Glycaemic index of foods: what it is and why it matters when you count calories

Short answer. The glycaemic index (GI, spelled glycemic index in the US) ranks how quickly the carbs in a food raise blood sugar compared with pure glucose, which scores 100. Low is 55 and under, medium 56–69, high 70 and above. GI says nothing about calories: brown rice (GI around 50) and white rice (around 73) are within a few calories of each other per 100 g. If you already count calories, GI is a useful second filter — low-GI foods with fibre keep you full longer, which makes a deficit easier to hold without the late-afternoon crash.

What the glycaemic index actually measures

Take a portion of a food containing 50 g of carbs, measure how far blood sugar climbs over the next two hours, compare it with 50 g of glucose. That ratio, from 0 to 100, is the GI. The higher it is, the faster and sharper the rise, the bigger the insulin response, and often the sooner hunger comes back.

The bands:

• Low, 55 and under: most vegetables, beans and lentils, berries, dairy, whole grains.
• Medium, 56–69: instant oats, banana, basmati rice, wholemeal bread.
• High, 70 and above: white bread, mashed potato, white rice, sweets, sugary drinks.

Glycaemic load: why watermelon is not the enemy

GI is measured on a portion containing 50 g of carbs, and a real portion often contains far less. That is what glycaemic load (GL) fixes: GI × carbs per portion ÷ 100. Low GL is under 10, medium 11–19, high 20 and up.

Watermelon has a GI around 75, but a 150 g slice holds only 8–9 g of carbs, so its GL is about 6 — low. Raw carrots score 35–40 with 7 g of carbs per 100 g, so their load is negligible. A "high GI" on a fruit or a vegetable rarely means anything in practice. Look at the portion.

GI table for everyday foods

Averages from published tables; real values shift with variety, ripeness and cooking. Exact numbers per food are in the Fat Track food database, where GI sits next to calories, macros and fibre.

What changes the GI of the same food

• Processing. The finer the grind and the longer the cooking, the higher the GI: steel-cut oats sit below instant, pasta al dente below pasta boiled soft.
• Ripeness. A green banana is around 40; a spotted one is over 60.
• What it is eaten with. Protein, fat and fibre in the same meal flatten the response. Bread with cheese and vegetables moves blood sugar less than bread on its own.
• Cooling. Cooked and chilled potatoes, rice and pasta carry more resistant starch, so their GI drops.
• Acidity. Vinegar, lemon or a fermented dairy food in the meal nudges the GI down a little.

Why bother with GI if I already count calories

Calories decide whether your weight moves. GI decides how comfortable that process is:

1. Fullness. A low-GI meal with fibre holds you longer, so you are less likely to overshoot by evening.
2. Steady energy. Fewer swings. The dip an hour after a sugary breakfast is routinely mistaken for hunger.
3. Fewer raids on the biscuit tin. That sugar dip is one of the most reliable triggers for wanting dessert.
4. Health. In prediabetes, type 2 diabetes and insulin resistance, GI and GL are practical tools a doctor may actually prescribe.

Myths worth dropping

• "Low GI means eat as much as you like." No. Nuts and dark chocolate are low GI and extremely calorie dense.
• "High GI is always bad." No. Fast carbs after training help recovery, and watermelon barely touches blood sugar because its load is so low.
• "Fruit is off limits because of sugar." Whole fruit is low to medium GI with fibre attached. The problem is juice and large amounts of dried fruit.
• "GI matters more than calories for weight loss." Trials that match calories and protein find only small differences between low-GI and high-GI diets. GI is about comfort and health; calories are about the result.

How to use it in practice

Do not recalculate the GI of every plate. Three rules cover it: choose whole starches over purées and refined flour, put carbs on the plate alongside protein and vegetables, and eat sweet things after a meal rather than on an empty stomach. Keep logging calories and macros as usual — a photo or a line of text to the Fat Track bot — and glance at GI and fibre in the database when you are choosing between two options. A ready-made shopping list with macros, fibre and GI does the same job at the supermarket.

Common questions

Do protein and fat have a GI? No. GI is only measured for foods with carbohydrate. Meat, fish, eggs and oil are not tested.

Brown rice or white rice? Calories are almost identical. Brown wins on GI and fibre. Basmati cooked al dente is a good middle option.

Does a healthy person need to track GI? Not as a routine number. As an explanation for why you are hungry an hour after breakfast, yes.

Look up the GI of the foods you actually eat in the database, set your calorie target in the calculator, and you have enough to build meals you do not want to escape from.