What is Glycemic index?

What is Glycemic index?

The glycemic index is a ranking of how quickly and how much a carbohydrate-containing food raises your blood glucose compared with a reference food, usually pure glucose, which is fixed at 100. It was developed in 1981 by David Jenkins and colleagues at the University of Toronto to help people with diabetes choose carbohydrates by their real blood-sugar effect rather than by whether they were labelled simple or complex.

A food with a high GI, such as white bread near 75 or boiled potato near 78, is digested and absorbed fast, so glucose floods the bloodstream and the level spikes and then falls quickly. A food with a low GI, such as an apple near 36 or boiled chickpeas near 28, releases glucose slowly, producing a gentler, more sustained rise.

The number is a property of the food tested in isolation on an empty stomach, not of the whole meal you actually eat. Two things determine it: how fast the starch or sugar is broken into glucose, and how quickly that glucose crosses into the blood. Fibre, fat, protein, acidity, food structure, and how much a starch has been cooked or processed all shift the value.

The GI is useful because carbohydrates that look similar on a label can behave very differently in the body, and steadier blood glucose is linked with better appetite control, more stable energy, and, over years, lower risk of type 2 diabetes and heart disease. It is one input into food choice, not a complete verdict on a food, which is why it is best read alongside glycemic load and total diet quality.

How it works

The glycemic index is measured in people, not in a test tube. In a standard test, at least 10 healthy volunteers eat a portion of the test food containing 50 grams of available carbohydrate (total carbohydrate minus fibre) after an overnight fast. Researchers then draw blood at set intervals over the next two hours and plot the rise in blood glucose.

The area under that curve above the fasting baseline, called the incremental area under the curve or iAUC, is compared with the same person's response to 50 grams of the reference glucose drink tested on a separate day. Dividing the food's iAUC by the reference iAUC and multiplying by 100 gives the food's GI, and the values from all subjects are averaged.

The method is standardised internationally as ISO 26642. Physiologically, the number reflects the race between digestion and disposal. High-GI foods are broken down fast by amylase and gut enzymes, so glucose enters the blood in a sharp wave; the pancreas answers with a large, fast surge of insulin to move that glucose into muscle, liver, and fat cells.

Low-GI foods slow that entry, so glucose trickles in and the insulin response is smaller and flatter. Anything that slows gastric emptying or enzyme access lowers the value: soluble fibre thickens gut contents, fat and protein delay emptying, acid such as vinegar or sourdough slows starch breakdown, and intact food structure, like whole grains versus flour, keeps starch physically locked away.

Processing does the opposite. Milling wheat into fine flour, puffing rice, or overcooking pasta gelatinises and exposes the starch, driving GI up. This is also why the GI of a single food is not fixed to one decimal: ripeness, cooking time, variety, and the exact test population all move it, so published values are averages with a real margin of error.

The formula

GI = (iAUC of test food ÷ iAUC of 50 g glucose) × 100

Classification: Low = 55 or less, Medium = 56 to 69, High = 70 or more. Both foods supply 50 g of available carbohydrate and glucose is fixed at 100.

How to apply it

• Add soluble fibre: Soluble fibre from oats, beans, lentils, and fruit thickens the contents of your gut and slows how fast glucose reaches the blood. Swapping white bread for whole, intact grains or adding beans to a meal lowers its overall glycemic response measurably.
• Pair carbs with protein and fat: Protein and fat slow gastric emptying, so the same carbohydrate raises blood glucose more gently in a mixed meal. Eating rice with chicken and vegetables, or toast with eggs, blunts the spike you would get from the carbohydrate eaten alone.
• Use acidity: Acid slows starch digestion. Sourdough bread, or adding vinegar or lemon to a meal, lowers the glycemic response of the accompanying starch. This is one reason sourdough tends to sit lower on the scale than standard white bread made from the same flour.
• Keep food structure intact: The more a starch is milled, puffed, or overcooked, the higher its GI, because processing exposes the starch to enzymes. Choose whole or minimally processed grains, cook pasta al dente rather than soft, and eat whole fruit instead of juice.
• Mind ripeness and cooking: A green banana has a much lower GI than an over-ripe one because its starch has not yet turned to sugar. Cooling cooked potato or rice raises resistant starch and lowers the glycemic response versus eating it piping hot.
• Judge the portion with glycemic load: GI describes 50 g of carbohydrate, not your serving. Always cross-check a high-GI food against its glycemic load, which scales GI to the actual amount you eat. A small portion of a high-GI food can have a modest real-world impact.

Worked example

Glycemic index alone can mislead because it ignores portion size. Glycemic load fixes that by scaling GI to the grams of carbohydrate in a real serving, using GL = GI x available carbohydrate per serving / 100. Watch how watermelon and white rice swap places once portion is accounted for.

Watermelon has a high GI but so little carbohydrate per slice that its glycemic load is low, so it barely moves your blood sugar in a normal serving. White rice is high on both counts. This is why GI and glycemic load must be read together, never GI on its own.

Glycemic index vs glycemic load

Use them together. GI tells you how a carbohydrate behaves gram for gram; glycemic load tells you how much that matters for the amount you actually eat. A high-GI food in a small portion can still have a low glycemic load.

