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 you eat them. A food with a GI of 75 will spike your blood sugar fast, while a food at 32 will release glucose slowly and steadily. The scale uses pure glucose (or sometimes white bread) as the reference point, set at 100.
How GI Is Measured
To determine a food’s GI, researchers give a group of healthy volunteers a portion of that food containing 50 grams of available carbohydrate. They then measure blood sugar levels at regular intervals over the next two hours and plot the rise and fall on a graph. The area under that curve is compared to the area produced by the same 50 grams of carbohydrate from pure glucose. The result is a single number representing how that food stacks up against the reference.
Foods fall into three categories based on their score:
- Low GI: 1 to 55
- Medium GI: 56 to 69
- High GI: 70 and higher
What Happens in Your Body
When you eat a high-GI food, the carbohydrates break down quickly during digestion and flood into your bloodstream. Your blood sugar rises sharply, and your pancreas responds by releasing a large burst of insulin to bring it back down. That rapid spike and crash can leave you feeling hungry again soon after eating, and over time, repeated spikes put stress on the systems that regulate blood sugar.
Low-GI foods work differently. They’re digested and absorbed slowly, so glucose trickles into your bloodstream rather than rushing in. The insulin response is smaller and more gradual. Blood sugar stays more stable, and energy levels tend to be steadier between meals.
GI Values for Common Foods
Some values from the International Tables of Glycemic Index give a sense of where familiar foods land. White bread scores 75, and boiled white rice is close behind at 73. Both are high-GI foods. Brown rice comes in at 68, placing it in the medium range. Watermelon scores 76, which surprises many people since it’s a fruit. Lentils sit at 32, firmly in the low-GI category.
These numbers show that “healthy” and “low GI” aren’t the same thing. Watermelon is nutritious, but its carbohydrates enter the bloodstream quickly. Lentils, on the other hand, release their energy slowly thanks to their fiber and protein content.
What Changes a Food’s GI
A food’s GI isn’t fixed. Several factors shift it up or down, sometimes dramatically.
Cooking and processing. Heat and moisture change the physical structure of starch, making it easier for digestive enzymes to break down. Boiled white rice has a higher GI than raw rice for exactly this reason. The more a food is processed, ground, or cooked, the faster your body can access the carbohydrates inside it. A whole grain kernel digests more slowly than flour made from the same grain.
Fiber content. Soluble fiber dissolves in water and forms a gel-like substance in your stomach that physically slows digestion. This gel makes it harder for enzymes to reach the starch, so glucose enters your bloodstream at a slower pace. Foods naturally rich in soluble fiber, like oats, beans, and lentils, tend to have lower GI scores.
Ripeness. This is especially visible in bananas. An unripe banana contains roughly 21 grams of starch per 100 grams of fruit and very little sugar. As the banana ripens, enzymes convert that starch into glucose, fructose, and sucrose. By the time the fruit is fully ripe, starch drops to about 1 gram per 100 grams, and the sugar content jumps by around 5 grams per 100 grams compared to the unripe fruit. A ripe banana will raise your blood sugar faster than a green one.
What you eat it with. Fat, protein, and acid (like vinegar or lemon juice) all slow stomach emptying, which lowers the overall glycemic response of a meal. A slice of white bread eaten alone behaves very differently in your body than the same bread eaten with avocado and eggs.
GI vs. Glycemic Load
One major limitation of the glycemic index is that it only measures speed, not quantity. It’s based on a standardized 50-gram carbohydrate portion, which doesn’t reflect how much of a food you’d actually eat. Watermelon scores 76, but you’d need to eat roughly 700 grams of it to consume 50 grams of carbohydrate. A typical serving is much smaller.
Glycemic load (GL) was created to fix this problem. It multiplies a food’s GI by the grams of carbohydrate in an actual serving, then divides by 100. This gives you a number that accounts for both how fast and how much a food raises blood sugar. A GL of 10 or under is considered low, 11 to 19 is medium, and 20 or above is high. Watermelon has a high GI but a low GL per serving, which makes GL more useful for real-world meal planning.
Health Benefits of Lower-GI Eating
A body of clinical research supports the benefits of favoring lower-GI foods, particularly for people with prediabetes or type 2 diabetes. Low-GI diets improve glycemic control, with the strongest effects seen in glycated hemoglobin (HbA1c, a measure of average blood sugar over two to three months), fasting insulin levels, and insulin resistance. Out of 13 randomized controlled trials assessing glycemic control, over half found that low-GI interventions produced significant improvements compared to standard diets.
The benefits extend beyond blood sugar. About half of the trials measuring body weight and composition found that low-GI eating led to greater reductions in weight, fat mass, and hip circumference. Several trials also found decreases in markers of inflammation, including C-reactive protein and interleukin-6, both of which are linked to cardiovascular risk. In one study, participants on a low-GI diet showed favorable shifts in gut bacteria, with increases in beneficial species like Bifidobacterium and Lactobacillus. Another trial found that people eating a low-GI diet reported higher quality of life, including better mental health scores.
These results are encouraging, but they come with a caveat: long-term adherence matters, and the benefits can diminish over time. One three-year trial found that HbA1c and weight improved during the first 9 to 15 months of a low-GI diet but gradually returned toward baseline by the study’s end.
Practical Limitations
The glycemic index is a useful tool, but it works best as a general guide rather than a precise instrument. Individual blood sugar responses to the same food vary significantly from person to person, influenced by genetics, gut bacteria, sleep, stress, and even the time of day. Two people eating the same bowl of rice can have meaningfully different blood sugar curves.
Most people also don’t eat single foods in isolation. A real meal contains a mix of carbohydrates, fats, and proteins that interact in ways that shift the glycemic response of the whole plate. The GI of individual ingredients becomes less predictive once they’re combined. Thinking in terms of overall dietary patterns, choosing whole grains over refined ones, pairing carbohydrates with protein or healthy fat, and eating plenty of fiber-rich vegetables, captures the spirit of low-GI eating without requiring you to memorize a table of numbers.