posted August 09, 2000 06:25 PM
By Rick MendosaIntroduction:
What is the Glycemic Index (GI)?
The glycemic index ranks foods on how they affect our blood sugar levels. This index measures how much your blood sugar increases in the two or three hours after eating.
The glycemic index is about foods high in carbohydrates. Foods high in fat or protein don't cause your blood sugar level to rise much.
A lot of people still think that it is plain table sugar that people with diabetes need to avoid. The experts used to say that, but the glycemic index shows that even complex carbohydrates, like baked potatoes, can be even worse.
When you make use of the glycemic index to prepare healthy meals, it helps to keep your blood sugar levels under control. This is especially important for people with diabetes, although athletes and people who are overweight also stand to benefit from knowing about this relatively new concept in good nutrition.
Recent studies of large numbers of people with diabetes show that those who keep their blood sugar under tight control best avoid the complications that this disease can lead to. The experts agree that what works best for people with diabetes-and probably the rest of us as well-is regular exercise, little saturated fat, and a high-fiber diet. That is excellent advice-as far as it goes.
The real problem is carbohydrates. The official consensus remains that a high-carbohydrate diet is best for people with diabetes. However, some of the experts, led by endocrinologists like Dr. Richard K. Bernstein, recommend a low-carbohydrate diet, because carbohydrates break down quickly during digestion and can raise blood sugar to dangerous levels.
Many high-carbohydrate foods have high glycemic indexes, and certainly are not any good in any substantial quantity for people with diabetes. Other carbohydrates break down more slowly, releasing glucose gradually into our blood streams and are said to have lower glycemic indexes. Does a substantial quantity of these foods with lower glycemic indexes belong in your diet? Only your personal experience can answer that question.
Before the development of the glycemic index beginning in 1981, scientists assumed that our bodies absorbed and digested simple sugars quickly, producing rapid increases in our blood sugar level. This was the basis of the advice to avoid sugar, a proscription recently relaxed by the American Diabetes Association and others.
Now we know that simple sugars don't make your blood sugar rise any more rapidly than some complex carbohydrates do. Of course, simple sugars are simply empty calories, and still should be minimized for that reason.
Many of the glycemic index results have been surprises. For example, baked potatoes have a glycemic index considerably higher than that of table sugar.
A more pleasant surprise is the very low glycemic index of a tasty bean called chana dal, which is the subject of a separate Web page that I maintain at http://www.mendosa.com/chanadal.html.
Another pleasant surprise is pearled barley, which has a glycemic index of 36. That's much lower than any other grain. For example, brown rice has a glycemic index of 79, and wheat is even higher.
Hulless barley-particularly the non-waxy variety-almost certainly has an even lower glycemic index than pearled barley, because pearling removes some of the fiber. A great source of hulless barley that I have used is Bud and Jean Clem's Western Trails Inc. in Bozeman, Montana, phone (800) 759-5489, e-mail [email protected]. Another source of hulless barley is Bob's Red Mill Natural Foods Inc. in Milwaukie, Oregon, phone (503) 654-3215, e-mail [email protected].
Scientists have so far measured the glycemic indexes of about 300 high-carbohydrate foods. The key is to eat little of those foods with a high glycemic index and more of those foods with a low index.
Where can you find what these foods are? The easiest way is to refer to another Web page that I maintain. That page at http://www.mendosa.com/gilists.htm summarizes the academic research on this important dietary concept.
The GI is especially useful to diabetics who want to plan their diets to minimize the incidence of high blood sugar, or spikes. It measures how fast the carbohydrate of a particular food is converted to glucose and enters the bloodstream. The lower the number the slower the action.
The numbers are percentages with respect to a reference food. They are given here with respect to white bread. In other words, on the scale white bread equals 100, which is what is generally used in the United States. Multiply the GI on this scale by 0.7 to convert to the value on the scale where glucose = 100.
If you are trying to lose weight-something of great interest to many type 2 diabetics-you may also want to study my
Satiety Index page at http://www.mendosa.com/satiety.htm. The satiety index, along with the glycemic index, are two of the most exciting tools that we have to control our diabetes.
Perhaps the best current defense of the glycemic index is a commentary by Dr. Thomas M.S. Wolever, "The Glycemic
Index: Flogging a Dead Horse?" in the journal Diabetes Care. Since Diabetes Care chose to not to make it available on-line
while making a rebuttal available on-line, I have scanned in an extract of Dr. Wolever's commentary with his permission at http://www.mendosa.com/wolever.htm.
