The Glycaemic Index and Glycaemic Load: What They Mean for Eating Plans

What GI and GL Actually Measure

Glycaemic index was developed in the early 1980s by researcher David Jenkins and colleagues as a way to rank carbohydrates by their physiological effect rather than their chemical structure. Before GI, dietary advice treated all complex carbohydrates as equivalent — a simplification that research showed was inaccurate.

The GI of a food is measured by having study participants eat a standardized portion and tracking blood glucose responses over two hours, then comparing those readings against pure glucose (GI = 100). White bread is sometimes used as an alternative reference food at GI = 100.

Glycaemic load was introduced later to address a clear limitation: a food could score high on GI but be eaten in such small portions that its real-world blood sugar impact was modest. Watermelon, for example, has a GI around 72 — classified as high — but a standard 120 g serving contains relatively few available carbohydrates, giving it a GL of roughly 4, which is low. The formula is straightforward: GL = (GI × grams of available carbohydrate per serving) ÷ 100. A GL under 10 is considered low; 20 or above is high.

Factors That Shift a Food's GI

GI is not a fixed property stamped onto an ingredient at the farm. Several variables shift the number considerably:

• Processing and particle size: Finely milled flours produce higher GI products than stone-ground or intact grain equivalents because digestion proceeds faster.
• Cooking: The more a starch is gelatinized by heat and moisture, the more rapidly it is digested. Al dente pasta has a measurably lower GI than fully cooked pasta.
• Ripeness: A ripe banana has a higher GI than an underripe one, as starch converts to sugars during ripening.
• Food combinations: Fat, protein, and soluble fiber slow gastric emptying and blunt glucose spikes. A baked potato eaten with sour cream and cheese will produce a different response than the same potato eaten plain.
• Individual biology: Emerging research, including continuous glucose monitoring studies, confirms that people's postprandial (after-meal) glucose responses to identical foods vary substantially based on gut microbiome, insulin sensitivity, and other factors.

How Eating Plans Use GI and GL

Several structured dietary frameworks explicitly incorporate GI and GL concepts. The low-GI diet label is perhaps the most direct application: it steers people toward minimally processed grains, legumes, non-starchy vegetables, and most fruits while reducing refined starches and added sugars.

The broader Mediterranean and DASH (Dietary Approaches to Stop Hypertension) eating patterns are not explicitly GI-based, but their emphasis on whole grains, legumes, and vegetables means their carbohydrate sources tend to cluster in the low-to-medium GI range.

Research published in journals including The Lancet and Diabetes Care has found associations between low-GI dietary patterns and improved glycaemic control, lower LDL cholesterol, and modest reductions in cardiovascular risk markers. However, researchers note that disentangling GI effects from the overall quality of a diet — which tends to improve when people choose lower-GI foods — is methodologically challenging. Most nutrition scientists view GI and GL as useful but incomplete tools.

This article is for general informational purposes only and does not constitute medical or dietary advice. Consult a registered dietitian or qualified healthcare professional before making changes to your eating plan, particularly if you have a medical condition such as diabetes.