The concept of glycemic index is meant to help determine what foods raise blood sugar a lot vs. what foods raise blood sugar a little. Dr. Jennie Brand-Miller's searchable database can be found here.
I have to admit that glycemic index provided me with a sense of false assurance for some years. It screwed up my health until I came to understand the issues a lot better.
For those of you just starting out in nutritional conversations, glycemic index (GI) represents a comparison of the blood glucose area-under-the-curve (AUC) over 2 hours after consuming 50 grams of the food in question compared to the AUC of glucose or white bread. Volunteers involved in developing these values are healthy people who are generally of normal weight.
Glucose, by definition, has a GI of 100. An equal quantity of sucrose (50% glucose, 50% fructose) has a GI of 60, lower than glucose. An equal quantity of whole wheat bread has a GI of 68-77 (Yes: The GI of whole wheat is higher than sucrose). Non-carbohydrate foods, such as eggs or avocado, have no GI since they do not impact on blood glucose.
Because the GI is also sensitive to how much carbohydrate is contained, the concept of Glycemic Load (GL) was introduced:
GL = (GI x amount of carbohydrate) / 100
GL is therefore the GI that incorporates the glycemic potential of the food of interest. GI does not vary with portion size; GL varies with portion size.
Let's take whole wheat pasta, a food regarded by most people as a healthy choice. Whole wheat pasta has a GI of 55--fairly low--and a GL of 29. A serving of 180 g (approximately 6 oz cooked) provides 50 g carbohydrates.
People who advocate that low-glycemic index foods would say that this is a desirable profile and should therefore replace high-glycemic index foods.
I say WRONG. First of all, most of us are not slender 20-somethings. We will therefore not show the same response as a young, slender person (like the GI volunteers), but will show exagerrated blood sugar responses. So this much low-glyemic index whole wheat pasta will typically yield a blood sugar of 120-200 mg/dl in non-diabetic people, high enough to trigger glycation. Sure, a high-glycemic index food, such as white flour birthday cake with plenty of sugary icing, might trigger a blood sugar of 140-250 mg/dl, much worse. But that doesn't make the lower blood sugar following pasta any less bad--it's still terrible.
Another issue: GI is assessed over a 2-hour timeline. What if blood sugar remains high in a sustained way, say, over 6 hours? That's precisely what whole wheat pasta will do: Keep blood sugar high for an extended period.
So not only does a low-glycemic index food like pasta increase blood sugar in most of us extravagantly, it does so in a sustained way.
Lastly, low-glycemic index pasta still triggers small LDL particles to an extreme degree, as I discussed in the previous Heart Scan Blog post, Small LDL: Complex vs. simple carbohydrates.
Don't be false reassured by the notion of low GI or GL. In fact, I'd go so far as to say that NO glycemic index is a GOOD glycemic index (or load). The foods we want to dominate our diet are the foods that aren't even listed in the GI database.