Saturday, May 24, 2008

The Secrets of the Glycemic Index

All Carbohydrates Are Not Created Equal

If you're confused about carbohydrates, you're not alone. Over the past several years, opinions about the role of carbohydrates in a healthy diet have ranged from "eat more for optimal health" to "nothing could be worse for your health." The truth about carbohydrates, however, lies somewhere in between. The fact is the type of carbohydrates that we eat is one of the foremost predictors of health. As you will see, a diet high in the wrong kinds of carbohydrates can lead to obesity, insulin resistance, diabetes, cardiovascular disease, and many of the other health problems that are so pervasive today. On the other hand, a diet that includes the right carbohydrates can help prevent these same diseases and put you on the road to excellent health.

Over the past couple of decades, the medical establishment has paid little attention to the impact of carbohydrates on health and wellness. But a growing body of evidence has made this a topic that can no longer be ignored. The fact is there are good and bad carbohydrates, and making the right choices is crucial to your pursuit of a healthy body weight and optimal health.

What makes some carbohydrate-containing foods better choices than others? One of the most important factors is the rate at which they raise blood sugar levels-or their glycemic index. This chapter will introduce you to this revolutionary way of looking at carbohydrates and show you why all carbs are not created equal. The following chapters will helpyou apply the glycemic index to your everyday life and create simple and satisfying meals that will enhance your health for years to come.

A Brief History of Carbohydrates

To truly understand how carbohydrates affect our health, it's important to look at how the carbohydrates we eat have changed over time. Throughout most of history, the only carbohydrate foods that were available were the wild roots, tubers, fruits, vegetables, and nuts that people foraged for. These foods were loaded with fiber and nutrients, and they were slowly digested and absorbed to provide a slow-release, sustained form of energy.

With the advent of agriculture about 10,000 years ago, people learned to cultivate grains such as wheat, rice, corn, oats, and barley. These foods, which quickly became mainstays in the human diet, were consumed in their natural unprocessed forms. Whole, cracked, or coarsely ground grains were made into porridges or baked into hearty whole-grain breads. These foods, too, were high in fiber and nutrients.

While the introduction of cereal grains substantially changed the human diet, the past 200 years have had an even greater impact on the types of carbohydrates available in the food supply-starting with the invention of high-speed grain mills in the early 1800s. Using this technology, millers learned to remove the fibrous bran and nutritious germ from grains and to make finely ground flour from just the starchy endosperm portion of the grain. People eagerly adopted this new flour, which had a very long storage life and made softer and lighter breads, cakes, and pastries. Unfortunately, this new white flour was also virtually devoid of the vitamins, minerals, and fiber found in whole grains. And its superfine texture makes it quickly digested and absorbed in the body, causing a rapid release of glucose and insulin into the blood. The past fifty years have brought the most dramatic changes of all to our food supply. For instance:

Products made from quickly digested white flours-such as breads, bagels, crackers, pretzels, and baking mixes-have become staples in most people's diets.

New technologies for processing grains-such as explosion puffing, extruding, and flaking-have been developed. Products made using these technologies, including breakfast cereals, snack foods, and a wide variety of "instant" and "quick-cooking" foods, are also rapidly digested, causing a fast rise in blood glucose and insulin levels. Like white-flour products, these foods make up a large part of many people's diets.

Consumption of refined sugars is at an all-time high.

The serving sizes of refined-carbohydrate foods like muffins, bagels, candy bars, and sodas have grown to enormous proportions.

This deluge of quickly digested nutrient-poor carbohydrates represents much of what's wrong with today's diets. Currently, about 85 percent of all grain products eaten by Americans are refined. And together, refined grains and sugars compose close to 40 percent of all calories eaten! What can you do to bring your diet back into balance? Learning about the glycemic index is a great place to start.

What is the Glycemic Index?

The glycemic index (GI) is a ranking of foods based on their potential to raise blood sugar levels. The higher the GI of a food, the faster the resultant rise in blood sugar after eating it. And the higher the GI, the higher the body's insulin response tends to be. Why is this important? High levels of blood sugar and insulin in the body have been linked to many of the health problems that are so common today.

