The glycemic index
The GI principle was first developed as a strategy for guiding food choices for people with diabetes. An international GI database is maintained by Sydney University Glycemic Index Research Services in Sydney, Australia. The database contains the results of studies conducted there and at other research facilities around the world.
A basic overview of carbohydrates, blood sugar and GI values is helpful for understanding glycemic index diets.
Carbohydrates, or carbs, are a type of nutrient in foods. The three basic forms are sugars, starches and fiber. When you eat or drink something with carbs, your body breaks down the sugars and starches into a type of sugar called glucose, the main source of energy for cells in your body. Fiber passes through your body undigested.
Two main hormones from your pancreas help regulate glucose in your bloodstream. The hormone insulin moves glucose from your blood into your cells. The hormone glucagon helps release glucose stored in your liver when your blood sugar (blood glucose) level is low. This process helps keep your body fueled and ensures a natural balance in blood glucose.
Different types of carbohydrate foods have properties that affect how quickly your body digests them and how quickly glucose enters your bloodstream.
Understanding GI values
There are various research methods for assigning a GI value to food. In general, the number is based on how much a food item raises blood glucose levels compared with how much pure glucose raises blood glucose. GI values are generally divided into three categories:
- Low GI: 1 to 55
- Medium GI: 56 to 69
- High GI: 70 and higher
Comparing these values, therefore, can help guide healthier food choices. For example, an English muffin made with white wheat flour has a GI value of 77. A whole-wheat English muffin has a GI value of 45.
Limitations of GI values
One limitation of GI values is that they don't reflect the likely quantity you would eat of a particular food.
For example, watermelon has a GI value of 80, which would put it in the category of food to avoid. But watermelon has relatively few digestible carbohydrates in a typical serving. In other words, you have to eat a lot of watermelon to significantly raise your blood glucose level.
To address this problem, researchers have developed the idea of glycemic load (GL), a numerical value that indicates the change in blood glucose levels when you eat a typical serving of the food. For example, a 4.2-ounce (120-gram, or 3/4-cup) serving of watermelon has a GL value of 5, which would identify it as a healthy food choice. For comparison, a 2.8-ounce (80-gram, or 2/3-cup) serving of raw carrots has a GL value of 2.
Sydney University's table of GI values also includes GL values. The values are generally grouped in the following manner:
- Low GL: 1 to 10
- Medium GL: 11 to 19
- High GL: 20 or more
A GI value tells us nothing about other nutritional information. For example, whole milk has a GI value of 31 and a GL value of 4 for a 1-cup (250-milliliter) serving. But because of its high fat content, whole milk is not the best choice for weight loss or weight control.
The published GI database is not an exhaustive list of foods, but a list of those foods that have been studied. Many healthy foods with low GI values are not in the database.
The GI value of any food item is affected by several factors, including how the food is prepared, how it is processed and what other foods are eaten at the same time.
Also, there can be a range in GI values for the same foods, and some would argue it makes it an unreliable guide to determine food choices.