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The Glycemic Index
By Gary F. Zeolla
Note: This article was revised and expanded and incorporated as a chapter in the book God-given Foods Eating Plan.
The glycemic index is a measure of how much a standard amount of a particular food raises blood glucose levels. A high glycemic food causes a rapid and large increase in blood sugar, and then a rapid drop in blood sugar, while a low glycemic food causes a lower and more gradual increase and then gradual decrease in blood sugar. The blood sugar spike caused by high glycemic foods causes a spike in insulin levels. Consumption of high glycemic foods on a regular basis leads to constantly elevated insulin levels (Brand-Miller, pp. 34-35).
These facts are important. Studies show that consuming a low glycemic diet can aid in weight loss. Also, high insulin levels are associated with increased risk of heart disease. And those with diabetes or insulin resistance (a.k.a. Syndrome X) can benefit by keeping their blood glucose levels on an even and low level (Brand-Miller, pp. 182-185, 35). Also, eating a high-glycemic meal will blunt human growth hormone release and increase cortisol levels (Faigin, pp.239-240; Brand-Miller, p. 230).
So understanding and applying the glycemic index can aid in one's health in many ways. This article will look at various aspects of the glycemic index. For a more detailed discussion of this subject, I would recommend the book The New Glucose Revolution, by Jennie Brand-Miller, Thomas Wolever, Kaye Foster-Powell, and Stephen Colagiuri. This book also contains the most up to date and complete glycemic index tables available.
How it is Measured
It is very important to understand how the glycemic index of foods is calculated. Not knowing this has caused many to draw wrong conclusions as to the application of the glycemic index.
When a particular food is tested, an amount of the food that contains 50 grams of carbohydrate is fed to 8-10 people. Over the next 2 hours, the test subjects' blood glucose levels are checked every 15-30 minutes. These readings are then plotted on a graph. The test is then repeated 2-3 times. And all of the values are averaged and compared to a standard food.
Earlier glycemic index ratings were calculated using white bread as the standard. But more recent test now uses pure glucose as the standard. Glucose is assigned a value of 100. With only a couple of exceptions, this is the highest rating of any food. All other foods then would have glycemic index rating from 0-99, based on how their glycemic response compares to glucose. So the higher the value is, the greater the glycemic response.
A food is considered to have a low glycemic rating when its value is 0-55, an intermediate rating is 56-69, and a high rating is 70 or greater (Brand-Miller, pp. 32-33).
The important point of the testing method is that all foods are tested by using whatever amount of the food provides 50 grams of carbohydrate. For some foods, this would be about an average serving size. For instance, an average sized baked potato contains about 50 grams of carbs. But for other foods, 50 grams of carbs would constitute a somewhat large serving. It would take 3-1/2 slices of bread to provide 50 grams of carbs. And for other foods, the amount one would have to consume to eat 50 grams of carbs would be rather ridiculous. For instance, to eat 50 grams of carbs from raw, cut-up cauliflower, you would have to consume 20 cups! That's five quarts of cauliflower. I seriously doubt anyone has ever consumed that much cauliflower in one sitting.
In fact, with foods like cauliflower being so low in carbs it is impossible to actually test them. For another example, to get 50 grams of carbs from celery would require the eating of 32 stalks of celery. It's hard to get ten people to eat that much celery 2-3 times. So foods like cauliflower and celery are just assigned a value of 0. Their consumption would have minimal impact on blood glucose levels. And foods that contain no carbs, like meat, don't need to be tested. With no carbs, there is no blood sugar response from eating them, so the value is automatically 0.
But for foods with a significant amount of carbs, it is not just the glycemic index of the food but also the amount of the food eaten that determines much blood glucose levels are raised. Two slices of bread would raise blood sugar levels twice as much as one slice would.
Wrong Conclusion in Regards to Carrots
Not accounting for how the glycemic index of foods is calculated led to some drawing wrong conclusions from the glycemic index. Carrots in particular received a "bad rap" when early studies of the glycemic index showed they had a high glycemic rating of 92. So many popular diet books recommended avoiding carrots due to their "high sugar" content.
However, ½ cup of cooked carrots contains only 8.7 grams of carbs. So to consume 50 grams, you would have to eat almost 3 cups of carrots. Now maybe someone who really likes carrots might consume this much at one sitting, but most would probably stop after a cup or so. And that amount of carrots would not cause a significant blood sugar response.
To correct this problem, the "glycemic load" was invented. This new value is calculated by accounting for both the glycemic index of a food and how many grams of carbs are in an average serving (Brand-Miller, p. 37). So in the charts in the about mentioned book, the values for the glycemic load is given along with the glycemic index.
Another important point is that when a food is measured, the test subjects eat only that food. But most people when they eat, consume a mixture of foods. And what matters is what the glycemic response of the entire of the meal is.
