Glycemic Index

The glycemic index (GI) is, admittedly, a somewhat technical concept, but it simply refers to a way of comparing how different foods affect blood glucose (blood sugar) levels—particularly how quickly those changes occur.

In research, the method has typically been as follows:

Study participants consume portions of different foods, each containing 50 grams of carbohydrates. Blood samples are then collected repeatedly over a period of 120 minutes to measure blood glucose levels. The resulting measurements are plotted on a graph with time on the horizontal axis and blood glucose concentration on the vertical axis.

The blood glucose response curve for each food is then compared with that of white bread. If the two curves overlap completely, both foods are assigned a glycemic index of 100. A food that produces a higher peak than white bread receives a GI value above 100. Conversely, if it raises blood glucose more slowly, producing a flatter response curve, its GI value will be below 100.

It should be noted that the standard used in Sweden is not always directly comparable with GI values reported elsewhere, since the reference food assigned a value of 100 may differ. In some systems, the reference is white bread, while in others it is pure glucose.

Glucose and Insulin
What does this mean in practice? If a food has a high glycemic index, such as instant mashed potatoes with a GI of 116, the pancreas responds by releasing a large amount of insulin into the bloodstream. The purpose of this response is to keep blood glucose levels within a relatively narrow range—high enough to provide the brain and nervous system with the energy they require, but not so high as to become harmful.

When blood glucose rises sharply, a small portion of the excess glucose can be stored in the liver as glycogen, the body’s short-term carbohydrate reserve. Most of the excess, however, is taken up by cells under the influence of insulin and converted into fatty acids, a more compact form of energy storage. The combined action of glucose and insulin also suppresses the breakdown of existing fat stores, reducing fat oxidation.

Ideally, this process returns blood glucose to a stable equilibrium. However, if a food has a particularly high glycemic index, the insulin response may become so strong that blood glucose falls below its pre-meal level. To compensate for this drop, the body may trigger hunger signals, prompting another meal—often another rapidly digested source of carbohydrates—to restore blood glucose levels.

By contrast, if a food has a very low glycemic index, such as soybeans with a GI of 25, digestion and absorption occur much more gradually. Glucose enters the bloodstream slowly, producing no sharp peaks that require large insulin releases. Instead, the glucose is steadily consumed by cells to support cellular respiration, whether in muscle tissue or the energy-demanding cells of the nervous system.

Only after the energy supplied by the soybeans has been largely utilized does the body respond with normal sensations of hunger. At that point, it can also begin releasing glucose from its glycogen stores, a process in which the hormone glucagon plays a central role. If additional energy is needed, the body can also compensate by increasing fat metabolism, a process stimulated in part by the hormone adrenaline.

GI applies only to carbohydrates
The concept of the glycemic index (GI) is not, however, a universal solution for maintaining good health. It applies only to the way carbohydrates behave in the body. Since carbohydrates—which include cellulose, starches, and sugars—are the body’s only source of glucose, the primary fuel for many cells, they are of particular interest in relation to body weight.

According to the text, more recent research has shown that an excess intake of sugar is one of the primary factors causing people to move away from their genetically programmed ideal body weight and become overweight.

The glycemic index is determined by measuring the body’s blood glucose response to a specific food. This relatively recent method has produced several findings that challenged earlier assumptions about carbohydrates. Previously, nutritionists believed that foods such as bread and potatoes were especially healthy because their carbohydrates consist largely of long starch molecules, which were expected to release glucose slowly and steadily into the bloodstream.

Measurements of the glycemic index have shown, however, that glucose from both bread and potatoes is released relatively rapidly. According to the text, this rapid release may contribute to obesity and type 2 diabetes.

Because GI measures the rate at which glucose becomes available, the same raw ingredient can produce very different values depending on how it has been prepared. Potatoes provide a good example. Different potato varieties may have a GI ranging from 80 to 93. If the potatoes are mashed, making them easier for digestive enzymes to break down, the GI rises to around 98. Turning them into French fries increases the GI further to approximately 107. Baking potatoes may raise the value even higher—to 117 or even 135, depending on whether they are cooked in a conventional oven or a microwave.

Somewhat surprisingly, potato chips have a GI of only about 77. According to the text, this is because they contain two major components—fat and carbohydrates—and the presence of fat generally slows the release of glucose into the bloodstream.

The same principle applies to mixed meals. Boiled potatoes eaten together with a protein-rich steak or fish produce a much smaller glycemic response than potatoes eaten by themselves, because the accompanying protein and fat slow digestion and glucose absorption.

Eating according to the GI table
From the discussion above, it may be clear that you do not necessarily need to make dramatic changes to your diet. The key is simply to consult a glycemic index (GI) table to see how different foods affect blood glucose levels, then gradually choose more foods with low GI values while reducing your intake of those with high GI values. Here are a few examples:

Choose (Low to moderate GI)

• Dark chocolate (over 60% cocoa) — GI 40
• Pearl barley, cooked — GI 36
• Fresh peach — GI 40
• Boiled potatoes (waxy/floury varieties) — GI 80/93
• Sweet corn kernels, frozen — GI 67
• Wild rice — GI 60
• Brown rice — GI 79
• Whole wheat kernels, cooked — GI 59

Avoid (High GI)

• Chocolate candy, milk chocolate — GI 70–91
• Barley bread — GI 95
• Muesli — GI 60–94
• Sweetened canned peaches — GI 83
• Baked potato — GI 135
• Instant mashed potatoes — GI 118
• Cornflakes — GI 121
• Taco shells — GI 97
• Rice cakes — GI 117
• Puffed rice cereal — GI 132
• Gluten-free bread — GI 129
• Baguette — GI 136