What is the glycemic index, and why isn’t it the whole story on carbohydrates?
The Short Answer
The glycemic index (GI) measures how quickly a food raises blood sugar on a scale of 0-100, with pure glucose as the reference at 100. Low-GI foods (under 55) raise blood sugar slowly; high-GI foods (over 70) spike it rapidly. But GI has limitations: it doesn’t account for portion size, ignores the insulin response, varies based on food preparation and combinations, and doesn’t capture the full metabolic picture. It’s a useful tool, but not the only tool.
Something to Sit With
The glycemic index offers one lens on carbohydrates — how fast they hit your bloodstream. But a food that scores well on GI can still be problematic, and a food that scores poorly might be fine in context.
Numbers are useful. They’re just not the whole story.
Learn More
How the glycemic index works. The GI was built in 1981 by David Jenkins and colleagues, who tested 62 common foods on groups of five to ten healthy fasting volunteers, measuring the two-hour blood-glucose area under the curve as a percentage of the equivalent glucose load.[1]
Foods are now commonly categorized as:
- Low GI: 55 or less
- Medium GI: 56-69
- High GI: 70 or more
Those cutoffs are a later convention layered on top of Jenkins’s method, not part of the original study.
The logic is straightforward: high-GI foods spike blood sugar rapidly, triggering a large insulin response that can lead to a subsequent crash, increased hunger, and metabolic stress over time. Low-GI foods produce a gentler, more sustained blood sugar curve.
Limitation 1: Portion size. The GI is based on 50 grams of carbohydrate, not a realistic serving. Watermelon has a high GI, but a typical serving contains relatively few carbs — you’d need to eat a lot to get 50 grams. Conversely, a low-GI food eaten in large quantities can still produce a significant blood sugar load.
The glycemic load (GL), also a later addition, attempts to address this by multiplying GI by carbohydrate content per serving. A food with high GI but low carb content per serving can have a low GL.
Limitation 2: Insulin response. Blood sugar and insulin don’t always move together. Some foods — particularly dairy products — produce an insulin response higher than their blood sugar impact would predict. The insulin index, developed by researchers at the University of Sydney, measures insulin response directly.[2] A food can have a moderate GI but a high insulin index.
Since insulin is the key hormone for fat storage and metabolic health, focusing only on blood sugar misses part of the picture.
Limitation 3: Context matters. The GI of a food changes based on:
How it’s prepared. Cooking generally increases GI. Al dente pasta has a lower GI than overcooked pasta. A whole apple has a lower GI than applesauce, which has a lower GI than apple juice.
What you eat it with. Fat and protein slow digestion and lower the effective GI of carbohydrates eaten alongside them. Fiber is the exception people expect least: in Jenkins’s data it did not predict glycemic response at all (r = −0.220, not significant), and wholemeal bread at 72% scored no lower than white at 69%.[1] What did predict the response was fat and protein.[1] A potato eaten alone has a higher GI than the same potato eaten with butter and steak.
Individual variation. People respond differently to the same food. A 2015 study in Cell by Zeevi and colleagues found enormous individual variation in blood sugar responses to identical foods, driven by factors including gut microbiome, genetics, and metabolic health.[3]
Limitation 4: GI isn’t everything. A food can be low-GI and still problematic. Fructose scored only 20 in Jenkins’s original set — which is why table sugar came in at a moderate 59 despite being a “sugar”[1] — but excessive fructose is metabolically harmful, promoting fatty liver and insulin resistance. Ice cream has a moderate GI due to its fat content, but that doesn’t make it health food.
Conversely, some high-GI foods — like potatoes — are whole foods with nutritional value that have been eaten by healthy populations for centuries.
How to use GI wisely. The glycemic index is most useful as one input among many:
Prefer whole foods over processed versions. The nutrition is better whatever the GI does — and as the bread comparison shows, the GI does not always follow.
Combine carbohydrates with protein, fat, and fiber. This naturally moderates blood sugar response.
Pay attention to your own response. Your reaction to specific foods matters more than published GI values.
Don’t let GI override common sense. A low-GI candy bar is still a candy bar. A high-GI carrot is still a carrot.
Further reading:
- Jenkins DJ et al., “Glycemic index of foods: a physiological basis for carbohydrate exchange”[4] — American Journal of Clinical Nutrition, 1981
- Brand-Miller JC et al., “Glycemic index and obesity”[5] — American Journal of Clinical Nutrition, 2002
- Zeevi D et al., “Personalized nutrition by prediction of glycemic responses”[6] — Cell, 2015
- The Obesity Code by Jason Fung, MD[7] — Chapters on insulin and blood sugar
Written by AI, directed and approved by me. How this is made →