T wenty years ago, the world had never heard of the glycemic index (GI) because it didn't exist. Nutritionists classified carbohydrates as either simple (sugars) or complex (starches) and that was that. Then, leading American nutritionist Phyllis A. Crapo, RD, proposed a "glycemic index" in her March/April 1984 paper published in Nutrition Today. She suggested that each carbohydrate food should have a number indicating the degree to which it causes blood sugar to rise. While the GI is very helpful in understanding the "power" in your carbohydrates, it requires some education to fully understand and use properly.

GI BASICS

The GI is a measure of how fast and how high a carbohydrate food raises your blood sugar within two to three hours of consumption relative to the rate that pure glucose does. The GI is actually a numerical comparison with the glucose rate. For example, a food that raises blood sugar at the same rate that glucose does would have a GI of 100. One that raises blood sugar just 50% in comparison would have a GI of 50, and so on. GI guidelines per portion of a single food are...

• Low GI = 0 to 54
• Medium GI = 55 to 69
• High GI = 70 or higher

The lower the better when it comes to GI.

The GI is important because the pancreas produces insulin in response to the level of blood glucose. When blood sugar rises quickly, insulin levels do as well, to store the excess glucose for later use. When insulin levels spike in this way, they also plummet quickly. This leads to swings in blood sugar levels, a sudden drop in energy and, often, renewed hunger. It also may, over time, increase the likelihood of reduced insulin sensitivity, a precursor to diabetes. The idea behind diets that focus on the GI is that eating low-GI foods stabilizes blood sugar, with the result that you experience greater, longer satisfaction and are less tempted to rummage around in the refrigerator.

THE NUMBERS BEHIND THE GI

According to Jane Higdon, PhD, research associate at the Linus Pauling Institute at Oregon State University, most people miss a critical component of GI. It's based on portion sizes of any food that supplies 50 grams of carbohydrates, because the glucose that foods are measured against contain 50 grams of carbohydrates and the measurement is gram-to-gram. For some foods (e.g., fudge), that portion size is very small, while for others the portion size may be very large. Case in point: Carrots. You may have heard that carrots have a very high GI -- 92 on some charts. But carrots are a low-carb food. To meet the 50 grams of carbohydrates that the GI measures would require eating 2.2 pounds of carrots!

HOW TO LOWER A FOOD'S GI

To make matters even more complex, there are other factors in addition to the number of carbs in a food that affect a GI. In particular, the GI is affected by anything that causes a food to digest more slowly.

Properties that slow a food's digestion -- and lower its GI -- include Its fiber (unpeeled apples have a lower GI than peeled ones), and fat content. (You actually can lower the GI of popcorn by cooking it in vegetable oil rather than air popping it, because adding fat slows down digestion.) Another property is something she calls "three-dimensionality." She says this doesn't exactly describe fiber, but rather the chunkiness in refined foods -- for example, the bits of grain that remain in coarsely processed flour. The degree of ripeness in the case of fruit also changes the GI (green bananas have a lower GI than ripe ones).

THE GLYCEMIC LOAD

To clarify the role of carbs in the diet, scientists built on the GI to create a second measure, the glycemic load (GL). Dr. Higdon explains that the GL is a better way to measure because it shows the glucose response to a typical serving and it takes into account an entire meal.

To figure out the GL of foods, scientists came up with a somewhat complex mathematical equation. It is not necessary to know the equation, but it is important to remember that the GI of each food is completely different from the GL of the same food. GL guidlines per portion of a single food are...

• Low GL = 0 to 10
• Medium GL = 11 to 19
• High GL = 20 and higher

To put the GL into play, let's return to the example of the carrots. Although their GI is 92, their GL is a lowly 5, reflecting the actual glucose response to a typical serving.

THE GI AND GL IN USE

Whether you use GI or GL to monitor your consumption, keep in mind that the jury is still out on their effectiveness as diet tools. Studies show that people feel fuller after a low-GI meal, but there is no research yet that proves it is effective for long-term weight control. It can, however, be instrumental in keeping blood sugar stable. Of course, don't use the GI to determine all of your food choices. Total calories still count. Furthermore, some foods, such as brown rice with medium-to-high GI and GL, are nutritional bonanzas and should be part of a regular diet (brown rice has a GI of 55 and a GL of 18).

Dr. Higdon recommends using the information that both measures provide, with the goal to eat low-GL meals. Example: If you want a baked potato (GI 85, GL 26), surround it with low GI/GL foods, such as a salad, vegetable and a protein, keeping the total GL of the meal low. GL daily intake should range from a low of 60 to a high of 180. You can lower the GI's impact of a carb if you eat protein first. GI is impacted by both gastric emptying time and speed of digestion in the intestines -- and by adding healthy fats, such as butter on popcorn, potatoes or pasta.

You can lower the ranking of pasta (which falls in the middle range of the GI) if you cook it al dente or mix whole-wheat pasta with regular pasta, and make a sauce with lots of vegetables and some protein. Eat plenty of low-GI foods, such as beans, legumes and fruits, throughout the day, and you'll easily keep within or even below the recommended daily GL of 100. To find the GI and GL of any carbohydrate, click on "GI Database" at www.glycemicindex.com.

Source

• Jane Higdon, PhD, research associate, Linus Pauling Institute, Oregon State University, Corvallis.