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.

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
• Cookies
• White bread
• Watermelon
• Pineapple
• 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.

Eating lots of foods with a high glycemic index can keep your blood sugar levels elevated. This can cause you to become resistant to insulin, the hormone that converts blood sugar into energy for cells. Insulin resistance will result in type 2 diabetes. Two large United States population studies found a diet containing high glycemic index foods and high fat content can double the risk of type 2 diabetes.

Foods with a high glycemic index also have been linked to cardiovascular disease. In fact, a low glycemic index diet can have a more positive impact on cardiovascular health than any other dietary factor. A 2012 review of available scientific evidence found that a high glycemic index diet increases the risk of cardiovascular disease by 23 percent. The risk ran higher for women than men.

A high glycemic index diet also is associated with obesity. People who eat foods with a high glycemic index tend to have a higher body mass index. Conversely, people who eat low glycemic index food tend to lose weight and gain a healthier BMI. However, portion sizes do matter -- eating a lot of low glycemic index food will cause you to gain weight if you take in more calories than you burn.

So how can you use the glycemic index to plan meals and improve your health? It isn't easy. According to Ginn-Meadow, dietitians find that it is most helpful to calculate a meal's total "glycemic load" to determine the overall effect that it will have on your blood sugar levels.

"Instead of consuming all foods high in carbohydrates at one meal, we also incorporate protein and heart-healthy oil," says Ginn-Meadow. "When we do that, we're able to lower that glycemic load."

To best use the glycemic index to guide your dietary decisions, you should:

• Generally stick to foods with low or medium glycemic index values.
• Include a mix of healthy foods with low and high glycemic index values when planning meals.
• Keep in mind that many nutritious foods have a higher glycemic index than foods with little nutritional value. Oatmeal has a higher glycemic index than chocolate, for example.
• Take portion sizes into account. The number of calories you eat matters just as much as the glycemic index of the food.

"If someone is trying to improve their meal quality, it can be a tool to use," says Ginn-Meadow. "That doesn't mean you can't have something high in glycemic index. It just means you should eat [foods with a low glycemic index] more often." As is often seen in most complicated biological systems, too much or too little of a dietary component is not good for the system; moderation of a dietary component, even one as important as glucose, is the better choice.