The Glycaemic Index: Does It Actually Matter for Everyday Eating?

What the Glycaemic Index Actually Measures

The glycaemic index was developed in the early 1980s by researcher David Jenkins and colleagues at the University of Toronto as a way to classify carbohydrates by their actual physiological effect — not just their chemical structure. Before GI, nutritionists largely assumed simple sugars raised blood sugar quickly and complex carbohydrates did so slowly. The research revealed the picture was more complicated.

GI testing works by feeding study participants a portion of food containing 50 grams of available (digestible) carbohydrate, then measuring blood glucose response over two hours. That response is compared against the same person's response to 50 grams of pure glucose, which anchors the scale at 100. The resulting score reflects only speed and magnitude of the blood sugar rise — nothing else.

It's worth noting what GI does not measure: fat, protein, fibre, vitamins, or overall nutritional value. A food can score low on the GI scale and still be heavily processed. Understanding GI is one lens among many — see our overview of calorie count vs. nutrient density for a broader picture of food quality.

Why Glycaemic Load Is Often the More Useful Number

GI's biggest practical limitation is that it ignores portion size. Carrots have a GI of around 70 — technically high — yet a standard serving contains so few carbohydrates that the real blood sugar impact is minimal. This is where glycaemic load (GL) closes the gap.

GL is calculated by multiplying a food's GI by the grams of available carbohydrate in a typical serving, then dividing by 100. A GL of 10 or below is considered low; 20 or above is high. Using GL, carrots score around 2 — low — which matches most people's real-world experience of eating them without a blood sugar spike.

Applying both figures together gives a more honest assessment. Watermelon's GI sits near 76, but its GL per typical serving is roughly 8, because a standard slice is mostly water. Using GI alone would make watermelon appear problematic; GL reveals it's largely a non-issue for most people.

Factors That Shift a Food's GI in Practice

GI values are measured under laboratory conditions that don't always mirror how people eat. Several real-world variables substantially alter glycaemic response:

• Cooking and processing: Longer cooking times generally raise GI. Pasta cooked al dente has a noticeably lower GI than the same pasta cooked soft. Highly processed grains — such as instant oats versus steel-cut oats — digest faster and raise blood sugar more steeply.
• Ripeness: A ripe banana has a higher GI than an under-ripe one, as starch converts to simpler sugars during ripening.
• Food combinations: Eating carbohydrates alongside protein, fat, or fibre slows digestion and blunts blood sugar response. A baked potato eaten alone has a very different effect from the same potato eaten as part of a mixed meal.
• Cooling cooked starches: Refrigerating cooked rice or potatoes increases resistant starch content, lowering their effective GI when reheated or eaten cold.

These variables mean published GI tables offer estimates, not fixed values. Dietary fibre plays a particularly significant role in moderating blood sugar responses, making fibre-rich eating one of the most effective strategies for managing GI in practice.

When GI Genuinely Matters — and When It Doesn't

GI is most clinically relevant for people managing conditions where blood sugar control is central — including type 2 diabetes, prediabetes, and polycystic ovary syndrome (PCOS). Research published in peer-reviewed nutrition journals consistently shows that lower-GI dietary patterns support better blood glucose management in these populations. Anyone in this situation should work with a registered dietitian or physician rather than relying on general information alone.

For the general healthy population, however, the evidence for GI-specific tracking producing meaningful advantages over simply eating well is less compelling. Whole dietary patterns — emphasising vegetables, legumes, whole grains, lean protein, and healthy fats — naturally skew low-GI without the need for meticulous scoring. The role of fibre in blood sugar regulation is one reason why: high-fibre diets tend to produce lower glycaemic responses across the board.

A useful habit for everyday eating is to treat GI as a general guide rather than a strict rulebook. Choosing less-processed carbohydrates, pairing carbohydrates with protein or fat, and eating plenty of vegetables will achieve much of the benefit associated with low-GI eating — without needing a reference chart at every meal. For more help interpreting the numbers on your food packaging, our guide to reading nutrition labels can help you connect GI principles to real products.

This article is for general informational and educational purposes only and is not a substitute for personalised medical or dietary advice. Consult a qualified healthcare professional or registered dietitian for guidance specific to your health needs.