Dr. Andrew Weil, the noted writer and lecturer on integrative medicine, has come out strongly for the utility of the glycemic index. See "What's Up with The Zone?" at http://cgi.pathfinder.com/drweil/. He writes that "we should pay attention to the glycemic index of carbohydrate foods - that is, the ease with which the body converts them to blood sugar. High-glycemic foods like rice cakes, bread, and potatoes stress the body's insulin system and probably are chief culprits in obesity."
What about Portion Size? And how is GI Determined?
The glycemic index is about the quality of the carbohydrates, not the quantity. Obviously, quantity matters too, but the measurement of the glycemic index of a food is not related to portion size. It remains the same whether you eat 10 grams of it or 1000 grams. That's because to make a fair comparison tests of the glycemic indexes of food usually use 50 grams of available carbohydrate in each food. You can eat twice as many carbohydrates in a food that, for example, has a glycemic index of 50 than one that has a glycemic index of 100 and have the same blood glucose response.
Basically, test foods are fed to various people, some with diabetes, others without, in portions that contain 50 grams of available carbohydrates. The 50 g carbohydrate portion is specified in Dr. Wolever's methodology paper (see bibliography below) as 50 grams of available carbohydrates. "That means it excludes the fiber," Professor Brand-Miller writes me. "We have always used a 50 gram available carbohydrate portion and often relied on manufacturers to give us the composition data. I am aware of only one instance where we been given incorrect information and therefore inadvertently included the fiber in the 50 gram carbohydrate portion...but there may be some papers from developing countries where the data is not reliable."
For example, to test boiled spaghetti, the scientists give their subjects 200 grams of spaghetti, which
according to standard food composition tables provide 50 grams of available carbohydrate. The
scientists compare this response with the volunteer's response to a reference food. Nowadays, the
reference food is usually white bread, which volunteers tolerate better than the glucose used in earlier
studies. Both for the test and for the reference foods the volunteer's response over the next two or three hours is calculated. Rather than measuring a single point, they make the more precise measurement of the area under the curve. Then, they repeat the whole process on different days to reduce the effect of day-to-day variations.
Next, the area under the response curve for the test food is expressed as a percent of the mean value for the reference food for the same subject. Finally, these percentages from each subject are averaged together to obtain the GI for that food. For more information, see Wolever, Thomas M.S. et al. "The Glycemic Index: Methodology and Clinical Implications," listed in the bibliography below.
Are there Other Important Diet Considerations?
The glycemic index should not be your only criterion when selecting what to eat. The total amount of carbohydrate, the amount and type of fat, and the fiber and salt content are also important dietary considerations. The glycemic index is most useful when deciding which high-carbohydrate foods to eat. But don't let the glycemic index lull you into eating more carbohydrates than your body can handle, particularly if you have diabetes. The number of grams of carbohydrate we consume is awfully important. Make sure you know the carbohydrate content of the foods you eat-study the nutritional information on the package.
But first you need to decide the composition of your diet in terms of carbohydrate, fat, and protein. Almost all the experts agree that we should minimize our intake of saturated and trans fat and eat a lot more fiber than we do. Some other fats, particularly those from cold-water fish and essential fatty acids such as found in large amounts in flax oil, seem to be beneficial. Beyond that, the battle rages between those who would have us eat more protein and those who say that carbohydrates should provide most of our calories. I'm no expert and am genuinely puzzled myself, although I have begun to cut back on my carbohydrates and eat more protein. Generally, foods high in fat and protein have lower glycemic indexes than foods high in carbohydrate. In a real sense, the glycemic index is not applicable to high-fat and/or high-protein foods.
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.
Factors such as variety, cooking, and processing may effect a food's GI. Foods particularly sensitive to these factors include bananas, rice, and potatoes (for a fuller discussion of the GI of rice and potatoes see the section below). A 1992 study by Hermansen et al. reported that the GI for under-ripe bananas was 43 and that for over-ripe bananas was 74. In under-ripe bananas the starch constitutes 80-90 percent of the carbohydrate content, which as the banana ripens changes to free sugars. Particle size is also an important factor, according to a 1988 study by Heaton et al. The researchers found that the GI of wheat, maize, and oats increased from whole grains (lowest GI), cracked grains, coarse flour, to fine flour (highest GI).
In addition, the glucose response to a particular food may be somewhat individual. So it is probably a good idea to carefully watch your own blood glucose level after eating foods you have questions about and determine if they have high or low GI for you.
So, the idea of glycemic index is a very useful one. But if you find a specific food produces an unexpected result, either high or low, take note of it and incorporate that into your meal planning.
