Whoa there! A single serving is how many grams? Usually, there are only 28 grams per ounce, and most serving sizes are done on a per ounce basis.
The glycemic index is quite controversial. For example, carrots for years had an extremely high glycemic level, but they were obviously good for you. Ends up the people they measured had to eat about 2 pounds of carrots each to get the required amount of starch. Walter Willett came up with the term Glycemic Load which takes the glycemic index and divides it by the serving size. That gave carrots a much lower number. A few years later, a new study found that carrots really didn’t have that high a glycemic index anyway. The original study was mistaken.
The problem of the glycemic index is how complex it is to measure. A typical measurement involves about 10 to 20 people (usually college students). They have to first fast and have their baseline blood glucose level measured. After that, they are fed a fixed amount of pure glucose or sucrose (depending upon the scale), and their blood glucose levels are measured again several times for two to three hours.
That is for the baseline. The same subjects must come back later to actually test the actual substance. Again, they fast, and are fed a specific amount of the substance being tested. The amount fed depends upon the starch level of the substance. The lower the starch level, the more they much eat. Low carb foods such as meat are not measured because the amount a person would have to eat would be enormous.
After consuming that substance (which must be eaten in a set amount of time), the subjects blood glucose levels are measured at fixed intervals. The results of that are compared to the previous glucose/sucrose measurements, and averaged among the subjects. That becomes the glycemic index.
There are two glycemic indexes because one is based upon sucrose and another upon glucose. In the sucrose based measurement, you can have a substance that has a glycemic index above 100 (which means that the substance affects your blood glucose more than sucrose). No substance has a glycemic index above 100 based upon the glucose scale.
This reveals why the glycemic index is not used in official measurements. If I give a dozen labs a box of Cap’n Crunch and asked them to tell me the number of calories, sugars, starches, fiber, vitamins, etc., they’d pretty much all agree. If I asked them to find the glycemic index, they’d all come up with different measurements. Also, why the labs could give me the basic nutritional information in a matter of days, the glycemic index measurements will take a few weeks. And, be quite expensive.
The glycemic index depends upon the subjects used, the lab, the protocol, and many other variables. It is lengthy and takes a long time to work out. This is why you will not ever see any governmental body requiring the glycemic index on food.
This isn’t to say that the glycemic index is useless. The glycemic index does show that many starches are actually more digestible than plain sucrose, and that by saying that eating something that is “low in sugar”, but is composed of highly processed starches isn’t necessarily better for you.
The glycemic index shows us that eating something with oil and protein can slow down the breakdown of starch, and that less unprocessed carbohydrates tend to have a lower glycemic index. And, how something is cooked can have a dramatic impact on the glycemic index. Overcooking pasta can double its glycemic index. Boiling potatoes and mashing them gives an extremely high glycemic index. Mixing them with milk and butter lowers it. Roasting the potatoes (that is putting it in the over without foil) also lowers the glycemic index.
Most important of all, the glycemic index has shown that the difference between starches and sugars are not all that different. If your cereal contains whole grain flour and a lot of fiber, the starch in it is probably not like sugar. For example, Raisin Bran would not be 43 grams of sugar. However, if you were eating something made from processed oat flour or corn flour that contains little fiber, that starch is pretty close to being nothing more than sugar.