The Glycaemic Index (GI)

The Glycaemic Index (GI) — Why Some Carbs Hit Harder

Not all carbohydrates behave the same way. Some break down to glucose very easily and enter your bloodstream fast, sending glucose and insulin levels soaring. Others take longer to digest or produce different simple sugars, such as fructose, which follows a slower route through the body.

That difference is described by the Glycaemic Index, or GI.
Glucose itself is given a score of 100, and every other food is measured against it.

What you’ll learn

• What the glycaemic index measures
• Why slow-release carbs help with energy and appetite
• How fibre, protein, and fat slow things down
• Why even low-GI foods can add up if you eat a lot of them

Deep dive

What GI means

The Glycaemic Index (GI) measures how quickly a food raises blood sugar compared with pure glucose.
In studies, volunteers eat a portion of food that provides 50 g of available carbohydrate, and their blood-glucose rise is compared with the same amount of pure glucose eaten on another day.

High-GI foods are digested and absorbed quickly; low-GI foods take their time.

• High GI → Fast glucose release → Bigger insulin spike
• Low GI → Slow glucose release → Smaller insulin rise

Protein, fat, and fibre lower the GI of a meal because they slow digestion and glucose absorption.

Examples (rough guide):

What affects how fast glucose hits the blood

• Processing: finely milled or puréed foods digest faster.
• Cooking time: softer = faster; firmer or al dente = slower.
• Cooling: cooked starches that cool (like leftover potatoes or rice) form resistant starch, which slows digestion.
• Add protein or fat: both slow how quickly the stomach empties and how fast glucose is absorbed.
• Add fibre: vegetables, beans, and whole grains help even out the release.
• Acidity: vinegar or lemon juice can slightly flatten the curve.

These effects combine, so the GI of a meal can be much lower than that of a single food.
For example, eating fibrous vegetables such as cabbage or broccoli before potatoes slows digestion and lowers the overall post-meal glucose response.
(Several studies show that eating non-starchy vegetables or protein before starch can reduce blood-glucose rise by 20–30%.)

Why slow glucose helps

When glucose enters the blood slowly, insulin rises more gently.
This keeps energy levels steadier, hunger lower, and fat-burning easier between meals.
If you eat or drink sugar-rich foods, insulin rises sharply, then drops just as fast — leaving you tired and craving more.

Think of it like a ramp versus a rollercoaster.
The total amount of carbohydrate might be the same, but how fast it arrives makes all the difference.

But total calories still matter

Even low-GI foods contain energy.
A wholemeal loaf of bread or a mountain of brown rice will still deliver more calories than your body may need — it just does so more slowly.

Slow carbs aren’t free carbs.
If your goal is weight loss, portion size still counts, even when you choose the “better” version.

The aim isn’t to eliminate carbs — it’s to control the rate and the amount at which they reach your bloodstream.

Other simple carbohydrates

Soft drinks often contain the simple sugar fructose.
It has a low GI because it does not raise blood glucose directly — instead, the liver processes it first.
In small amounts (as in whole fruit) that’s fine, but large intakes from sweetened drinks can overload the liver.
Excess fructose is then converted into fat in the liver, which can later be stored in fat cells.

So while fructose takes a more scenic route than glucose, the destination can be the same if you consume too much.

Key point

• GI measures how fast carbs release energy, not how much.
• Slow and steady carbs help keep energy and mood stable.
• But portion size still rules: even healthy foods can overshoot your calorie needs.