Also note that the numbers vary from study to study. This may be due to variations in the individuals in a particular study, other foods consumed at the same time, or different methods of preparation, since your body can absorb some foods better when they are well cooked.
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 (or tomatoes or similar foods) 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.
Mixed Meals
Some people wonder if the glycemic index can predict the effect of a mix meal containing foods with very different indexes. Studies have shown that it does that job very well, too.
More than fifteen studies have looked at the glycemic index of mixed meals. Twelve of them showed an excellent correlation between what was expected and what was actually found.
You can quite readily predict the glycemic index of a mixed meal. Simply multiply the percent of total carbohydrate of each of the foods by its glycemic index and add up the results to get the glycemic index of the meal as a whole. Professor Brand-Miller has an example on page 29 of her book The G.I. Factor. The report of a Joint FAO/WHO Expert Consultation "Carbohydrates in Human Nutrition" (cited in the Web bibliography below), has a comprehensive explanation and example in its section 6.3.
The three studies that did not show the expected correlation came from a group of researchers who were not using standardized methodology for working out the glycemic index from the area under the curve. In addition, their meals were high in fat, which tends to reduce the impact of any one carbohydrate food.
Pizza
Many people have noticed that pizza seems to keep their blood glucose level high longer than just about any other food. While the reason remains a mystery, this folk wisdom now has scientific confirmation.
Ahern et al. compared the effect on insulin-dependent patients of a pizza meal with a control meal that included high glycemic index foods. They found that although the initial glucose increase was similar for the two meals, the GI continued to rise and was significantly increased from four to nine hours after the pizza meal compared with the control meal.
Rice and Potatoes
Rice and potatoes are some of the foods most tested for their glycemic indexes. They are important both because most of us tend to eat a lot of rice and potatoes and because they can have a high glycemic index. Professor Brand-Miller reports the results of 49 studies of rice and 24 studies of potatoes. The results for rice ranged all the way from 54 to 132 and for potatoes from 67 to 158.
What could possibly cause such tremendous variation? According to Professor Brand-Miller, for rice one of the most important considerations is the ratio of amylose to amylopectin. She says that "the only whole (intact) grain food with a high G.I. factor is low amylose rice, such as Calrose rice...However, some varieties of rice (Basmati, a long grain fragrant rice, and Doongara, a new Australian variety of rice [which is not available in the United States] have intermediate G.I. factors because they have a higher amylose content than normal rice."
Wallace Yokoyama, a research chemist working for the U.S. Department of Agriculture in Albany, California, gave me a comprehensive explanation. There are, says this noted rice specialist, four types of rice: long-grain, medium-grain, short-grain, and sweet rice. Sweet rice is also known as sticky or waxy rice. It makes the best sauces and gravies, and is usually the rice used in Asian restaurants. Sweet rice has no amylose, Yokoyama says. In other words, it is the rice that has the highest glycemic index. The three other types of rice have lower glycemic indexes. Among these types, long-grain rice has the highest amylose content and short-grain the lowest.
Of course, each of these three types of rice may be brown or white. Brown rice has a lower glycemic index than white rice, everything else being equal. Therefore brown long-grain rice-or if you can find it-brown Basmati rice-will probably be your best best for a rice with a lower glycemic index. White Basmati rice had a glycemic index of 83 in one study. Brown Basmati rice can be expected to have a somewhat lower index, but we don't know precisely what it is, because the studies haven't been done yet.
Richard Jackson maintains in e-mail to me that my statement that there are three basic types of rice is "somewhat incorrect." He says that there is also a sweet rice used in oriental cooking. "It is not only very much stickier than standard Asian milled rice (such as Kokuro Rose Brand)," he writes, "but is perceptably sweeter when eaten. It is typically fermented prior to cooking, whereas standard Japanese-style milled rice is not. I think if more research is done into this factor, the data may prove that the difference between sweet rice and regular Asian-style rice is different on the scale of caloric values as pertains to ingestion by diabetics."
Among potatoes, new and some white potatoes have the lowest indexes.The explanation is almost certainly the low starch content of these potatoes. It comes from Potatoes: A Country Garden Cookbook by Maggie Waldron, Collins Publishers San Francisco, 1993. Here is how the different varieties of potatoes differ in starch content, according to the book:
High Starch Potatoes
Russet: varieties include Arcadia, Burbank, and Idaho.