The glycemic response of each food in a meal needs to be averaged together, taking into account what percentage of the total carbs each food contributes to the meal. Moreover, the presence of fat and protein will slow digestion and thus blunt the glycemic response. So eating a cup of carrots with a meal containing a serving of a lower glycemic food like a sweet potato and a serving of a food containing protein and fat (like meat) would mean the overall glycemic response of the meal would be rather low (Brand-Miller, p. 36).
Moreover, the glycemic index is only one aspect of a food that should be considered. A low glycemic food is not necessarily "healthy" and a high glycemic food is not necessarily unhealthy. Other aspects of the food need to be considered as well. In the case of carrots, they are a storehouse of vitamins and minerals, in particular beta carotene (pro-vitamin A). So avoiding carrots due to a misunderstanding of the glycemic index would lead people to avoid a very excellent source of this essential nutrient.
And finally on carrots, the early studies on carrots were flawed. More recent studies have shown that carrots actually have only a moderate glycemic rating of 47 (Brand-Miller, p.59).
Given the above points, all of the condemnations of carrots based on its glycemic index were misguided.
Estimating Glycemic Ratings
It is difficult to estimate the glycemic value of a food just by looking at it. Many factors can affect the glycemic rating. The only way to know for sure what the glycemic index for a particular food is would be to look up the value in a book like The Glucose Revolution. However, some generalities can be drawn.
For starters, for the most part, the less processed a food is, the lower it's glycemic rating will be. For instance, corn has a glycemic rating of 60, cornmeal of 68, and corn flakes of 92 (Brand-Miller, p. 265). Oranges have a glycemic rating of 42 while orange juice has a rating of 52.
In regards to bread, one would think that whole-wheat bread would have a lower glycemic rating than white bread. However, if the whole wheat flour is finely ground, then the ratings are about the same, both around 70. The reason for this is both are processed into fine flour using the relatively newer method of using high-speed steel rollers. But the more traditional method of stone-grounding flour produces courser flour. So stone-ground whole wheat has a rating of only 53 (Weil, p. 56, 61-62).
On my for One and All Web site and in my book God-given Foods Eating Plan, I recommend eating foods in the most "natural" state possible while generally avoiding highly processed foods. My main reason for recommending doing so is that natural foods are much more nutrient dense than processed foods. But the lower glycemic rating of natural foods would be another reason to eat less processed foods.
There are also classes of foods that tend to have lower glycemic ratings. Most vegetables have very low glycemic ratings. In fact, as discussed above, most vegetables are so low in carbs that they cannot even be tested. There are some exceptions, with white potatoes being the most notable. But for the most part, vegetables can be eaten with little concern for their glycemic rating, or calorie or carb content for that matter. So when it comes to vegetables like broccoli, cauliflower, spinach, lettuce, zucchini, etc, eat away.
Similarly, most fruits have low to moderate ratings. Cherries have the lowest rating of 22, while watermelon has the highest rating of 72. The latter would put watermelon in the high glycemic food category. But like carrots, the amount of carbs in watermelon is low enough that a slice or two would not produce a significant blood sugar response. You shouldn't go overboard on fruit, but overall they are a great food to eat
It's hard to test the glycemic value for nuts and seeds. The carb level is very low in comparison to the amount of fat in them. So too many calories would need to be consumed in one sitting to test them. To test almonds, for instance, would require the eating of 1-3/4 cups, which would provide almost 1500 calories. That's much more than the average person would generally eat at one sitting. So most nuts and seeds are just assigned a value of 0. But peanuts and cashews have been tested since they are somewhat higher in carbs than other nuts, and their ratings are 14 and 22, respectively. So they have very low glycemic values.
Yogurt is another healthy, natural food. And it's glycemic rating is a very low 14.
In my book and on the Web site, I strongly recommend the consumption of vegetables, fruits, nut and seeds, and yogurt due to their high nutrient contents and associations with the reduction in risk of degenerative diseases like heart disease and cancer. And their low glycemic ratings are yet another reason to eat these foods. Detecting a pattern here? Foods that have a reputation for being healthy tend to have low glycemic ratings.
Of course, there are exceptions. For the most part, cold breakfast cereals have relatively high glycemic ratings. As with steel-milled whole wheat bread, the high degree of processing leaves even cereals made with whole grains with high glycemic ratings. Corn flakes was mentioned above as having the very high rating of 92. But shredded wheat doesn't far much better with a rating of 75, while puffed rice cereals have a rating of 83.
And some very unhealthy foods have low to moderate glycemic ratings. Sugar, as in normal table sugar (sucrose), has a moderate rating of 55. So any food high in added sugar would tend towards a moderate rating. And since the presence of fat lowers the glycemic response, foods high in fat would have lower ratings. So white chocolate, being basically a mixture of sugar and fat, has a moderate rating of 44. But this doesn't make white chocolate a healthy food.
Using the Glycemic Index
So how does one use the glycemic index to improve your diet? Eating more healthy, low glycemic foods like vegetables, fruits, nuts and seeds, and yogurt would be a good place to start. But in addition to this, there are many places in ones diet where a simple substitution would significantly lower the glycemic response of a meal.