The glycemic index has been the subject of scientific research for over twenty years. It was originally developed as a dietary strategy to help people with diabetes gain better control over their blood sugar levels. Today the GI is widely accepted in Canada, Australia, and much of Europe, and its use has expanded to include roles in treating obesity, cardiovascular disease, and various other health problems. Health professionals in the United States have been slow to adopt this revolutionary way of classifying carbohydrates. However, this is rapidly changing as mounting evidence on the benefits of the GI make this a topic that can no longer be ignored. The health effects of high- versus low-GI foods are summarized below:

High-GI Foods

Are quickly digested, causing a rapid rise in blood sugar and insulin levels.

Provide short bursts of energy that may be quickly followed by hunger and a roller-coaster pattern of overeating.

Promote excess insulin secretion, which may increase the risk for diabetes, cardiovascular disease, and some types of cancer and may contribute to a variety of other health problems.

Low-GI Foods

Are slowly digested, allowing for a gradual rise in blood sugar and insulin.

Provide a slow-release form of energy that sustains you between meals and promotes a healthy body weight.

Protect the body from the harmful effects of too much insulin.

You might look at this comparison and deduce that you should eliminate entirely foods that have a high glycemic index and eat only low-GI foods. Fortunately, going to extremes is not necessary. And as you will discover, just because a food has a low GI does not necessarily mean it is a healthful choice. However, replacing some of the high-GI carbohydrates in your diet with healthful lower-GI carbohydrates should be a primary strategy for anyone who wants to achieve a healthy body weight and maximize his or her health.

Ranking Foods on the Glycemic Index

Determining the GI of a food is a fairly complicated process (see the inset "How Do Researchers Determine the GI of a Food?" on page 11 for details), so the GI of every food is not known. However, researchers have tested a variety of common foods, some of which are shown on page 9. A more extensive listing of the GI values of foods can be found in the appendix. These tables list the glycemic indexes of foods when compared to pure glucose, which has a GI of 100. When comparing foods, the following scale will help you put the GI in perspective:

Very low G1 = 39 or lower
Low GI = 40 to 54
Moderate GI = 55 to 69
High GI = 70 or higher

A look at Table 1.1 may surprise you. Many foods that are often thought of as "health foods"-rice cakes and baked potatoes, for instance-have very high indexes, while "junk foods" like potato chips and chocolate have relatively low indexes. Is there any rhyme or reason to the glycemic index? Yes. The GI of a food is influenced by a variety of factors, including the degree to which the food is processed; how long the food is cooked; the kind of starch, sugar, or fiber the food contains; and the food's acidity. In general, anything that speeds the rate at which a food is digested and absorbed will increase the GI of a food. The section "Factors That Affect the Glycemic Index of a Food" on page 13 provides more details about what factors can raise or lower the GI of a food.

Of course, the glycemic index cannot be the only factor that determines which foods you should eat. As you can see from looking at Table 1.1, just because a food has a low GI does not necessarily mean it is good for you. It's important to consider all the nutritional qualities of a food when planning your diet. This book will help you make the best choices based on this philosophy.

While the GI should not be the only criterion used for choosing foods, some generalities can be drawn from Table 1.1 that can help guide you in choosing foods:

Foods That Raise The Glycemic Index Of Your Diet

Breakfast cereals
Processed snack foods like chips, crackers, and pretzels

Foods That Lower The Glycemic Index Of Your Diet

Minimally processed whole grains
Dairy products

Realize that some variation exists within these lists. For instance, not all kinds of bread and potatoes have a high GI. The remaining chapters of this book will help you make these distinctions and help you to plan varied and satisfying meals and snacks.

What effect do sweets have on the glycemic index of your diet? Many candies, cakes, cookies, and sodas have a moderate GI. However, these foods are very concentrated sources of carbohydrate, and the workload they place on the pancreas is considerable. Since sweets are often high in calories and low in nutritive value, they should be eaten with your total carbohydrate and nutrition goals in mind.

Perspective on Portions

How do portion sizes affect the glycemic index? The more carbohydrate you eat in a meal, the more insulin your pancreas must secrete to process the carbohydrate. For instance, eating a 4-ounce bagel will cause twice the insulin response as eating a 2-ounce bagel. Choosing low-GI foods will minimize the amount of insulin that you secrete when you eat carbohydrates, but portions are still important. Chapter 2 will give you an idea of how much carbohydrate is right for you.

