Nutrition by the Numbers: Making Sense of the Glycemic Index

Can glycemic measures help you make smart carb choices?

By Elaine Magee, MPH, RD
WebMD Weight Loss Clinic - Expert Column

Reviewed By Kathleen Zelman, MPH, RD/LD

Low glycemic, high glycemic ... If you pay any attention to nutrition news, you've heard these terms more and more lately.

But what does it all mean? And is the glycemic index or GI -- essentially, a number that says how much your blood sugar rises after you eat a particular food that contains carbohydrates -- really the be-all, end-all to nutrition and health?

Well, not exactly.

High-GI foods (generally, things like white bread and white rice) give you a quick blood sugar boost that also fades quickly, leaving you hungry again. Lower-GI foods (think whole grains, produce, and beans) keep you feeling full longer as your blood-sugar levels rise more slowly.

But many researchers don't consider the glycemic index a valid tool. That's because it's based on how blood sugar rises in response to one particular food, such as carrots or rice. But we don't sit down to just a bowl of carrots or a plate of rice, do we? We eat foods together, as dishes and meals.

The presence of fat or fiber in a meal also influences how quickly our bodies metabolize the carbohydrates. So do some other factors, like how long noodles are cooked, or how finely grain is ground. (Slightly undercooked noodles are absorbed more slowly and have a lower GI; while the more finely a grain is ground, the more quickly its carbs are absorbed.)

What Exactly Is the Glycemic Index?

The glycemic index calculates how high your blood sugar rises in two hours after you eat a food containing roughly 50 grams of carbohydrates, compared to how much it rises after you eat a 50 gram serving of white bread or 50 grams of pure glucose (sugar).

The higher the GI for a certain food, the faster your body absorbs the carbs from that food. A lower GI means a food has a slower rate of carbohydrate absorption, and thus lower blood sugar and insulin peaks.

Here's where the controversy kicks in: whether low-GI foods lead to weight loss, lower blood sugar levels, and/or a reduced risk of heart disease and cancer compared with high-GI-foods.

After reviewing the existing research, the American Institute for Cancer Research (AICR) concluded that there was not enough evidence to recommend that people change their diets based on the glycemic index.

Enter a potentially more useful tool: the glycemic load, a more accurate measure of a food's effect on blood sugar levels.

A Better Measure

Think of the glycemic load as the glycemic index with attitude.

The GI tells you how quickly a particular carbohydrate in food makes your blood sugar rise, but it doesn't take into account how many carbohydrates are found in a serving. That means that some healthy, but relatively lower-carb, foods -- like carrots -- end up with a high GI number.

The glycemic load, meanwhile, takes the number of carbs per serving into consideration along with the food's glycemic index. To find a food's glycemic load, you basically multiply its GI value by number of carbohydrates per serving.

So the glycemic load allows us to compare the likely effect on blood sugar of realistic serving sizes of different foods.

What Influences the Glycemic Load/Index?

Many factors help determine your body's glycemic response to a particular food, including:

  • Physical form, such as a whole apple vs. applesauce. Mashing foods tends to give them a higher glycemic index/load.
  • Ripeness. The riper the fruit, the higher its glycemic index.
  • Fiber -- particularly viscous fiber, a type of soluble fiber found in oats, barley, and other foods. Generally, the higher the fiber, the lower the glycemic index/load.
  • Acidity. The higher a food's acidity, the lower its glycemic index/load.
  • Processing. The more processed or refined a food, generally, the higher its glycemic index/load will be. When a grain is in a more "whole" form, your body's digestive enzymes have a tougher time breaking it down, which lowers the glycemic response to it. (There are some notable exceptions: pasta, and parboiled and basmati rice tend to have lower glycemic indexes, especially if they're not overcooked.)
  • Whether protein and fat were eaten with the food. The presence of high amounts of protein and fat will decrease the glycemic index/load.

The following foods, even in large amounts, when eaten alone are not likely to cause a significant rise in blood sugar because they contain little carbohydrate: meat, poultry, fish, avocados, salad vegetables, eggs, fish, and cheese.

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