It seems strange that the most important work on the glycemic index comes from Australia and Canada. Why not from the United States, where we have otherwise made so much progress in treating diabetes?
Not all carbohydrates are created equal.
The first book to bring us the facts about the glycemic index attributes the American lag to academic politics. It sounds like another example of the NIH factor. No, it's not the National Institutes of Health, but rather that it was Not Invented Here.
For those who are open-minded enough to accept findings from other countries, the book is The G.I. Factor: The Glycaemic Index Solution, published in Australia and New Zealand by Hodder Headline Australian Pty Limited in April 1996. Its authors are Dr. Jennie Brand Miller, an associate professor and head of the teaching and research staff of the Human Nutrition Unit of the Department of Biochemistry at the University of Sydney; Kaye Foster-Powell, a dietitian and nutritionist; and Dr. Stephen Colagiuri, the director of the Diabetes Centre and head of the Department of Endocrinology, Metabolism and Diabetes at the Prince of Wales Hospital in Randwick, New South Wales, Australia.
The glycemic index (to use the American spelling) is simply a ranking of foods based on their immediate effect on blood glucose levels. It measures how much your blood glucose increases over a period of two or three hours after a meal.
Dr. David Jenkins, a professor of nutrition at the University of Toronto, Canada, first developed the concept of the glycemic index to help determine which foods were best for people with diabetes. His pathbreaking study, "Glycemic index of foods: a physiological basis for carbohydrate exchange," appeared in March 1981. In the subsequent 15 years literally hundreds of clinical studies in the United Kingdom, France, Italy, Canada and Australia have proved the value of the glycemic index. The United States remains "one of the last bastions of opposition," according to Brand Miller. That's especially strange since Jenkins's original work and several subsequent studies, including some of Brand Miller's appeared in The American Journal of Clinical Nutrition, a publication of the American Society for Clinical Nutrition.
Jenkins went on to author at least 15 more clinical studies of the effects of the glycemic index. In recent years Brand Miller has begun to play the role as the leading advocate of the glycemic index. She has authored or co-authored more than 150 publications, including 50 on the glycemic index. Her "International tables of glycemic index," which appeared last year in The American Journal of Clinical Nutrition was—before the publication of The G.I. Factor—the most authoritative compilation of knowledge of this important concept.
People with diabetes, athletes, and people who are overweight all stand to benefit from a knowledge of the glycemic index. But people with diabetes stand to benefit the most, partly because we have the most to gain from understanding and applying the concept to our daily lives.
Nowadays, after years of pernicious diet advice telling us that we should follow a high-fat diet or one low in simple sugars, the experts are beginning to reach a consensus that what works best for people with diabetes over the long haul is a high-carbohydrate and high-fiber diet (or perhaps a high-protein diet). That advice is excellent—as far as it goes.
Generally, foods high in fat and protein have lower glycemic indexes than foods high in carbohydrate. Don't let that fool you. These foods appear in a falsely favorable light, according to The G.I. Factor. People with diabetes have many other excellent reasons to minimize fat in our diet.
The problem is that even among the complex carbohydrates not all are created equal. Some break down quickly during digestion and can raise blood glucose to dangerous levels. These are the foods that have higher glycemic indexes. Other carbohydrates break down more slowly, releasing glucose gradually into our blood streams and are said to have lower glycemic indexes.
Before the development of the glycemic index, scientists assumed that our bodies absorbed and digested simple sugars quickly, producing rapid increases in our blood glucose levels. This was the basis of the advice to avoid sugar, a proscription recently relaxed by the American Diabetes Association and others.
Contrariwise, the experts thought that our bodies absorbed starches such as rice and potatoes slowly, causing only small rises in blood glucose. Clinical trials of the glycemic index have also proven that assumption to be false.
The glycemic index, or what the book calls the G.I. factor, gives us a simple handle on the problem. It is a ranking of foods on a scale of 0 to 100, telling us whether a food will raise blood glucose just a little, moderately or dramatically. The lower the number the slower the action.
To date hundreds of volunteers, both people with diabetes and others, have participated in the clinical trials that form the basis of our knowledge of the glycemic index. The test food that they eat is 50 grams of carbohydrate. For example, to test boiled spaghetti, the scientists give their subjects 200 grams of spaghetti, which according to standard food composition tables provides 50 grams of carbohydrate. The scientists compare this response with the volunteer's response to a reference food. Both for the test and for the reference foods the volunteer's response over the next two or three hours is calculated. Then, they repeat the whole process on different days to reduce the effect of day-to-day variations.
However, one source of confusion remains, the reference food. Some studies set 50 grams of pure glucose as equal to 100, others use 50 grams of white bread. It is, however, easy to convert from one to the other using a conversion factor (white bread has a glycemic index of 70 when glucose is the reference food).