Glycemic Index Versus Glycemic Load

Recognizing that both the GI of the carbohydrate-containing food and the amount of carbohydrate eaten affect blood insulin levels, researchers have coined the term glycemic load to describe these two factors considered together. Glycemic load is a better indicator of total insulin demand and the workload of the pancreas than just glycemic index by itself. This term is becoming more popular in the scientific literature, so when you see it, just realize that it reflects both the type and the amount of dietary carbohydrate.

Factors That Affect the Glycemic Index of a Food

Table 1.1, which lists the glycemic index of a variety of common foods, reinforces the statement that all carbohydrates are not created equal. However, at first glance, the glycemic index may not seem to make much sense. Why do two starchy foods like pasta and potatoes have such different indexes? And why does fruit have a lower GI than bread? Differences in cooking and processing methods; the chemical structure of the starches, sugars, and fibers in foods; and the presence of fat, protein, or acid can all markedly affect the GI of a food. Knowing more about how these factors affect the digestibility of foods will help you make sense of the GI.

Milling, Grinding, And Processing Of Grains

Modern food-processing techniques, such as grinding, pulverizing, puffing, extruding, and otherwise destroying the natural intact form of whole grains, make whole grains easier to digest and absorb. This is why most breads, breakfast cereals, snack chips, and crackers have such a high glycemic index. This is also why thinly cut instant oats have a higher GI than thicker cut old-fashioned oats.


During cooking and baking, the starches in foods like grains, pasta, breads, and muffins absorb water. This causes the starch granules to swell and rupture, a process known as gelatinization. Gelatinized starch is readily attacked by digestive enzymes and very quickly digested and absorbed. Bread has a high GI partly because the starch in the finely ground flour used to make bread is easily gelatinized. And soft, overcooked pasta has a higher GI than firm, al dente pasta because the overcooked pasta absorbed more water during cooking.

Many of the processing methods used to make extruded, flaked, or puffed cereals and snack foods involve steam-cooking at very high temperatures and pressures. This fully gelatinizes the starch in these foods and contributes to their high glycemic indexes.

The Type Of Starch Present

Starch is a storage form of glucose found in plant foods. Because starch is composed of hundreds or thousands of glucose molecules that are strung together in chains, it is often referred to as complex carbohydrate. Scientists have long believed that because starch has a complex structure, it is more slowly digested than simple sugars. However, the glycemic index has proven this notion to be false.

There are two main kinds of starch present in plant foods-amylose and amylopectin. When these starches are digested, their glucose molecules are liberated and absorbed, causing a rise in blood sugar. However, because of the differences in their chemical structures, these two starches have very different effects on blood sugar.

Amylopectin's structure resembles the branches of a tree and so it is easily attacked by digestive enzymes. Starchy foods that contain a high proportion of amylopectin-like baking potatoes and sticky short-grain rice-are quickly digested and produce rapid rises in blood sugar levels. Amylose, on the other hand, consists of a long, straight chain of tightly packed glucose molecules that resists digestion. Foods high in amylose-such as new potatoes and basmati rice-are absorbed more slowly and have lower glycemic indexes.

The Type Of Sugar Present

Many people are surprised to learn that with the exception of glucose (GI = 100), most sugars have low to moderate glycemic indexes. Fructose, the sugar that occurs naturally in fruits, is very slowly absorbed, giving it a GI of only 23. Lactose, the sugar naturally present in milk and dairy products, has a GI of 46. This is one reason why most fruits and dairy products have such low glycemic indexes. Sucrose (white table sugar), a combination of equal parts fructose and glucose, has a GI of 65. The fact that sucrose is part fructose is one reason why many sweets have a moderate GI.


The naturally occurring acids in fruits, as well as the acids in fermented foods like yogurt, buttermilk, and sourdough bread, slow the rate of digestion and contribute to the low GI of these foods. Likewise, adding just 4 teaspoons of vinegar or lemon juice to a meal can lower the GI of the meal by about 30 percent. For this reason, using vinegar and lemon juice to flavor foods can be a powerful way to lower the GI of your diet.

Excerpted from

Good Carb Cookbook: Secrets of Eating Low on the Glycemic Index
by Sandra Woodruff
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