Glycemic index
The glycemic index (GI) ranks carbohydrate-containing foods by the incremental area under the blood-glucose response curve after a standardized amount of available carbohydrate, expressed relative to a reference food such as glucose. Glucose is assigned a GI of 100. The index describes an average response under test conditions, not the exact response of every person or the nutritional quality of the food.
GI testing compares equal amounts of digestible carbohydrate, not equal serving sizes or equal calories. The glycemic load extends the idea by also considering how much available carbohydrate a usual portion contains. This is why a food can have a high GI but a modest effect at a small serving, or a lower GI but still supply a large carbohydrate load in a large serving.
Processing, particle size, ripeness, cooking, cooling, and the type of starch or sugar can change GI. Fiber, fat, protein, acidity, and other foods in a mixed meal can alter gastric emptying and the overall glucose response. Sleep, recent exercise, metabolic health, and person-to-person variation add further uncertainty. International experts therefore describe GI as a standardized measure with useful applications, while also emphasizing correct testing and context in their scientific consensus statement.
In training nutrition, GI is one possible tool rather than a universal rule. A rapidly digested carbohydrate may be convenient during or between closely spaced, demanding sessions. At ordinary meals, preference, gastrointestinal comfort, total carbohydrate, food variety, and the time available for digestion may matter more. A low-GI food is not automatically nutrient-dense, and a high-GI food is not automatically unsuitable.
GI should also not be confused with a person's blood sugar level or with insulin itself. It cannot diagnose impaired glucose regulation, predict one individual's continuous-glucose-monitor trace, or show how well a strength program is working. Use it to compare carbohydrate sources in a defined context, not as a stand-alone score for health or muscle growth.