What is Glycemic Index (GI)?

GI compares how quickly and how much a food raises blood glucose versus a reference.

At a Glance

Glycemic Index (GI) compares how quickly and how much carbohydrate foods raise blood glucose. Using glucose (GI=100) or white bread as the reference, healthy participants eat a portion of a single food that provides 50 g available carbohydrate. The 2‑hour blood glucose AUC is measured and expressed as a percentage of the reference.

0 55 69 100 Apple ~36 Brown rice ~68 Glucose 100
GI bands: Low <55; Medium 55–69; High ≥70.
Low GI Medium GI High GI

How GI is Measured

  • Reference food: glucose (or white bread) set to GI=100.
  • Fixed carbs: test portion provides 50 g available carbohydrate.
  • Sampling window: multiple readings over 2 hours to get AUC.
  • Expression: (test AUC ÷ reference AUC) × 100.

Note: GI reflects quality (effect per unit carb), not quantity eaten.

Bands and Examples

Low
Apple (~36)
Medium
Brown rice (~68)
High
White bread (~75–85)

What Affects GI

  • Processing & particle size: finer grind and gelatinization raise GI.
  • Ripeness & variety: riper fruit often has higher GI; varieties differ.
  • Cooking method/time: cooking and cooling (retrogradation) change GI.
  • Nutrients: fiber, fat, and protein slow digestion and lower GI.
  • Individual response: people vary in absorption and insulin response.

Using GI Day to Day

  • Prefer low/medium: choose lower‑GI staples and snacks when possible.
  • Smart pairing: eat carbs with protein, healthy fats, and veggies.
  • Watch portions: same GI, different amounts can change impact.
  • Consider GL too: Glycemic Load reflects total impact per serving. See GI vs GL.

Limitations

  • Single foods: GI comes from single‑food tests; mixed meals differ.
  • Standardized setup: 50 g carb and the 2‑hour window don’t match all meals.
  • Study variation: labs and populations can yield slightly different values.
  • Not overall health: GI doesn’t capture full nutrition quality.

Takeaways

GI helps compare carbohydrate foods by their effect on blood glucose: Low (<55), Medium (55–69), High (≥70). Day to day, prefer lower GI, mind portions, pair with fiber/protein, and consider GL for total impact per serving.

Back to Articles