Glycemic Index

What is the Glycemic Index?

The glycemic index is a ranking of carbohydrates based on their immediate effect on blood glucose (blood sugar) levels. It compares foods gram for gram of carbohydrate. Carbohydrates that breakdown quickly during digestion have the highest glycemic indexes. The blood glucose response is fast and high. Carbohydrates that break down slowly, releasing glucose gradually into the blood stream, have low glycemic indexes.

What is the Significance of Glycemic Index?

• Low GI means a smaller rise in blood glucose levels after meals
  
• Low GI diets can help people lose weight
  
• Low GI diets can improve the body's sensitivity to insulin
  
• High GI foods help re-fuel carbohydrate stores after exercise
  
• Low GI can improve diabetes control
  
• Low GI foods keep you fuller for longer
  
• Low GI can prolong physical endurance

What is Glycemic Load?

• Glycemic load builds on the GI to provide a measure of total glycemic response to a food or meal
  
• Glycemic load = GI (%) x grams of carbohydrate per serving
  
• One unit of GL ~ glycemic effect of 1 gram glucose
  
• You can sum the GL of all the foods in a meal, for the whole day or even longer
  
• A typical diet has ~ 100 GL units per day (range 60 - 180)
  
• The GI database gives both GI & GL values

How to Switch to a Low GI Diet

• Use breakfast cereals based on oats, barley and bran
  
• Use "grainy" breads made with whole seeds
  
• Reduce the amount of potatoes you eat
  
• Enjoy all types of fruit and vegetables (except potatoes)
  
• Eat plenty of salad vegetables with vinaigrette dressing
  
  
  

Glycemic Index Range

• Low GI = 55 or less
  
• Medium GI = 56 - 69
  
• High GI = 70 or more
  

Measuring the GI

To determine a food's GI rating, measured portions of the food containing 10 - 50 grams of carbohydrate are fed to 10 healthy people after an overnight fast. Finger-prick blood samples are taken at 15-30 minute intervals over the next two hours. These blood samples are used to construct a blood sugar response curve for the two hour period. The area under the curve (AUC) is calculated to reflect the total rise in blood glucose levels after eating the test food. The GI rating (%) is calculated by dividing the AUC for the test food by the AUC for the reference food (same amount of glucose) and multiplying by 100 (see Figure 1). The use of a standard food is essential for reducing the confounding influence of differences in the physical characteristics of the subjects. The average of the GI ratings from all ten subjects is published as the GI of that food.

The GI of foods has important implications for the food industry. Some foods on the Australian market already show their GI rating on the nutrition information panel.Terms such as complex carbohydrates and sugars, which commonly appear on food labels, are now recognised as having little nutritional or physiological significance. The WHO/FAO recommend that these terms be removed and replaced with the total carbohydrate content of the food and its GI value. However, the GI rating of a food must be tested physiologically and only a few centres around the world currently provide a legitimate testing service.

Glycemic Index Foods