Ok, time to finish this mess up because I have something special for next time. Last time I looked at some of the major issues with the Glycemic Index including the fact that a single meal, typically of a single carbohydrate, was tested after an overnight fast which is really not how people eat in the real world. People typically eat meals containing other nutrients (protein, fat, fiber) which impact on the GI, there is a second meal effect and, simply, the general approach to determining GI is a little bit contrived in the first place.
I also looked at one of the big assumptions regarding GI which has to do with the insulin response. Given the impact of blood glucose on insulin release, it was always kind of assumed that insulin was responding similarly. That is, a higher GI meant a higher insulin response. But this does not exactly turn out to be true for reasons discussed in that part.
I had already mentioned that the GI for diabetics had been kind of abandoned due to being too complex and unrealistic with the focus being more on total carbohydrate intake than anything else. That said, studies do find that lower GI diets have a small but measurable effect on metabolic parameters of diabetes compared to higher GI diets.
I’d note again that weight loss (and regular exercise) always improves issues related to insulin resistance and it’s often hard to tell whether effects are occurring from the low GI diet or the diet causing weight loss. There is an additional issue related to this that I will come back to below.
And that brings me to perhaps one of the largest problems with the GI concept.
Problem 4: The Quantity of the Test Carbohydrate
When I first described how GI was determined/measured I mentioned that the test was invariably based around 50 grams of white bread (originally glucose) as the test carbohydrate and then 50 grams of the test food. Whether I mentioned it explicitly or not, it’s important to realize that the 50 grams refers to the amount of digestible carbohydrates, not just the mass of the food.