What Is the Glycemic Index?
The Glycemic Index (GI) is a numerical scale from 0 to 100 that ranks carbohydrate-containing foods based on how quickly they raise blood glucose levels after eating, compared to pure glucose (rated at 100). It's a practical tool — not a perfect one — for understanding how different foods affect your blood sugar.
How the Scale Works
- Low GI (55 or below): Slower digestion, gradual blood sugar rise. Examples: lentils, oats, most fruits, non-starchy vegetables.
- Medium GI (56–69): Moderate impact. Examples: whole wheat bread, brown rice, sweet corn.
- High GI (70 and above): Rapid blood sugar spike. Examples: white bread, white rice, most breakfast cereals, sugary drinks.
For people with diabetes, prioritising low-to-medium GI foods can help reduce post-meal blood sugar spikes and improve overall glucose control.
The Glycemic Load: A More Complete Picture
GI alone doesn't tell the whole story. A food's Glycemic Load (GL) accounts for both the GI and the portion size — giving a more real-world picture of its impact.
For example, watermelon has a high GI (around 72), but its carbohydrate content per serving is low, making its glycemic load quite modest. So eating a moderate portion of watermelon won't spike blood sugar the way the GI number alone might suggest.
GL formula: GL = (GI × grams of carbohydrate per serving) ÷ 100
Practical Low-GI Food Swaps
| Instead of (High GI) | Try (Lower GI) |
|---|---|
| White bread | Dense wholegrain or sourdough bread |
| White rice | Basmati rice, quinoa, or cauliflower rice |
| Cornflakes | Rolled oats (porridge) |
| Baked potato | Boiled baby potatoes or sweet potato |
| Sports drinks | Water or unsweetened sparkling water |
Factors That Change a Food's GI
GI is not fixed in stone — several factors shift it up or down:
- Cooking method: Al dente pasta has a lower GI than overcooked pasta. Cooling cooked potatoes lowers their GI through resistant starch formation.
- Ripeness: Riper fruit generally has a higher GI (e.g., a very ripe banana vs. a slightly green one).
- Pairing foods: Eating carbohydrates alongside protein, healthy fat, or fibre slows digestion and lowers the overall glycemic response of the meal.
- Processing: Finely milled or heavily processed foods usually have higher GIs than their whole-food counterparts.
Limitations of the Glycemic Index
The GI is a useful guide, but it has real limitations:
- It doesn't account for individual variation — two people can have very different blood sugar responses to the same food.
- GI values are tested in isolation, but we rarely eat a single food alone.
- It says nothing about a food's overall nutritional value (a chocolate bar may have a moderate GI but is still high in saturated fat).
The most reliable approach is to monitor your own blood glucose before and after meals to understand your personal response to specific foods.
Getting Started
You don't need to memorise the GI of every food. Start with a few simple habits: choose whole grains over refined ones, add protein or fat to carbohydrate-rich meals, eat plenty of non-starchy vegetables, and limit sugary drinks. These changes alone can make a meaningful difference in day-to-day blood sugar management.