I don't know how to calculate it. But I do know that the combination matters. Try this experiment. Eat a bagel plain for breakfast. Wait as long as you can for lunch. Note when you start feeling like you are starving and your hands get shaky (blood sugar crash). Next day eat the bagel with peanut butter. Note how you feel after the same amount of time. I predict the blood sugar crash will later, or won't happen at all.

I have found that the secret to sustaining a vegan diet is beans (resistant starch). They release sugar into your blood over many hours, blunting the blood sugar roller coaster that would otherwise be caused by eating a lot of fruits and vegetables without a lot of meats and fat.

I have done a version of the above proposed experiment with a foot long Subway sandwich, veggies only (no meat, no cheese, no mayo). I feel like I am starving in 3 hours. Add hummus, I feel fine for 6 hours. I know it adds some calories, but I'm convinced it is more about the resistant starch. According to this video https://nutritionfacts.org/video/beans-and-the-second-meal-e... eating beans in one meal even reduces the blood sugar spike in later meals!

Another interesting thing. There is a Ted Talk about how glycemic response varies by individual due to different makeup of gut bacteria. I think it is this one: https://www.youtube.com/watch?v=0z03xkwFbw4. The guy claims he can analyze your gut bacteria and tell you which foods will spike your blood sugar. Subway might be bad for me but fine for you!

I've actually tried that peanut butter bagel experiment myself and didn't notice any significant difference. Of course experiments with a population of 1 and no control group are meaningless.

In the end your metabolism will have its own way of digesting which may or may not be close to the very general glycemic load. Which is compounded by your metabolism reacting differently based on time of day, what you ate before, what after, whether you're sick or just in a different state.

With that out of the way, of course it depends which things you eat together. Potatoes are high on the index but if you combine them with cheese (mmmm cheese) it will slow down digestion and thus lower the glycemic load. Similarly, bread has a high load. But add peanut butter and you slow it down.

As a general rule: Yes, add fat to slow digestion. There may be methods to calculate the load of a compund meal but don't count too much on them. The glycemic index is an average of how people react to foods. It's already impractical to study all the meals in that fashion and still you wouldn't know how you individually reacted to them.

Edit: If you want to watch how your body reacts, you can get a set of Freestyle Libre sensors from Abott which allows you to monitor your blood glucose levels. They're not super accurate, but they easily allow you to track up/down movements which is enough to see how long it takes you to digest a meal.

A portion or meal differs for like anyone. When I make a slice of bread its like twice the size the one my SO makes. That goes for everything. So you have to calculate it for yourself.

I dont have much knowledge about this kind of stuff; i "researched" what GI and GL actually means and pasted it for others also curious.