The glycemic index (GI) is a scale from 0 to 100 that ranks foods by how quickly they raise your blood sugar after eating. Foods scored 0 to 55 are low GI, 56 to 69 are medium GI, and 70 or above are high GI. Once you understand those three categories and a few nuances behind the numbers, you can use the GI to make more informed food choices.
What the Numbers Actually Mean
Every GI value is measured against a reference food, usually a pure glucose drink, which is set at 100. Researchers give volunteers a portion of a test food containing a specific amount of carbohydrate, then track how their blood sugar rises over the next two hours. That blood sugar curve is compared to the curve produced by the same amount of carbohydrate from glucose. The result is expressed as a percentage: a food with a GI of 73, like white rice, raises blood sugar about 73% as much as pure glucose does.
Some older GI tables use white bread as the reference food instead of glucose, which shifts all the numbers upward. If you’re comparing values from different sources and one food seems oddly high, check whether the table used glucose or white bread as its baseline. Most modern databases standardize to glucose.
The Three GI Categories
The simplest way to use the index is to sort foods into three buckets:
- Low GI (0 to 55): These foods raise blood sugar slowly and steadily. Examples include lentils (32), raw apples (36), chickpeas (28), rolled oat porridge (55), and white spaghetti (49).
- Medium GI (56 to 69): A moderate rise in blood sugar. Brown rice (68), couscous (65), sweet potato (63), and muesli (57) fall here.
- High GI (70 and above): These foods cause a rapid spike. White rice (73), boiled potatoes (78), cornflakes (81), instant mashed potatoes (87), and white bread (75) are all in this range.
Low-GI foods are generally the ones to favor if you’re trying to manage blood sugar, though context matters, as explained below.
GI Values for Common Foods
Having a mental map of where everyday foods land on the scale helps you read GI tables quickly. Here are some reference points from international GI databases:
Among grains, there’s a wide spread. Barley sits very low at 28, while rice crackers top out at 87. Whole wheat bread (74) is surprisingly close to white bread (75), a detail that catches many people off guard. Corn tortillas (46) and chapatti (52) score much lower than sliced bread of either type.
Fruits tend to cluster in the low-to-medium range. Raw apples average 36, oranges 43, bananas 51, and mango 51. Watermelon is an outlier at 76, though that number is misleading on its own (more on that shortly). Pineapple lands at 59. Dates, despite tasting very sweet, come in at just 42.
Legumes are consistently among the lowest-GI foods. Soya beans score 16, kidney beans 24, chickpeas 28, and lentils 32. Dairy is similarly low: full-fat milk at 39, skim milk at 37, and fruit yogurt at 41. Rice milk, however, jumps to 86.
Why the Same Food Can Have Different GI Values
If you’ve ever seen conflicting GI numbers for the same food, it’s not an error. Several factors shift a food’s GI considerably.
Cooking and processing are major ones. Instant oat porridge (79) scores much higher than rolled oat porridge (55) because the finer particles are digested faster. Instant mashed potatoes (87) spike blood sugar more than french fries (63), partly because frying adds fat, which slows digestion.
Ripeness matters too. Ripe fruits generally have low GIs ranging from about 13 to 36, but overripe fruits jump to 29 to 58. A ripe banana might score around 51, while a very ripe, brown-spotted sweet banana can climb to 58, crossing into medium-GI territory. As fruit ripens, fiber breaks down and sugar content increases, both of which push the GI upward.
Other factors that influence GI include the type of starch in the food (some starches resist digestion longer), the protein and fat content (both slow glucose absorption), acidity (adding vinegar or lemon can lower GI), fiber content, and even how long a food has been stored. This is why published GI values are averages, and your personal response may vary.
GI vs. Glycemic Load
The glycemic index has one major blind spot: it doesn’t account for portion size. GI is measured using a fixed amount of carbohydrate (usually 50 grams), but you rarely eat that exact amount in a real meal. This is where glycemic load (GL) fills in the gap.
The formula is straightforward: multiply the food’s GI by the grams of carbohydrate in your actual serving, then divide by 100. For example, watermelon has a high GI of 76, but a typical serving contains so little carbohydrate that its GL is only about 5, which is very low. You’d need to eat several cups of watermelon to get the blood sugar spike the GI number implies.
GL values are categorized similarly: low is 10 or below, medium is 11 to 19, and high is 20 or above. When reading GI tables, checking the GL column (if one is included) gives you a more realistic picture of what a normal serving will do to your blood sugar.
GI Labels on Packaged Foods
In some countries, particularly Australia, you may see a certified GI Symbol on food packaging. This is a certification trademark that requires the product to have a low GI verified through standardized laboratory testing. But the symbol means more than just low GI: foods must also meet strict criteria for calories, saturated fat, sodium, and in some cases fiber and calcium. The actual GI number appears in the nutrition information panel, so you can check it yourself rather than relying on the symbol alone.
In most other countries, GI labeling is not yet standard on food packages. You’ll typically need to look up values in published GI databases or apps rather than finding them on the label.
What GI Can and Can’t Tell You
The GI is useful for comparing carbohydrate-containing foods head to head, but it doesn’t measure overall nutritional quality. A chocolate bar scores 40 (low GI), which is lower than boiled potatoes at 78, but that doesn’t make chocolate the healthier choice. Fat and sugar content, vitamins, minerals, and fiber all matter independently of GI.
GI also only applies to foods that contain carbohydrates. Meat, fish, eggs, cheese, and oils don’t have a GI value because they contain little to no carbohydrate. And the total amount of carbohydrate you eat in a meal often matters more than the GI of any single food in it.
That said, consistently choosing lower-GI options within a food category can make a meaningful difference. In one clinical study of people with poorly controlled diabetes, switching to a low-glycemic diet for 10 weeks reduced HbA1c (a measure of average blood sugar over months) by 1.1 percentage points, bringing participants from poorly controlled levels down to a significantly better range. Fasting blood sugar dropped by about 17%, and participants also lost an average of 3.3 kilograms. These results came from changing food quality, not cutting calories drastically.
Practical Tips for Using GI Tables
When you look up a food in a GI table, start by checking which reference food the table uses (glucose or white bread) so you’re comparing apples to apples. Then note whether the value is for the raw or cooked version, since cooking often raises GI. Look for a GL column or calculate it yourself to account for your actual serving size.
Rather than memorizing numbers, it helps to learn a few low-GI swaps within the food groups you eat most. Barley or legumes instead of white rice. Rolled oats instead of instant. Whole fruit instead of juice. Corn tortillas instead of white bread. These swaps keep meals familiar while meaningfully shifting the glycemic profile of your plate.