Glycemic Load vs Glycemic Index: The Portion Is the Point

If you have ever looked up a food’s glycemic index, been alarmed, and then eaten it anyway — this one is for you.

Short answer: glycemic index (GI) is a property of the food. Glycemic load (GL) is a property of your serving. GI doesn’t change when you eat less; GL does. That single difference is why GI tables scare people away from foods that, at a normal portion, barely register.

What glycemic index actually measures

GI is a tested, published value: how quickly 50 g of available carbohydrate from that food raises blood glucose, compared with pure glucose (GI 100). It’s measured in laboratories, food by food, and collected in published tables such as the University of Sydney’s GI database.

Two things follow from that definition:

• GI is fixed per food (for a given variety and preparation). A smaller bowl doesn’t lower it.
• GI says nothing about how much carbohydrate a normal serving actually contains.

What glycemic load adds

Glycemic load fixes the second problem with one line of arithmetic:

GL = GI × available carbohydrate in your serving ÷ 100

That’s the whole formula. It’s why GL is the number that changes when your portion does — and why it is usually the more useful of the two for a real plate.

The watermelon problem

Watermelon has a high published GI — around the mid-70s. On a GI-only list it sits next to white bread, looking like something to avoid.

But a typical 120 g slice of watermelon holds only about 6 g of available carbohydrate. Run the formula: 76 × 6 ÷ 100 ≈ GL 5 — firmly low. The food is “high GI”; the serving is a non-event.

Food (typical serving)	Published GI	Carbs in serving	GL
Watermelon, 120 g	~76	~6 g	~5
White rice, 150 g cooked	~73	~38 g	~28
Spaghetti, 180 g cooked	~46	~44 g	~20
Spaghetti, 120 g cooked	~46	~29 g	~13
Lentils, 150 g cooked	~32	~24 g	~8

Values are published, rounded figures for illustration — varieties and preparation shift them.

Notice the spaghetti rows: same food, same GI, different portion — different story. That’s the entire argument for tracking load, not just index.

What GI and GL don’t tell you

Honesty matters more than a tidy story, so: neither number predicts your glucose response. Mixed meals (fat, protein, fiber), cooking and cooling, ripeness, and individual differences all move the real-world outcome. Published GI is a solid, tested starting point — it is not a forecast of your afternoon.

That’s why in 3sense Food every glycemic value carries its evidence label: GI is marked published, load is marked calculated at your logged portion, and if a food has no published GI, we say unknown rather than inventing one.

Try it on your own portion

The fastest way to internalize the difference is to move a serving slider and watch GI stay put while GL moves. Our glycemic load calculator does exactly that — pick a GI, set your grams, read the load. If you want the formula and the thresholds alone, there’s the 60-second version.