The GI institute explains how it works far better (and more poetically) than I can
the body is not only absorbing glucose from the gut into the bloodstream, it is also extracting glucose from the blood. Just as a gentle rain can be utilised better by the garden than a sudden deluge, the body can metabolise slowly digested food better than quickly digested carbohydrate. Fast-release carbohydrate causes "flooding" of the system and the body cannot extract the glucose from the blood fast enough. Just as water levels rise quickly after torrential rain, so do glucose levels in the blood. But the same amount of rain falling over a long period can be absorbed into the ground and water levels do not rise
When they have compared GIs of foods tested in people with diabetes with those tested on non diabetics the results correlate well. (though obviously some people with D will start and possibly reach higher levels than the non D, the relative GI of the foods remains the same)
I could actually see an argument as to why this might not work so well with injected insulin, however the injected insulin we use doesn't work nearly as quickly as high GI foods . I find that lower GI carbs , as long as they aren't made lower by the addition of high amounts of fat can be managed more easily. (ie foods like pizza)
I gave the Univ of Sydney link in the other thread but will give it again. As I said there the FAQS, and the newsletter will tell you al lot about what the GI is about.
http://www.glycemicindex.com/faqsList.php#1
Many GI diet plans (as opposed to just using the index) suggest adopting the half plate veg, 1/4 starch and 1/4 protein (+ daily: some dairy, usually low fat ie skimmed or semi skimmed milk and some fruit)
Jenny Brand -Miller (professor at Sydney Uni and doyenne of the GI diet) is a bit more specific in her books and suggests specific amounts for different weights.
Eg she suggests (for weight reduction and this isn't from her diabetes book which I can't find) that a woman weighing less than 70kg (me) should eat 3 portions of low GI carb rich foods (20-30g a portion), 5 portions veg (80g portion 150g of veg soup is a portion) 2 fruits( each medium or 80g), 3 protein portions (each providing 10-15g protein) 2 fat servings ie oils cheeses, butter , seeds etc ( each10g fat)
Looking at that again (haven't looked at it for a long time) I probably eat a bit more protein , about the same amount of added fat (but more with the protein) and often larger than 80g portions of veg . I think that it's worth noting that many of the carbs in the diet come from veg/fruit/dairy and not all from starchy carbs.
A man and someone who weighed more would though be advised relatively higher amounts of everything.