a while to appreciate. And that flaw created some absurd nutritional guidance.

The Problem with GI Alone

GI is measured by feeding people a portion of food containing exactly 50 grams of digestible carbohydrate, then measuring blood glucose for two hours and comparing the result to 50g of pure glucose. The GI score is this comparison, expressed as a percentage. Pure glucose is 100. White bread is around 70. Lentils are around 32.

The test uses a 50g carbohydrate dose. But real foods contain very different amounts of carbohydrate per gram of food, and real people eat different portion sizes. GI ignores both variables.

Watermelon illustrates this perfectly. Its GI is about 72, the same as white bread. That sounds alarming. But watermelon is about 91% water. A normal 120g serving contains only 6 grams of carbohydrate. White bread has roughly 13g of carbohydrate in a single slice.

To get 50g of carbohydrate from watermelon, you’d need to eat about a kilogram of it. That’s a five-pound watermelon slice for one GI measurement. Nobody does that.

What Glycemic Load Fixes

Glycemic load was introduced precisely to address this. The formula is straightforward.

GL = (GI x grams of carbohydrate per serving) / 100

For watermelon at a normal serving: (72 x 6) / 100 = 4.3. That’s a very low GL, in the same range as most non-starchy vegetables.

For white rice, a cup of cooked white rice contains about 45g of carbohydrate and has a GI around 72. GL = (72 x 45) / 100 = 32.4. That’s high.

Carrots were famously maligned for years because of a high raw GI, around 71 in older tables. But a serving of carrots contains only 6g of carbohydrate. GL around 5. The carrot panic was entirely a product of using GI in isolation (Foster-Powell et al., 2002, Am J Clin Nutr).

Comparing Real Foods by GL

A few foods illustrate why GL gives a more useful picture.

Cornflakes: GI 81, GL around 24 per cup. Genuinely high.

Lentils: GI 32, GL around 12 per half cup. Low-moderate.

Boiled new potatoes: GI around 62, GL around 12 per 150g serving. Moderate, not problematic.

Baked potato: GI around 85, GL around 26 for a medium potato. High.

Dates: GI around 42 (the fibrous structure slows absorption), but GL around 18 for just 3-4 dates due to high carbohydrate density. Deceptive.

Brown rice vs. white rice: Both have similar GI values (around 66-72). Brown rice has slightly more fiber, which slows digestion a bit, but the GL difference per serving is modest. The often-cited “white rice is terrible, brown rice is great” comparison is somewhat overstated from a glycemic standpoint.

The Evidence on Low-GL Diets

Research linking low-GL diets to health outcomes is promising but not definitive. Willett et al. (2002, Am J Clin Nutr) found associations between high dietary GL and type 2 diabetes risk in observational studies. The associations are consistent across many cohort studies but don’t prove causation.

Intervention trials have shown that low-GL diets improve glycemic control in people with type 2 diabetes and prediabetes. The effect on weight in healthy people is more modest and inconsistent across trials (Augustin et al., 2015, Nutr Metab Cardiovasc Dis).

Critically, what you pair carbohydrates with matters enormously and often dwarfs the GI or GL of the carb itself. Fat, protein, and fiber in the same meal all reduce the glycemic response. A plain white potato is high-GL. A potato eaten with butter, protein, and a side salad creates a much lower glycemic response than the isolated GL value suggests. Meal composition is at least as important as the GI or GL of individual foods.

Limitations of Both Measures

Both GI and GL have real limitations.

Individual responses to the same food vary considerably. The same white rice can cause very different blood glucose responses in different people, depending on gut microbiome, metabolic health, and even the time of day. Research by Zeevi et al. (Cell, 2015) showed that personal glucose responses are highly individual and poorly predicted by GI.

Both measures only apply to individual foods. Mixed meals are harder to predict.

Food preparation changes GI. Al dente pasta has a lower GI than overcooked pasta. Firm, underripe banana has a lower GI than ripe banana. Cooled cooked potatoes have a lower GI than hot potatoes because cooling converts some starch to resistant starch. GL tables can’t capture all this variation.

Neither measure accounts for fiber, vitamins, minerals, or other aspects of food quality. White bread and whole grain bread have similar GI values, but differ enormously in nutritional quality. Using GL as the only guide to diet quality is an error.

The best use for GL is as a tie-breaker and a check on portions, not as the primary framework for food choices.

This article is for educational purposes only. It’s not medical advice. Talk to your doctor or a registered dietitian before making significant changes to your diet.

The mechanics of how carbohydrates are broken down and absorbed are covered in carbohydrate metabolism. If you’re thinking about glycemic response in the context of insulin resistance specifically, insulin resistance science explains the underlying cellular mechanisms. The role of fiber type in slowing glucose absorption is covered in fiber types.