The glycemic index (GI) is a scale from 0 to 100 that ranks carbohydrate-containing foods by how quickly they raise your blood sugar after eating. Pure glucose sits at the top with a score of 100, and every other food is measured against it. A low-GI food causes a slow, gradual rise in blood sugar, while a high-GI food causes a sharp spike.
How the Scale Works
Foods are grouped into three categories based on their GI score:
- Low GI: 55 or below
- Medium GI: 56 to 69
- High GI: 70 or above
Most legumes, pasta, fruits, and dairy products fall into the low-GI category. Breads, breakfast cereals, rice, and snack foods tend to land higher on the scale, though both high- and low-GI versions exist within each of those groups. Whole-grain versions of bread and cereal are not automatically low-GI, which surprises many people.
How GI Is Measured
To assign a GI value, researchers feed volunteers a portion of food containing 50 grams of available carbohydrate. They then draw blood samples at regular intervals over two hours: before eating and at 15, 30, 45, 60, 90, and 120 minutes after. The resulting blood sugar curve is compared to the curve produced by 50 grams of pure glucose eaten on a separate day. The ratio between the two curves, expressed as a percentage, becomes the food’s GI score.
For foods that are very low in carbohydrate density (think non-starchy vegetables), eating enough to reach 50 grams of carbs would mean consuming an unrealistically large portion. In those cases, researchers use a 25-gram carbohydrate load instead, with a matching 25-gram glucose reference.
Why GI Alone Doesn’t Tell the Full Story
The glycemic index measures carbohydrate quality but ignores quantity. Watermelon, for instance, has a high GI (around 72), yet a typical slice contains relatively little carbohydrate. That means eating watermelon doesn’t actually spike your blood sugar the way its GI number suggests.
This is why nutritionists developed a companion measure called glycemic load (GL). GL multiplies a food’s GI by the grams of carbohydrate in a realistic serving, then divides by 100. The result accounts for both the type and amount of carbohydrate you’re actually eating.
- Low GL: 10 or below
- Medium GL: 11 to 19
- High GL: 20 or above
A slice of watermelon has a high GI but a low GL. A large bowl of white pasta has a moderate GI but a high GL because of the sheer volume of carbohydrate. Using both numbers together gives you a much more accurate picture of how a meal will affect your blood sugar.
What Changes a Food’s GI
The same food can have a very different GI depending on how it’s grown, processed, and prepared. Several factors are at play.
Starch Structure
Starch comes in two main forms: amylose (long, straight chains) and amylopectin (branched chains). Foods higher in amylose are harder for your digestive enzymes to break apart, so they release glucose more slowly. One study on rice varieties found that switching from a low-amylose rice to a medium-high amylose variety dropped the GI from about 91 to 66. This is why basmati rice (naturally higher in amylose) consistently scores lower than sticky or short-grain rice.
Resistant starch takes this a step further. It passes through your small intestine without being digested at all, so it contributes zero blood sugar rise. Cooling cooked potatoes or rice overnight increases their resistant starch content, which measurably lowers the GI when you reheat them.
Cooking and Processing
The more a food is processed, the easier it is for your body to break down the starch, and the higher its GI climbs. Research on sweet potatoes illustrates this clearly: boiled sweet potato scored around 46 to 49, while baked and roasted versions jumped to the high 80s and low 90s. The same vegetable, prepared differently, can shift from low-GI to high-GI.
Milling grains into fine flour has a similar effect. A whole, intact grain of wheat forces your body to do more mechanical and chemical work during digestion. Pulverize that same grain into white flour, and the starch is exposed and rapidly absorbed.
Fiber, Fat, and Protein
Dietary fiber acts as a physical barrier in your gut, slowing the interaction between digestive enzymes and starch. This is one reason whole fruits (with their fiber intact) have lower GI values than fruit juices. Fat slows stomach emptying and can form complexes with starch molecules that resist digestion. Protein stimulates insulin release, helping keep blood sugar from rising too high. This is why a mixed meal almost always produces a lower blood sugar response than any single carbohydrate food eaten alone.
Natural Plant Compounds
Phenolic compounds and flavonoids, the same molecules that give berries, tea, and dark chocolate their color and slight bitterness, actively inhibit the enzymes that break starch into sugar. Meta-analyses have confirmed that foods naturally rich in these compounds tend to have meaningfully lower GI values. This partly explains why lentils and beans, which are loaded with both fiber and phenolics, are among the lowest-GI foods available.
Ripeness
As fruit ripens, resistant starch converts into simple sugars. A green banana has a substantially lower GI than a spotted, fully ripe one, even though the total carbohydrate content is similar. The sugars in ripe fruit are simply more accessible to your digestive system.
GI and Blood Sugar Management
For people with diabetes, choosing lower-GI foods is one strategy for smoothing out blood sugar swings. A meta-analysis of eight clinical trials involving 314 people with diabetes found that low-GI diets reduced HbA1c (a marker of average blood sugar over two to three months) by a statistically significant amount. Other reviews have estimated HbA1c reductions ranging from 0.15% to 0.5%, depending on how strictly participants followed the diet and how their meals were structured.
That said, the American Diabetes Association’s 2024 standards of care note that the evidence is mixed. Some systematic reviews found no significant impact on HbA1c, while others did. The inconsistency likely reflects differences in study design and how strictly “low-GI” was defined across trials. The ADA does not single out GI-based eating as superior to other approaches like carbohydrate counting or standard dietary education. In practice, GI is one useful tool among several for managing blood sugar, not a magic formula.
Your GI Response Is Personal
Published GI values are averages derived from small groups of test subjects, and individual responses vary considerably. A 2025 study in Nature Medicine confirmed that postprandial glucose responses (how your blood sugar behaves after a meal) depend on your gut microbiome, your ethnicity, your habitual diet, and the overall macronutrient composition of the meal. Specific gut bacteria were linked to higher or lower blood sugar responses to the same carbohydrate foods.
This means two people eating the same bowl of oatmeal can see meaningfully different blood sugar curves. GI tables are a useful starting point, but your own body’s response may not match the published number exactly. If you use a continuous glucose monitor, you can see this variability in real time and identify which foods work best for you personally.
Putting GI Into Practice
You don’t need to memorize GI tables to benefit from this concept. A few general patterns cover most of what matters. Swap refined grains for intact whole grains when you can. Pair carbohydrate-heavy foods with protein, fat, or fiber to blunt the blood sugar response. Choose legumes, most fruits, and dairy over white bread, sugary cereals, and processed snack foods. Cook pasta al dente rather than soft, since firmer textures slow starch digestion. And remember that portion size matters just as much as food choice, which is exactly the insight glycemic load provides.
GI is most useful as a comparison tool within food groups. Choosing one bread over another, one rice variety over another, or one breakfast cereal over another based on GI can make a real difference in your daily blood sugar patterns. Trying to compare across food groups (is an apple “better” than cheese?) is where GI breaks down, because the foods differ in too many other nutritional ways for a single number to capture.