Glycemic index (GI)

The glycemic index (GI) measures how quickly the carbohydrate in a food raises blood glucose compared with an equivalent amount of pure glucose. Foods are classified as high GI (70 and above), moderate GI (56–69), or low GI (55 or below) relative to glucose (which has a GI of 100). High-GI foods trigger rapid increases in blood sugar that decline quickly, while low-GI foods produce lower, more gradual rises.

The glycemic index (GI) measures how quickly the carbohydrate in a food raises blood glucose compared with an equivalent amount of pure glucose. Foods are classified as high GI (70 and above), moderate GI (56–69), or low GI (55 or below) relative to glucose (which has a GI of 100). High-GI foods trigger rapid increases in blood sugar that decline quickly, while low-GI foods produce lower, more gradual rises.

Explanation

GI values are determined by giving healthy volunteers a test food containing 50 g of carbohydrate and comparing the area under their blood glucose curve with the response after consuming the same amount of carbohydrate from pure glucose. Foods with carbohydrates that are easily digested and absorbed, such as refined starches or glucose, have high GI values; those with slowly digestible carbohydrates, such as most legumes, whole grains and fruits, have low GI values. High‑GI foods lead to sharp increases in blood glucose followed by a rapid fall and require a large insulin response. Low‑GI foods produce a slower rise in blood glucose and reduce the demand on the pancreas. Factors such as ripeness, degree of processing, cooking time and the presence of fiber, fat or protein can change a food’s GI. For athletes, the timing and type of carbohydrate matters: a study of trained cyclists found that eating a low‑GI meal (lentils) one hour before exercise allowed them to ride about 20 minutes longer than after a high‑GI meal (potatoes or glucose). The low‑GI meal provided a steady release of glucose without inducing hyperglycemia or a spike in insulin, which helped maintain blood glucose and free‑fatty‑acid levels during exercise.

Choosing carbohydrates by GI

High‑GI sugars like glucose and maltodextrin pass quickly through the digestive system, enter the bloodstream rapidly and stimulate glycogen resynthesis. Maltodextrin’s GI can exceed that of glucose, while fructose has a low GI of around 23 because it must be processed by the liver before it can be used by muscle. Sports drinks and gels often rely on high‑GI carbohydrates for quick energy during high‑intensity exercise, whereas low‑GI foods such as oats, beans, yogurt and many fruits are commonly eaten hours before an event to provide a steadier supply of glucose. Combining fast and slow carbohydrates takes advantage of different intestinal transporters, allowing athletes to absorb more total carbohydrate per hour. In everyday eating, pairing high‑GI items with fiber‑rich foods, and considering portion sizes, helps stabilize blood sugar.

A reliable understanding of the glycemic index allows athletes to match carbohydrate sources to the demands of training and competition. Low‑GI foods can support longer endurance activities by releasing glucose steadily, whereas high‑GI sugars are useful during and immediately after intense exercise when rapid fuel delivery is needed.

Related Terms: Glucose:fructose ratio, Carbohydrate per hour, Carbohydrate periodization, Gastric emptying, Intestinal absorption