Medium Starch Potatoes
Long White (aka White Rose or New White)
Peruvian Blue
Round White (aka Eastern): varieties include California, Canada, Chippewa, Delaware, Irish Cobbler, Katahdin, Kennebec, Long Island, Maine, and Superior. Yellow Finnish (aka Yellow Finn)
Yukon Gold
Low Starch Potatoes
Fingerling: varieties include Ruby Crescent and Russian fingerlings. These are "ideal steamed, roasted, or barbequed"-but not baked, according to Waldron. Round Red: varieties include La Soda, La Touge, Red Norland, Red Pontiac.
Waldron says that some reds, often called new potatoes, are harvested before they mature in the late spring and they last through the summer. (If seen later in the stores, they have been in cold storage.) When picked young they are low in starch and sweet in flavor, making them a poor choice for baking, but ideal for roasting and frying. When harvested later in the season, red potatoes grow to full size with a higher starch content.
Fructose and High Fructose Corn Syrup
An addition to the published glycemic indexes is high fructose corn syrup, which is endemic in U.S. processed foods. Fructose is not the same as high fructose corn syrup, Professor Jennie Brand-Miller emphasized in a message to [email protected], which he kindly faxed to me. "The former is pure fructose; the latter [high fructose corn syrup] is a mixture of fructose and glucose," she wrote. "In high fructose corn syrups, the fructose content is about 50 percent. Thus the GI of high fructose corn syrups is about the same as sucrose, i.e. 60-65 (if glucose = 100)." When white bread = 100, the GI of high fructose corn syrups is 85-92.
Soy Milk
Soy milk has a low glycemic index of 43, according to e-mail from Professor Brand-Miller. The tested soy milk, she writes, has 4.5 grams of carbohydrates, 3.5 grams of fat, and 3.5 grams of protein per 100 ml. A low factor for soy milk isn't surprising, since soybeans have a GI of 25. But consumers in the United States-where many different brands and flavors are available-need to be aware that not all soy milks are created equal.
My wife, who like me has type 2 diabetes, recently discovered that her blood glucose rose dramatically after a large cup of chai made with soy milk. That's when we paid attention for the first time to how many grams of carbohydrate that particular soy milk had. So then I looked through the nutrition information on the dozens of brands and flavors of soy milk (and rice milk and almond milk and oat milk, etc.) in our local natural foods store.
I was amazed to find that the carbohydrate content of these beverages varied from 4 grams per 8 oz. to 36 grams. The lowest is Westbrae Natural Foods' Westsoy 100% Organic Non Dairy Soy Beverage Unsweetened. It contains nothing but filtered water and organic whole soybeans. Pacific Original Non Dairy Beverage Unsweetened, also has nothing but filtered water and organic whole soybeans. It has one more gram of carbohydrates per 8 oz. than Westbrae's. But 3 grams of Pacific's carbohydrate are fiber, which means it has only 2 grams of available carbohydrate per serving as opposed to the 4 grams of available carbohydrate in a serving of Westbrae's brand.
You may also want to compare these numbers with cow's milk. It has 11 to 13 grams of carbohydrate per 8 oz, whether it is non-fat or full-fat. Also note that the carbohydrates in soy beverages have a lower GI than that of lactose, which is 65.
Differences from The G.I. Factor
The G.I. Factor lists values for both reference foods (white bread or glucose) as well as much other data that is omitted here. It lists whether the subjects are insulin dependent or non-insulin dependent diabetics or non-diabetics, the number of subjects, the reference food, the time period of the test (whether two or three hours), and the source of the study.
The G.I. Factor has 573 separate entries. But that's not 573 different foods. They are the results of 573 studies of the responses of volunteers fed 50 grams (rarely 25 grams) of one food or another or of mixed meals. Several of the studies are of similar foods prepared differently.
In the lists here, when more than one study of a specified food has been made, only the mean of those studies is listed below. However, The G.I. Factor lists both the mean and the results of the individual studies.
These Web pages condense this information into listings for about 300 foods. This is all of the different foods for which glycemic indexes have been published, according to The G.I. Factor.
Still, by being able to include about 300 different foods here, this is a substantial increase from the 103 listings that Tere Griffin and I laboriously collected when we first published this information on the Web in 1994. By coincidence, a major contributor to that early effort even before I became involved in it was Catherine Nord, who posted a list of the glycemic indexes of 37 foods to the Diabetic mailing list in April 1994, just before I joined the group. At the time she posted her list we had never communicated, but we were married in 1995.
UBBFriend: Email This Page to Someone!
George Spellwin's ELITE FITNESS Discussion Boards
posted August 09, 2000 06:25 PM
