Glossary · Health

Glycaemic Index

A ranking of carbohydrates on a scale from 0 to 100 according to how much they raise blood sugar levels after eating.

Definition

What 'Glycaemic Index' means

The glycaemic index (GI) measures how quickly a carbohydrate-containing food raises blood glucose. Low-GI foods (≤55) cause a gradual rise, while high-GI foods (≥70) cause rapid spikes. The GI is useful for managing diabetes and weight, but does not account for portion size; glycaemic load adjusts for that.

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All glossary terms

In depth

The longer answer

The glycaemic index (GI) is a ranking system that classifies carbohydrate-containing foods based on how quickly they raise blood glucose levels after eating. Foods are scored on a scale from 0 to 100, with pure glucose serving as the reference point at 100. You encounter GI values on nutrition labels, in diet books, and in diabetes management guidelines, where low-GI foods are often recommended for more stable blood sugar control.

In practice, the GI is determined by feeding human volunteers a portion of food containing 50 grams of available carbohydrate and measuring their blood glucose response over two hours. Low-GI foods (55 or less) include legumes, most vegetables, and whole grains; high-GI foods (70 or above) include white bread, sugary drinks, and many processed snacks. However, the GI does not account for typical portion sizes—a concept addressed by the glycaemic load (GL), which multiplies GI by the grams of carbohydrate per serving.

For someone eating a plant-based diet, focusing on low-GI whole foods like lentils, oats, and non-starchy vegetables can help maintain steady energy levels and reduce cravings. Yet the GI has limitations: it varies with ripeness, cooking method, and food combinations. A balanced approach that prioritises whole, minimally processed foods over isolated GI numbers is more practical for long-term health, as nutrient density and fibre content also matter greatly.

At a glance

Quick facts

Quick facts — Glycaemic Index
Detail	What to know
Scale range	0 to 100, glucose = 100
Low GI threshold	55 or less
High GI threshold	70 or above
Common low-GI foods	Legumes, oats, apples, lentils

In practice

What this means for you

When shopping, look for whole grains like barley or quinoa rather than refined white rice or bread for lower GI choices.
Pair high-GI foods with protein, fat, or fibre to slow digestion and blunt blood sugar spikes.
Check the glycaemic load (GL) on apps or labels for a more realistic measure of a food's impact.
For stable energy, build meals around beans, lentils, and vegetables instead of relying solely on GI scores.

FAQ

Common questions

Is the glycaemic index useful for weight loss?

It can be a helpful guide, but it's not a magic bullet. Low-GI foods tend to be more filling and less processed, which may help with appetite control. However, calorie balance and overall diet quality matter more for sustainable weight loss than GI alone.

Do all carbohydrates have a glycaemic index?

Only foods that contain enough digestible carbohydrate to test can be assigned a GI. Pure fats and proteins, like oils or meat, have no GI because they don't raise blood glucose directly. That's why GI is most relevant for starchy foods, fruits, and sugary items.

Can cooking change a food's glycaemic index?

Yes, significantly. Overcooking pasta or rice breaks down starches and raises their GI. Similarly, ripening fruit increases sugar availability and GI. Al dente pasta and slightly firm vegetables generally have a lower GI than their well-cooked counterparts.