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This myth is not about the glycemic index as a whole, but about some of the hype surrounding it and the misunderstandings which arise.
The glycemic index (GI) certainly has its uses and its a good rough guide as to how fast foods will raise your blood glucose. So its an indication of the speed that your BG will rise when you eat a particular food. Its usually based on glucose having a GI of 100 and a food which raises your BG slower, will have a lower GI. So wholemeal bread for example has a GI of around 70 (in the UK).
One of the most reputable sources for GI data is the University of Sydney GI testing service. http://www.glycemicindex.com/
However, there have been some books, articles and so on which tend to overestimate the importance of GI particularly with reference to diabetes. To understand why this is a problem, we must know certain things.
1) GI testing is not performed on diabetics. Its performed on 10 non-diabetics and the results are used to look at the average effect - a GI is calculated from these results. They do not perform the tests on the same people every time, so theres no real comparison between the results.
2) The GI does not tell you how long your BG will be raised for, or by how much. Its just an indication of the speed of the rise.
3) GI values are for individual foods. As soon as you combine that food with another in the form of a meal, the GI changes. Fats in particular can reduce the GI of foods.
4) Particularly on food packaging, labels like "low GI" are meaningless. They can mean whatever they want it to mean and the foods have not been tested in any standardised way
5) The GI is very very specific. There is no GI value for "an apple". Gala, golden delicious, granny smith etc all have different GI values.
6) Particularly with fruit & veg, factors such as the place where they were grown and their ripeness can make an enormous difference to the GI.
7) Cooking methods change the GI. Potatoes have a GI under 100, but baked potatoes have been clocked as high as 158.
So as you can see there are various problems with the GI - not that this renders it useless, but we need to bear in mind the above when looking at GI. Despite all the problems, it still remains a useful rough guide.
I mentioned in point 2 that the GI does not indicate how high or for how long your BG will be raised. The Glycemic Load does that. These two are related and together give a far better indication of the probable effects on the BG. For those who are more mathematically minded, the GL gives an indication of the area under the curve.
So a while ago GI became quite a buzzword and it seemed like everyone was talking about it. Books on GI diets were everywhere and in the main, the messages given out were reasonable on the basis that if you eat lower GI foods in general then you will be better off. But in some cases they did not take the GL into consideration, particularly if it was not aimed at diabetics - and to diabetics GL is the important one.
Look at it this way - the GI of wholemeal bread in the UK is between 68 and 72 depending on brand. Since glucose with a GI of 100 is bad, wholemeal bread is often referred to as "low GI". But the GI of table sugar is 65 - so its lower than bread. Does that mean its better for diabetics to eat 100g of sugar rather than 100g of bread?
No.
So if we look at these things too simplistically, its easy to get the wrong idea. This is particularly the case when foods are referred to as "low GI". A DSF member was told on a course that a particular chocolate cake was "low GI" despite:
a) She hadn't had it tested, so had no idea if this was true
b) Where does "low" start and end? Is it anything with a GI between 99 and 100? Or between 5 and 10?
c) Its clearly not low GI and the GL of any choc cake made with flour would be pretty bad.
Overall, despite the hype that makes the GI look like some magic bullet answer to diabetes, this is not the case. What we really want to know is how foods will affect our BG and what we can eat without raising our BG too much and for too long.
The only way to reliably do this is to try things for yourself and see with your meter what the actual effect of a meal was on your BG and learn from it. Perhaps try the meal again using a lower GI/GL version of one of the ingredients and see what that does to your BG.
So the GI and GL used together will not tell you what will work for you to keep your BG low. But they do point you in the direction of things to try.
Further information on GI and GL:
http://www.mendosa.com/gi.htm
This is a series of blog entries about diabetes myths. Diabetes Myth #1 - Its all your own fault Diabetes Myths #2 - Its all about sugar
Diabetes Myth #3 - Calories Count Diabetes Myth #4 - Cures Diabetes Myth #5 - The Diabetic Diet Diabetes Myths #6 and #7 - "Diabetes is progressive" and "Pills or Insulin mean failure" Myth #8 - Glycemic Index is the answer. Now whats the question? Diabetes Myths #9 - Cholesterol Diabetes Myths #10 - HbA1c is an Average
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