What Is the Glycemic Index?
The glycemic index (GI) is a numeric "rating" scale developed to help consumers make healthier food choices.
Essentially, GI value of any given food is an indicator of the relative glycemic impact (short-term impact on blood sugar levels) of portions of each food type containing that contain a fixed amount of carbohydrate (usually 50 g).
GI values can range from 0 (no reaction at all) to 100 (same impact as obtained by eating sugar).
Many common foods have GI values that are for all practical purposes essentially zero (see Home Page), while many others fall somewhere in the middle range (between 30-70).
How Does The Glycemic Index Help me Choose Foods?
All carbohydrate foods are not equal in terms of effect on blood sugar.
Some foods are capable of causing large and rapid "spikes" in circulating glucose levels, and these fluctuations can have adverse health impacts.
The glycemic index helps consumers distinguish "bad carb" foods (those with substantial glycemic impact) from "good carbs" (those with relatively low impact) by providing a simple numerical reference point.
What Are "Good" (or "Bad") GI Values?
In truth, that somewhat depends upon who you ask. There is no general consensus on a single GI value separating "good" vs. "bad" carbohydrates. Suggested scales for evaluating GI are admittedly arbitrary, and differ substantially among different authorities on the subject.
For example, the United States Department of Agriculture (USDA) scale - which was developed with participation of the commercial food industry - is far more generous in what is considered "low GI" than is the scale developed by Michel Montignac, a European pioneer on GI (Figure 1, below).
Montignac feels that a "low" threshold of 55 is actually too high, and fails to correspond to "physiological reality" – a direct result of undue influence by the US food industry, which did not want most of the products it marketed classified in the "high" GI range.
So, which "scheme" should you use? That's up to you of course. Opinions differ, and no two people have exactly the same metabolisms, genetic makeup, activity patterns and such. The conservative approach would be to err on the side of caution, and use the Montignac scale. If you do not very well tolerate foods with GI values above 50, your body will let you know.
How Is Glycemic Index Measured?
GI values are obtained from clinical studies of human subjects.
The average reaction of a group of subjects to any given "test food" is compared with their reaction to a "reference food", typically a glucose (sugar) solution, and a GI value is thereby derived.
Note that because these tests are usually based only upon one small group (less than 12) of human "test subjects", the results can hardly be expected to be representative of the 7 billion people now on Planet Earth!
Glycemic Index Caveats
The average dieter just introduced to the low glycemic concept is likely to have little understanding of the many pitfalls of applying GI to weight loss. In fact for these users, the glycemic index is probably the most misused and misunderstood guide to weight-loss dieting in the modern world.
Here, we list the top 5 caveats regarding practical application of the glycemic index:
1. GI values are approximations - NOT precise measures - of glycemic impact. Your reaction to a particular food can be expected to vary with such factors as age, activity levels, time of day, and other foods consumed.
2. Broad classes of foods cannot readily be assigned a single GI value. The glycemic index of potatoes, bananas, etc. will vary according to variety, stage of ripeness, manner of preparation and cooking, etc.
3. The published glycemic index value of a particular food is only applicable when that food is eaten alone. The glycemic index of foods in combination (i.e., meals) cannot be calculated from the GI of each component, and (unless specifically tested) remains unknown.
4. GI is of direct use as a food guide only when comparing portions of foods that contain the same amount of carbohydrate. The fact that watermelon has a far higher GI value than does pasta does NOT mean a bowl of spaghetti will have less glycemic impact than a slice of watermelon!
5. Glycemic load (next page) is a far more useful indicator than glycemic index for evaluating the glycemic impact of normal portion sizes of different foods.
Glycemic Index Values and Good Nutrition
Finally, we particularly emphasize that the GI value of a food tells us NOTHING about nutritional quality or caloric "density". For example a candy bar or cookie might have a low glycemic index (because of very high fat content), but contain lots of calories along with high levels of saturated fats and a large dose of table sugar, with few healthy nutrients. Clearly then, using glycemic index ALONE is NOT a smart way to choose your foods.
(Updated March 1, 2017.)