Watermelon Has a High Glycaemic Index and It Does Not Matter, Which Tells You What Is Wrong With the Number

It is calculated by multiplying the glycaemic index by the grams of available carbohydrate in the real portion and dividing by 100. Low is 10 or under, medium is 11 to 19, and high is 20 or above.

Applied to watermelon, a high index multiplied by a small carbohydrate content produces a low load. Applied to white rice, a moderate to high index multiplied by a large portion produces a high one. The load resolves the paradox and matches what a glucose monitor shows.

The practical rule: use the index to choose between similar foods, and the load to decide how much of them to eat. Basmati against sticky rice is an index question, which our piece on rice covers. How much rice is a load question.

Where does glycaemic index break down?

In several places that matter for real eating, and this is why it should inform decisions rather than dictate them.

• It is measured on foods eaten alone. Almost nothing is. Adding protein, fat or acid changes the response substantially, which is why the order of a meal alters glucose by more than a third without changing a single ingredient.
• It varies with cooking and processing. Pasta cooked al dente scores lower than the same pasta cooked soft. Cooling cooked starch lowers it. Finely milled flour scores higher than a coarse grain of the same cereal.
• Ripeness changes it. A green banana and a spotted one are meaningfully different foods.
• Individuals differ. Studies measuring post-meal responses in large groups find substantial person-to-person variation to identical meals, driven partly by gut microbiome differences. Published tables are population averages, and you are not one.
• Low index is not the same as healthy. Ice cream has a relatively low glycaemic index, because fat slows absorption. Chocolate scores lower than boiled potato. The index measures one property, not overall merit.

So how should you use it?

As a rough sorting tool rather than a scoring system, in three ways.

Use it to make swaps within a category. Choosing steel-cut oats over instant, basmati over jasmine, or wholegrain over white is a sensible index-driven decision, and the alternatives are close enough that the index is the relevant difference.

Use load to control the portion once the choice is made, since a low index food in a large enough quantity still delivers a large load.

And, where you have access to it, test yourself. With continuous glucose monitoring now recommended much more broadly, it is increasingly possible to find your own responses rather than to trust a table. A food that spikes you and a food that does not will not always match the published figures, and your own curve outranks any list.