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Glycemic index definition
Every meal you eat changes your blood sugar level. Digestion releases the glucose stored in food. That glucose then drips, flows, or floods into your bloodstream, depending on the type of food you've eaten.
The glycemic index measures the effect that food has on blood sugar levels. In essence, the index is "a rating system of how fast a food item can raise your blood sugar," says Angela Ginn-Meadow, a registered dietitian in Baltimore and a spokeswoman for the Academy of Nutrition and Dietetics.
Doctors calculate a food's glycemic index by measuring the rise in your blood sugar level two hours after eating it. The food is then ranked on a scale from 0 to 100. The top measure, 100, reflects the rise in blood sugar following consumption of pure glucose.
The glycemic index of your meals can affect your health. Consistently high blood sugar levels can cause type 2 diabetes. Food with a high glycemic index also has been linked to cardiovascular disease and obesity.
What are high and low glycemic foods?
It can be tricky to estimate the glycemic index of different foods. In general, foods high in carbohydrates have a high glycemic index because they break down into glucose more quickly. Foods higher in protein or fats have a lower glycemic index.
But the amount of fiber in a food can offset the carbohydrates it contains. That's because fiber slows digestion, which causes glucose to release more slowly into the bloodstream. "Foods that are higher in fiber don't spike your blood sugar," Ginn-Meadow says. For example, vegetables are mostly carbohydrate, but they also have very high fiber content.
Examples of foods with a high glycemic index include:
- Breakfast cereals
- Fruit juice
- Soft drinks
- White bread
- White potatoes
Other factors also can affect a food's glycemic index. These include:
- Processing. Processed foods tend to have a higher glycemic index because you digest them more easily. Whole wheat bread has about the same glycemic index as white bread. Fruit juice has a higher glycemic index than whole fruit.
- Cooking time. Foods cooked longer have a higher glycemic index. Pasta cooked until it is soft has a higher glycemic index than al dente pasta.
- Ripeness and storage time. Ripe fruits and vegetables tend to have higher glycemic indexes.
- Type. Sometimes glycemic index varies widely within a single food category. Converted long grain white rice has a lower glycemic index than brown rice. At the same time, short grain white rice has a higher glycemic index than either brown rice or long grain white rice.
- Foods eaten with it. Different foods in a meal can affect each others' glycemic index. For instance, dipping bread in olive oil will reduce the glycemic index of the bread.