For instance, the authors of The Glucose Revolution tell the story of a man with diabetes. His normal breakfast included a bowl of corn flakes. Afterwards, his blood sugar would soar to about 200 mg/dl. Most any other cold breakfast cereal would be almost as bad. But hot breakfast cereals tend to have lower glycemic ratings. Old-fashioned rolled oats, for instance, has a rating of only 44. And by simply substituting rolled oats for the corn flakes, his blood sugar rose to only 125 mg/dl (p. 208).
Potatoes were mentioned above as being a notable exception to vegetables having a low glycemic rating.
A baked potato has a high rating of 85. And, as noted above, just one average sized potato provides the 50 gram amount of carbs used for testing. So in the case of potatoes, the "bad rap" they've received due to the glycemic index is justified. But there are many alternatives to white potatoes. Sweet potatoes only have a rating of 44, yams, 37, and brown rice, 55. So substituting any of these for a potato would lower the glycemic rating of the dinner side dish.
Substituting stone-ground whole wheat bread for white or commercial whole wheat bread would be another simple step to take. Whole grain rye bread would be another alternative with a rating of 58, while whole grain pumpernickel would be even better with a rating of only 41.
Another alternative would be sprouted grain breads. Although such breads had not yet been tested at the time the book was written, the authors of The Glucose Revolution believed they were likely to have low ratings (p.101). And recently I purchased a loaf of "Ezekiel" bread, and it had a sticker attached to it reading "Low Glycemic: Certified by the Glycemic Research Institute, Washington, DC." So apparently, at least this brand of sprouted grain bread has now been tested and found to in fact be low glycemic.
Another important point is an area where I would disagree with the authors of The Glycemic Revolution. They basically recommend a high carbohydrate diet, with the carb level ranging from 45-65% of calories. Of course they recommend that the carbs come from mostly low glycemic foods. But still, as already indicated, it is not just the glycemic index of a food but the total amount of carbs eaten that determines the glycemic response of a food or meal.
Moreover, as also already indicated, the presence of fat or protein would moderate the glycemic response of any carbs eaten. For instance, in the example of the man with diabetes above, along with the corn flakes, he was consuming 8 ounces of milk, plus two slices of whole wheat toast, with a small amount of margarine.
Substituting stone-ground whole wheat bread for the whole wheat toast might have lowered his blood sugar response to breakfast even more. But even better would have been to substitute a low carb, high fat/ protein food for the toast, like say by mixing a small handful of nuts or peanuts into the oatmeal. A ¼ cup of nuts or peanuts would have about the same number of calories as two slices of toast with margarine, but his blood sugar response would have been even lower. Eating both cereal and toast for breakfast is simply too many carbs at once. Moreover, the nuts or peanuts would provide healthy monounsaturated fats, unlike the unhealthy trans fat in the margarine.
I'm not recommending a low carb diet per se. But I am saying that something like The Zone approach of consuming only 40% carbs, along with 30% protein and 30% fat, would be a better approach. The point is to avoid consuming only or mostly carbs at any given meal but to be include some fat and protein with the carbs.
Putting the above together, the best way to reduce the overall glycemic response of ones diet is to substitute low glycemic foods for higher glycemic foods, consume fewer carbs in general, and to balance any carbs eaten with some protein and fat.
Rightly used, the glycemic index can be a useful tool in designing a healthy diet. Keeping ones blood sugar on a even keel, while avoiding the high blood sugar and insulin spikes that comes with eating high glycemic foods would be beneficial to diabetics, those with insulin resistance, those who are trying to lose weight, and for anyone concerned about their health in general. But it is only one aspect of food that needs to be considered. Other factors, like the nutrient content of foods, need to be considered as well.
As for the book The New Glucose Revolution, I would highly recommend it for the detailed background information on the development of the glycemic index and for the comprehensive tables of the glycemic index and glycemic load ratings of foods. But I would shy away from its recommendations for consuming a high carb diet.
Brand-Miller, Jennie, Thomas Wolever, Kaye Foster-Powell, and Stephen Colagiuri. The New Glucose Revolution. Marlow & Company: New York, 2003.
Faigin, Rob. Natural Hormonal Enhancement. Extique Publishing: Cedar Mountain, NC, 2000.
Weil, Andrew. Eating Well for Optimum Health. Alfred A. Knopf: New York, 2000.
Carbohydrate values for foods taken from DietPower 2.4 software program, copyright 1992-2001 by DietPower, Inc.
The Glycemic Index. Copyright © 2005 by Gary F. Zeolla.
Disclaimers: The material presented in this article is intended for educational purposes only. The author is not offering medical or legal advice. Accuracy of information is attempted but not guaranteed. Before undertaking any diet, exercise, or health improvement program, one should consult your doctor. The author is in no way responsible or liable for any bodily harm, physical, mental, or emotional, that results from following any of the advice in this article.
The above article was posted on this site July 5, 2005.
It was updated October 8, 2005.
It originally appeared in the free email newsletter FitTips for One and All.
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