Glycemic load, or GL, answers a practical question: how much glycemic impact might this serving create? Glycemic index ranks the response to a standardized carbohydrate dose, while GL also includes serving size.
The short answer
Glycemic load combines carbohydrate quality with the amount actually eaten, making it more useful than GI alone for understanding a real serving.
The simple formula
GL equals GI multiplied by grams of available carbohydrate in a serving, divided by 100. A food with GI 60 and 20 g available carbohydrate has a GL of about 12. Common cutoffs are low at 10 or less, medium at 11–19, and high at 20 or more.
Why serving size changes the picture
Watermelon can have a relatively high GI but a normal serving contains limited carbohydrate, so its GL may be low. A very large serving of a medium-GI food can create a higher load.
Using GL sensibly
GL is not a good-or-bad label. Fast-digesting food may be useful around exercise. For everyday meals, overall quality, fiber, protein, micronutrients, and calories matter more. People with diabetes should follow individualized clinical guidance.
Practical checklist
Do not test unfamiliar food on an important training or competition day.
Adjust amount and timing to gut tolerance and session goals.
The quality of the whole daily diet matters more than one meal.
References
University of Sydney — Glycemic Index and Glycemic Load — https://glycemicindex.com/about-gi/