Understanding the Glycemic Index

Research shows that some carb-containing foods tend to result in a slower increase in blood glucose, while others cause a faster rise. The glycemic index (GI) attempts to identify which foods do what.


You've likely wondered if some carb-containing foods have less impact on blood glucose than others. The researchers who developed the GI concept did, too. The GI ranks carbohydrate-containing foods from 0 to 100. This score indicates how much a single food containing 50 grams of carbohydrate typically raises blood sugar compared with a standard 50 grams of pure glucose (which has a GI score of 100).

The glucose from low-GI foods, such as fresh peaches and kidney beans, tends to be digested and absorbed into the blood slower than glucose from high-GI foods, such as jelly beans and white bread. So for better blood glucose control, you might think it would make sense to always choose lower-GI foods -- but that is wrong.

Before you choose your foods by the numbers, use these tips to understand the GI and how it could work for you.

1. Apply the basics of good nutrition.

If you relied solely on the GI to plan your meals, you'd pick french fries (with a GI of 64) over a baked potato (GI 98) and premium ice cream (GI 37) over grapes (GI 46). The fat in the fries and ice cream slows down the rate of digestion, so you might see a slower rise in blood glucose after eating them. But the fries and ice cream are less nutrient-dense and will saddle you with more calories, saturated fats, and added sugars.

"The GI shouldn't be the sole determinant of food choices. No single nutrition factor should be used in isolation," says Jennie Brand-Miller, Ph.D., professor of nutrition at the University of Sydney in Australia and coauthor of The Low GI Handbook (Da Capo Lifelong Books, 2010). There's no reason to shun foods with high GI numbers either, or you could miss some important nutrients. Pumpkin, for example, has a GI rating of 75, but it also offers vitamin C, potassium, and beta-carotene.

2. Portions matter most.

"You can use the GI in addition to your usual meal plan," says diabetes consultant Patti Geil, RD, CDE, author of What Do I Eat Now? (American Diabetes Association, 2009). When combined with other methods of diabetes meal planning, using the GI to refine your carbohydrate choices can, on average, drop A1C by an additional 0.5 percentage point, a small but meaningful amount. For example, if your initial changes in food choices reduced your A1C to 7.5 percent, fine-tuning your carb choices with the GI could lower it further to 7.0 percent.

But if your plate is overloaded, the GI is unlikely to rein in your blood glucose. It doesn't matter how low a food's GI is -- if large portions result in too much food, your blood sugar will show it, as will your waistline.

3. Combine low- and high-GI foods.

The number flashing on your blood glucose meter after eating is a reflection of your entire meal, not just an individual food. Toby Smithson, RD, CDE, PWD type 1, frequently adjusts her insulin dose to bring down high blood glucose. Even with this flexibility, she prefers to avoid high-GI foods or to combine them with lower-GI foods. "The GI is another tool in your diet toolbox," Smithson says. While it's tempting to focus on one number, such as the GI, you still need to keep your main focus on choosing healthful foods, being physically active, and taking your medications as prescribed, she says.

4. Determine your own response.

GI values are established among people with normal glucose metabolism, but people with diabetes might not respond in the same ways. Your postmeal blood glucose is a picture of your entire meal, as well as your glucose-lowering medications and response. To learn how a meal affects you, check your blood glucose before the meal and again two hours later. If your blood glucose rises more than 40 mg/dl, it's an indication you should change your food choices, portions, or medication.