By goal

• Fat loss and appetite control: Favour lower-GI, higher-fibre carbohydrates such as oats, beans, lentils, and whole fruit, which slow glucose entry and tend to keep you fuller for longer. Steadier blood sugar means fewer sharp crashes that drive snacking, though total calories still decide fat loss.
• Endurance and training performance: Timing flips the usual advice. Lower-GI meals a few hours before training give sustained energy, while higher-GI carbs during and right after long, hard sessions refill muscle glycogen fastest. Here a rapid glucose rise is a feature, not a fault.
• Blood-sugar and diabetes management: Choosing lower-GI carbohydrates can reduce blood-glucose swings and is supported by diabetes guidelines as one useful tool. It works best combined with controlling total carbohydrate portions and overall diet quality. Anyone managing diabetes should individualise this with their clinician.

Common misconceptions

• "Low-GI foods are always the healthy choice." GI ranks blood-sugar speed, not nutrition. Ice cream and some chocolate sit in the low range because their fat slows digestion, while nutritious watermelon and boiled potato rank high. GI is one signal among many, and total diet quality matters far more than any single food's number.
• "A high-GI food will always spike your blood sugar." GI is measured for 50 g of carbohydrate eaten alone. In a real portion, or in a mixed meal with protein, fat, and fibre, the actual rise is often much smaller. Watermelon has a high GI but a low glycemic load, so a normal slice barely moves blood glucose.
• "Simple carbs are high-GI and complex carbs are low-GI." The simple-versus-complex split does not predict GI reliably. Fructose and lactose are simple sugars with low GI, while some complex starches like potato and white bread are high. GI was created precisely because that old classification failed to match real blood-sugar responses.
• "The glycemic index is a precise, fixed number for each food." Published GI values are averages with real variation. Ripeness, variety, cooking method, processing, and even the tested individuals shift the result, so the same food can span a range of 10 to 20 points. Treat GI as a broad band, not an exact score.

Related terms

Glycemic index FAQ

What is the glycemic index in simple terms?

The glycemic index is a 0 to 100 score that ranks how fast a carbohydrate food raises your blood sugar compared with pure glucose, which scores 100. High-GI foods spike blood sugar quickly, while low-GI foods release glucose slowly for a steadier rise.

What is a low, medium, and high glycemic index?

A food is low GI at 55 or less, medium GI from 56 to 69, and high GI at 70 or above. Low-GI foods like beans and apples raise blood sugar slowly. High-GI foods like white bread and potato raise it fast. Pure glucose is the reference at 100.

How is the glycemic index measured?

At least 10 people fast overnight, then eat a portion of the food containing 50 grams of available carbohydrate. Blood glucose is tracked for two hours and compared with the same people's response to glucose. The averaged ratio, times 100, is the food's GI, standardised as ISO 26642.

What is the difference between glycemic index and glycemic load?

Glycemic index rates a carbohydrate's blood-sugar speed for a fixed 50-gram amount. Glycemic load scales that to the carbohydrate in a real serving, using GI times serving carbs divided by 100. GI judges the food's quality; glycemic load judges the actual portion you eat.

Why is watermelon high GI but still considered fine?

Watermelon has a high GI near 76, but a normal slice contains very little carbohydrate, so its glycemic load is only about 5, which is low. Because portion carbohydrate is small, watermelon barely raises blood sugar in practice, showing why GI alone can mislead.

Does the glycemic index work for mixed meals?

Not reliably on its own. GI is measured for a single food eaten alone on an empty stomach. Adding protein, fat, fibre, or acid slows digestion, so a mixed meal usually produces a smaller blood-sugar rise than the GI of its carbohydrate would suggest.

Is a low-GI diet good for weight loss?

Lower-GI, higher-fibre carbohydrates can improve fullness and steady your energy, which may make eating less easier. Evidence for GI directly driving weight loss is mixed, and total calories still decide fat loss. Treat low GI as a helpful habit, not a magic rule.

What foods have the lowest glycemic index?

Legumes rank lowest, with peanuts near 13, kidney beans near 24, and chickpeas near 28. Most non-starchy vegetables, many whole fruits like apples and cherries, and dairy also sit low. High-fibre, minimally processed, and intact-grain foods tend to have the lowest values.

Does the glycemic index change with cooking or ripeness?

Yes. Riper fruit, longer cooking, and finer processing all raise GI by making starch and sugar easier to digest. A green banana ranks lower than a ripe one, and al dente pasta lower than soft. Cooling cooked potato or rice can also lower the response.

Does glycemic index affect insulin?

Yes. High-GI foods deliver glucose to the blood quickly, which triggers a large, fast insulin release to clear it. Low-GI foods release glucose slowly, so the insulin response is smaller and flatter. Steadier insulin is one reason low-GI eating is studied for metabolic health.

References

1. Jenkins DJ, et al. Glycemic index of foods: a physiological basis for carbohydrate exchange. Am J Clin Nutr, 1981. PubMed 6259925
2. Atkinson FS, Foster-Powell K, Brand-Miller JC. International tables of glycemic index and glycemic load values: 2008. Diabetes Care, 2008. PubMed 18835944
3. Vega-Lopez S, Venn BJ, Slavin JL. Relevance of the Glycemic Index and Glycemic Load for Body Weight, Diabetes, and Cardiovascular Disease. Nutrients, 2018;10(10):1361. PMC6213615
4. Glycemic Index and Glycemic Load. Linus Pauling Institute, Oregon State University
5. Glycemic index and glycemic load for 100+ foods. Harvard Health Publishing, Harvard Medical School
6. About Glycemic Index. University of Sydney GI Research Service
7. Glycaemic index (GI) and diabetes. Diabetes UK