The G.I. Factor uses a standard where glucose equals 100, but the comprehensive tables in the book give values for both standards. Let's hope that these tables themselves don't become another source of confusion. In one place the book says "we know the G.I. factor of more than 600 foods" and elsewhere it says "This table lists almost 600 separate entries."
In fact, it lists 573 separate entries, which are not 573 foods. They are instead the results of 573 studies of the responses of sets of volunteers to being fed 50 grams of one food or another or mixed meals. For example, three of these entries are for bulgur boiled for 20 minutes as tested by different researchers and on different subjects, some of whom have diabetes and some who don't.
Rice is a food that takes even more entries—precisely 49 times to be exact. The apparent duplication is actually to test responses to different types of subject, varieties and preparations. Just how many foods are actually included depends on definition, but it is more than the approximately 100 foods that Tere Griffin and I included on our World Wide Web page on The Glycemic Index, which was the most listed anywhere before Brand Miller published her "International tables."
The 573 entries in the table at the back of the book are a valuable resource, which the book claims correctly to be "the most complete list published to date anywhere in the world." It shows the food, its glycemic index on both the glucose and white bread standards, the number and type of subjects (whether NIDDM, IDDM, or non-diabetic), the reference food and time period (glucose or bread, 2 hours or 3 hours), and the source. The sources are 80 different studies in professional journals with complete citations.
You can, of course, approximate the effect that any untested food has on you. Simply, test yourself with your blood glucose meter an hour and a half or two hours after eating the food in question and compare the results with whatever standard you set for yourself. This will not be as accurate as glycemic index tests, but there is a close correlation between those tests of the total effect of a food for the two or three hour period after a meal and the approximate results that a home meter would report.
Most, but not all, of the foods tested are high in carbohydrates. Some may wonder at the gaps—why other high-carbohydrate low-calorie foods like celery have never been tested. The problem is a technical one for the testers, because they would be so hard put to get anyone to volunteer to eat 50 grams of carbohydrate from celery—it's just too much celery to think about! Essentially, from a glycemic index standpoint, celery and foods like it can be considered as free foods.
What is not a free food, clearly, is rice. Rice has a surprisingly high glycemic index, as high as 94 (where glucose = 100). Brown rice and basmati rice have much lower indexes (brown basmati rice, available here, but not in Australia and not yet tested anywhere, promises to have an even lower index).
With information that this book provides we are now able to choose not only different foods but also different varieties of the same food for their lower glycemic values. Apparently, all sorts of beans and rice varieties like basmati that are high in amylose compared with amylopectin have lower values than other foods. Amylose is a straight-chain molecule that is harder to digest than amylopectin, which has many branches.
Pasta has a low glycemic index. Why? Because it is prepared from semolina or cracked wheat, not wheat flour. The method of preparation counts, because the large particle size means that we absorb it more slowly.
On a personal note, I have made several changes in my diet on the basis of what I've learned about the glycemic index. One is to cook rice with half barley, because barley has such a low index, about 46.
Another is to eat a lot of chana dal, otherwise known as Bengal gram dal (Cicer arietinum). This is a split yellow legume, similar to garbanzo beans (chick peas) but smaller and sweeter. Clinical trials from India and South Africa show a glycemic index of 11 and 7 respectively, just about the lowest glycemic index of any food tested. This food, generally available in Indian groceries, not only has a remarkably low index but it also has a wonderful flavor.
Still another change is that I am now delighted to be able to eat fat-free ice cream and yogurt without feeling any guilt! Low-fat ice cream has an average index of 61, and artificially sweetened low-fat yogurt has an index of 14 (test results of non-fat ice cream and yogurt aren't in yet). Ice cream, despite of all its sugar, has much less effect on blood glucose than ordinary bread. This book quite fairly points out that sugar is not just empty calories but rather a source of pleasure and reward that also helps us to limit our intake of fatty foods and high glycemic index carbohydrates.
The G.I. Factor should find its primary audience among people with diabetes, although athletes and others can also profit from studying its lessons. At least a dozen studies have already looked at using the glycemic index as an aid in long-term diabetes control. All but one of these studies, the book reports, showed a clear benefit in improving blood glucose levels.
To make it even easier to apply the concept of the glycemic index the book includes 50 recipes. In addition to the standard computations of fat, carbohydrate and fiber, the recipes calculate the glycemic index of the recipe itself.
An edited version of this article appeared in Diabetes Interview, August 1996, pp. 1, 9, 12.
|This book, The G.I. Factor, has now been published in the United States as The Glucose Revolution. Marlowe & Company in New York published it in 1999 and revised it in 2003 as The New Glucose Revolution.|