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Picture an old-fashioned roller coaster with plenty of ups and downs. That's what your blood sugar and insulin levels look like over the course of a day. The highs that follow meals and snacks drop to lows later on. Learning to eat in a way that makes your blood sugar levels look more like a kiddie coaster with gentle ups and downs than a strap-'em-in, hang-on-tight ride with steep climbs and breathtaking drops can make a difference to your health.
How can you do this? A tool called the glycemic index (GI) can help. It rates carbohydrate-containing foods by how much they boost blood sugar (blood glucose). As someone with diabetes, I use the glycemic index as one strategy to keep my blood sugar under control. And there may be other benefits—low glycemic index diets have been linked to reduced risks for cancer, heart disease, and other conditions.
Carbohydrates are the main nutrient in bread, pasta, cereals, beans, vegetables, and dairy foods. All carbs are made up of sugar molecules. Some carbs, like sucrose (table sugar), are just a pair of linked sugar molecules, glucose and fructose. Other carbs, like the starches in potatoes, corn, and wheat, are a tangle of glucose molecules strung together in long chains.
How a carbohydrate-containing food affects blood sugar depends on how quickly the digestive system can break apart the food into its component sugar molecules. It also depends on the sugar molecules present.
The glycemic index measures how much a food boosts blood sugar compared to pure glucose. A food with a glycemic index of 28 boosts blood sugar only 28% as much as pure glucose; one with a glycemic index of 100 acts just like pure glucose. Over the past three decades, researchers have measured the glycemic index of several thousand foods. Click here to see the glycemic index of 100 foods. You can also look up glycemic index values from the University of Sydney's GI website.
Glycemic index and health
New studies on how the glycemic index of a diet affects health are published almost every week. Some of the latest include:
Using the glycemic index
Using the glycemic index to choose a healthier diet is easier than you might think. "It's actually quite simple," says Dr. Jennie Brand-Miller, a professor of human nutrition at the University of Sydney and an advocate of the glycemic index. "Swap high glycemic index foods for low ones." See the table below for examples of these swaps.
Brand-Miller and others suggest three categories of carbohydrate-containing foods:
Low glycemic index (GI of 55 or less): Most fruits and vegetables, beans (Brand-Miller calls beans "star performers"), minimally processed grains, pasta, low-fat dairy foods, and nuts.
Moderate glycemic index (GI 56 to 69): White and sweet potatoes, corn, white rice, couscous, breakfast cereals such as Cream of Wheat and Mini Wheats.
High glycemic index (GI of 70 or higher): White bread, rice cakes, most crackers, bagels, cakes, doughnuts, croissants, waffles, most packaged breakfast cereals.
Choosing healthy, low-GI foods is easier in Australia, where hundreds of foods carry the GI label.
A few caveats
You can't rely on the glycemic index alone for choosing a healthy diet. Some foods, like carrot and watermelon, have a high glycemic index, but a serving contains so little carbohydrate that the effect on blood sugar is small. Others, like sugary soda, have a moderate glycemic index because they contain a fair amount of fructose, which has relatively little effect on blood sugar. But they also pack plenty of glucose, which does boost blood sugar, cautions Dr. Frank Hu, professor of nutrition and epidemiology at Harvard School of Public Health.
The glycemic index of a particular food can also be influenced by what it is eaten with. Olive oil or something acidic, like vinegar or lemon juice, can slow the conversion of starch to sugar, and so lower the glycemic index.
The glycemic index isn't a perfect guide for choosing a healthy diet. But it offers useful information that can help you choose foods that have kinder, gentler effects on blood sugar.
Swaps for lowering glycemic index
Instead of this high glycemic index food
Eat this lower glycemic index food
||Brown rice or converted rice
||Peas or leafy greens