Glycemic Index: What It Measures and What It Doesn't

Glycemic index was developed in the early 1980s by Dr. David Jenkins and colleagues at the University of Toronto. The original goal was practical: give people with diabetes a better tool than vague “avoid sugar” guidance.

The idea was simple. Feed a person a food containing 50 grams of digestible carbohydrate. Measure blood glucose every 15 to 30 minutes for two hours. Compare that curve to the same test done with pure glucose (or white bread, depending on the reference standard being used). The ratio gives you the GI score.

Pure glucose = 100. Foods that raise blood sugar faster than glucose aren’t common, but they exist. Foods that raise it more slowly get lower numbers.

A low GI is generally below 55. Medium is 56 to 69. High is 70 and above.

How GI Is Actually Measured

The measurement process matters, because it reveals why GI numbers are more variable than most people realize.

Each test uses 10 or more healthy human subjects. They fast overnight. They eat the test food in the morning. Their blood glucose is tracked for two hours.

The variation between individuals is significant. The same person eating the same food on different days can get different results. And different labs studying the same food sometimes report GI values that differ by 20 points or more (Atkinson et al., 2008, Diabetes Care).

Ripeness changes things. A ripe banana has a higher GI than an unripe one because more starch has converted to sugar. Individual gut microbiome composition affects glucose response. A 2015 Weizmann Institute study found that two people eating identical foods could have completely opposite blood glucose responses, driven largely by differences in their gut bacteria (Zeevi et al., 2015, Cell).

This variability doesn’t make GI useless. But it does mean you should treat GI values as estimates, not precise measurements.

Glycemic Load: The More Useful Number

Here’s the problem with GI alone: it doesn’t account for how much of a food you actually eat.

Watermelon has a GI of around 72. That’s in the high range. But watermelon is 92% water. A typical serving of watermelon contains only about 11 grams of digestible carbohydrate. To hit the 50 grams of carbohydrate the GI test uses, you’d need to eat about 800 grams of watermelon.

Glycemic load (GL) fixes this. The formula is:

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

Watermelon’s GL per typical serving is around 8, which falls in the low range. The high GI number was almost irrelevant to real-world blood sugar impact.

This table shows why focusing on GI alone can be misleading. Brown rice’s moderate GI still produces a high glycemic load because we eat a lot of it in one sitting. White bread’s high GI produces a modest glycemic load per slice.

What Changes a Food’s GI in Practice

Beyond the lab measurement, a lot of real-world factors shift how a food actually affects blood sugar.

Fiber content: Fiber slows gastric emptying and blunts the rate of glucose absorption. This is one reason whole fruit has a lower GI than fruit juice, and whole grain bread has a lower GI than white bread. As explained in the fiber types guide, soluble fiber in particular forms a gel that slows digestion.

Fat and protein in the meal: Eating carbohydrates alongside fat and protein slows the overall digestion rate. A potato eaten plain behaves differently than a potato eaten as part of a meal with chicken and olive oil. This is one reason studying single foods in isolation is only partly useful.

Cooking method and cooling: Al dente pasta has a lower GI than soft-cooked pasta because less starch granule disruption means slower digestion. Cooling cooked rice or potatoes then reheating them increases resistant starch content and lowers GI. Resistant starch acts like soluble fiber in the gut.

Ripeness and processing: Highly processed foods with disrupted food structure tend to raise blood sugar faster. This is one reason a whole apple behaves differently from applesauce, and applesauce differently from apple juice, even when the carbohydrate content is similar.

What the Evidence Shows for Health Outcomes

Blood sugar management in diabetes: This is where GI evidence is strongest. Low-GI diets have moderate evidence for improving HbA1c (a marker of average blood glucose over three months) in people with type 2 diabetes. A systematic review in Diabetes Care found that low-GI diets produced a statistically significant but modest reduction in HbA1c compared to higher-GI diets (Ajala et al., 2013). Most major diabetes organizations include GI as a useful tool, not a rigid rule.

Weight management: Evidence here is more mixed. Some trials show benefits from low-GI diets for weight loss, possibly because low-GI foods tend to be more filling. But when studies control total calorie intake carefully, the GI difference often disappears. A large meta-analysis in the American Journal of Clinical Nutrition found that low-GI diets led to modest improvements in body weight, but the effect was small and heterogeneous (Thomas et al., 2007).

Cardiovascular risk: Some large prospective studies (following people over time) show associations between higher dietary glycemic load and cardiovascular disease risk, but this association weakens or disappears after adjusting for fiber intake, suggesting fiber may be the active variable, not GI per se.

Where the evidence gets murky: People with metabolic syndrome or insulin resistance may respond differently to glycemic load than metabolically healthy people. The Framingham Heart Study Offspring cohort data suggested that dietary glycemic load was more strongly associated with cardiovascular risk in people who were already insulin resistant (McKeown et al., 2004, Diabetes Care). Individual response matters here.

The Practical Reality

GI is a useful conceptual tool, but it’s easy to over-apply it. A few honest points:

The foods with the highest GI tend to be highly processed carbohydrates: white bread, crackers, breakfast cereals, sweet drinks. The foods with the lowest GI tend to be whole foods rich in fiber, fat, or protein: legumes, most fruits, non-starchy vegetables. If you just avoid ultra-processed foods and eat mostly whole foods, you’ve probably gotten most of the benefit of a low-GI diet without ever looking up a number.

Artificial sweeteners are sometimes positioned as a zero-GI alternative to sugar, which is true on the glycemic index. But there are separate questions about their effects on the gut microbiome and insulin signaling covered in the food additives coverage of artificial sweeteners.

The bioavailability guide adds useful context here: how a food is processed and paired with other nutrients affects not just blood sugar but how all its nutrients get absorbed. Another reason the whole meal matters more than any single number.

GI is a starting point for thinking about carbohydrates. It’s not a complete framework for diet quality. The most consistent finding across nutrition research isn’t that low-GI is best or low-carb is best or any particular food rating system is best. It’s that dietary patterns built around whole, minimally processed foods tend to produce better outcomes than those built around refined and processed ones.

That principle doesn’t require memorizing a single number.