Limitations of the Glycemic Index and the Glycemic Load
Some proponents of the Glycemic Index (including many diet books authors) would like you to believe that GI and GL are all that matters when selecting which foods to eat. In reality, diet is a more complex issue than that. I agree that the Glycemic Index is a marvelous tool for ranking carbohydrates (and much better than the old “simple” and “complex carbohydrate” designations). However, there are also many limitations to GI and GL, which are explained in this section. Consider this the warning that those diet book authors don’t want you to hear…
1. Scarcity of GI data
Although methods for determining Glycemic Index have been in existence for more than 20 years, GI values have so far only been determined for about 5% of the foods as calculated with the earlier described formula (see prcemievious Article in this Blog How Glycemic Load Improves Glycemic Index). Seemingly similar foods can have very different GI values, so it’s not always possible to estimate GI from either food type or composition. This means that each food has to be physically tested. GI testing requires human subjects, and is both relatively expensive and time-consuming. The fact that only a very limited number of researchers currently do GI testing compounds this problem. Food manufacturers continue to introduce thousands of new foods each year. Since GI testing is neither required nor common (at least in the U.S.), this problem is likely to get worse rather than better.
2. Wide variation in GI measurements
The Glycemic Index table(in the previous mentioned article) shows a single value of GI for each food. In reality, though, the measurements are not so precise. Reported values are generally averages of several tests. There’s nothing wrong with that methodology, but individual measurements can vary a significant amount. For example, baked Russet potatoes have been tested with a GI as low as 56 and as high as 111! The GI for the same fruit has even been shown to increase as the fruit ripens. This amount of variation adds a great deal of uncertainty to GI calculations.
3. GI values affected by preparation method
The Glycemic Index gets even trickier when you take into account the changes in value that occur in response to differences in food preparation. Generally, any significant food processing, such as grinding or cooking, will elevate GI values for certain foods, because it makes those food quicker and easier to digest. This type of change is even seen with subtle alterations of the preparation, such as boiling pasta for 15 minutes instead of 10.
4. GI values affected by combination with other foods
While tests for Glycemic Index are usually done on individual foods, we often consume those foods in combination with other foods. The addition of other foods that contain fiber, protein, or fat will generally reduce the Glycemic Index of the meal. The GI of this “mixed meal” can be estimated by taking a weighted average of the GI’s of the individual foods in the meal. However, this averaging method may become less accurate as the total percentage of carbohydrate decreases. Therefore, foods like pizza often create a higher glycemic response than the simple weighted average of the ingredient GI’s would predict. So be careful with quick meals. A stir fry can be just as quick and is much better for your health.
5. Individual differences in glycemic response
The rate at which different people digest carbohydrates also varies, so there are some individual differences in glycemic response from person to person. In addition it has been shown that one person’s glycemic response may vary from one time of day to another. And finally, different people have different insulin responses (i.e. produce different levels of insulin), even with an identical glycemic response. This fact alone means that a diabetic can not rely completely on the Glycemic Index without monitoring his own blood sugar response. (This, of course, is a limitation of any food index, and not a specific limitation of GI.) Your physical activity and the way you eat and your metabolism works has a large effect on the rate in which people digest carbohydrates.
6. Reliance on GI and GL can lead to overconsumption
It’s important to remember that the Glycemic Index is only a rating of a food’s carbohydrate content. If you use GI and GL values as the sole factor for determining your diet, you can easily end up overconsuming fat and total Calories. See example below…
Example – How the Glycemic Index can encourage overeating:
Apples have a GI of 38 (as shown in the table above), and a medium-size apple, weighing 138 grams, contains 16 grams of net carbohydrates and provides a Glycemic Load of 6. This is a low GL, and most would consider the apple to be a very appropriate snack. But now look at peanuts. A 4-oz serving not only weighs less than the apple, but has a much lower GI (14), and provides an even lower GL of 2. Based on Glycemic Load alone, you would have to believe that the peanuts were a better dietary choice than the apple. But if you take a look at the Calories contained in these two foods, you’ll see that the apple contains approximately 72 Calories, while the peanuts contain more than 500! Those 400+ extra Calories are NOT going to help you lose weight.
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