What is Glycemic Index?
The Glycemic Index (GI) ranks carbohydrate-containing foods by how quickly they raise blood glucose after eating, on a scale where glucose = 100. It helps compare the blood-sugar impact of different carbs.
How GI is defined & measured
- Test subjects consume a portion of the test food containing 50 g of available carbohydrates (or a fixed portion in some protocols).
- Blood glucose is measured at intervals (e.g., 0, 15, 30, 45, 60, 90, 120 minutes) over 2 hours.
- Compute the area under the curve (AUC) for the blood glucose response.
- GI = (AUC of test food ÷ AUC of reference food) × 100.
GI categories
| GI value | Category | Practical effect |
|---|---|---|
| ≤ 55 | Low GI | Slow, gradual rise in blood sugar |
| 56–69 | Medium GI | Moderate rise |
| ≥ 70 | High GI | Rapid spike in blood sugar |
Factors that change a food's GI
- Type of carbohydrate: simple sugars (glucose) increase GI faster than complex carbs or fructose.
- Processing & cooking: more processed or softer foods → higher GI (e.g., instant oats vs steel-cut oats).
- Fiber content: soluble fiber (like β-glucan in oats) lowers GI.
- Fat & protein: slow gastric emptying → lower GI.
- Ripeness: riper fruit has higher GI.
- Acidity & pairing: vinegar, lemon, or combining carbs with protein/fat lowers GI response.
Common GI examples (approx.)
| Food | Approx. GI | Category |
|---|---|---|
| Glucose | 100 | Reference |
| White bread | 75 | High |
| White rice (short-grain) | 73 | High |
| Basmati rice | 50–58 | Low–Medium |
| Boiled potato | 78 | High |
| Whole wheat pasta | 45–50 | Low |
| Steel-cut oats | 42–55 | Low |
| Instant (flavoured) oats | 65–75 | Medium–High |
| Apple | 36 | Low |
| Watermelon | 72 | High (but low GL) |
| Lentils | 21–29 | Low |
Glycemic Load (GL) — why portion matters
GL adjusts GI by the actual carbohydrate amount in a typical serving.
Formula: GL = (GI × carbs per serving in grams) ÷ 100
| GL range | Interpretation |
|---|---|
| ≤ 10 | Low GL |
| 11–19 | Medium GL |
| ≥ 20 | High GL |
Example: Watermelon GI ≈ 72 but 1 cup has ≈ 6 g carbs → GL ≈ 4 (low).
Clinical relevance & use
- Diabetes: low-GI diets help reduce post-meal glucose spikes and may improve long-term control.
- Weight control: lower GI meals may increase satiety and reduce subsequent calorie intake.
- Cardiovascular health: low-GI diets can improve lipid profile and reduce CVD risk factors.
- Sports: high-GI carbs are useful immediately after intense exercise to quickly refill muscle glycogen.
Limitations & cautions
- GI is measured under controlled conditions — it varies between people and even on different days.
- GI ignores other nutrients (fiber, vitamins, minerals) — a low-GI cookie is not necessarily healthy.
- Portion size matters — use GL for real-world impact.
- Mixed meals (protein/fat/fiber) change GI — GI tables for single foods are guides, not exact predictions.
Quick practical tips
- Prefer whole, minimally processed carbs (whole grains, legumes, fruits).
- Combine carbs with protein, healthy fats, and fiber to reduce the glycemic response.
- Choose lower-GI grain options (e.g., steel-cut/rolled oats vs instant oats).
- Use GL when planning meals, especially for diabetes management.
FAQ
- Is GI the same for everyone?
- No — factors like insulin sensitivity, gut microbiome, food combinations, and physical activity change individual responses.
- Should diabetics only eat low-GI foods?
- Low-GI foods help, but overall diet quality, portion control, and medications/exercise are also critical. Discuss personalized plans with a clinician or dietitian.
- Are fruits with high GI bad?
- Not necessarily — many fruits have vitamins, fiber, and low carbohydrate per serving; check GL and total nutrition.