A glycemic index (GI) diet is an eating plan based on how foods affect your blood sugar level. The glycemic index is a system of assigning a number to carbohydrate-containing foods according to how much each food increases blood sugar. The glycemic index itself is not a diet plan but one of various tools — such as calorie counting or carbohydrate counting — for guiding food choices.
The term “glycemic index diet” usually refers to a specific diet plan that uses the index as the primary or only guide for meal planning. Unlike some other plans, a glycemic index diet doesn’t necessarily specify portion sizes or the optimal number of calories, carbohydrates, or fats for weight loss or weight maintenance.
The purpose of a glycemic index diet is to eat carbohydrate-containing foods that are less likely to cause large increases in blood sugar levels. The diet could be a means to lose weight and prevent chronic diseases related to obesity such as diabetes and cardiovascular disease. However, you might be able to achieve the same health benefits by eating a healthy diet, maintaining a healthy weight, and getting enough exercise.
Check with your doctor or health care provider before starting any weight-loss diet, especially if you have any health conditions, including diabetes.
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. Depending on your health goals, studies of the benefits of GI diets have produced mixed results.
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.
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. Comparing these values, therefore, can help guide healthier food choices. GI values are generally divided into three categories:
- Low GI: 1 to 55
- Medium GI: 56 to 69
- High GI: 70 and higher
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. 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.
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. 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.
Results of a 16-year study that tracked the diets of 120,000 men and women showed that a low GI diet may also promote weight loss and help maintain weight loss. However, data from another study indicated a substantial range in individual GI values for the same foods. This range of variability in GI values makes for an unreliable guide when determining food choices.
Blood glucose control
Some clinical studies have shown that a low-GI diet may help people with diabetes control blood glucose levels, although the observed effects may also be attributed to the low-calorie, high-fiber content of the diets prescribed in the study.
Reviews of trials measuring the impact of low-GI index diets on cholesterol have shown fairly consistent evidence that such diets may help lower total cholesterol, as well as low-density lipoproteins (the “bad” cholesterol) — especially when a low-GI diet is combined with an increase in dietary fiber. Low- to moderate-GI foods such as fruits, vegetables, and whole grains are generally good sources of fiber.
One theory about the effect of a low-GI diet is appetite control. The thinking is that high-GI food causes a rapid increase in blood glucose, a rapid insulin response and a subsequent rapid return to feeling hungry. Low-GI foods would, in turn, delay feelings of hunger. Clinical investigations of this theory have produced mixed results.
Also, if a low-GI diet suppresses appetite, the long-term effect should be that such a diet would result over the long term in people choosing to eat less and better manage their weight. The long-term clinical research does not, however, demonstrate this effect.
The Bottom Line
Selecting foods based on a glycemic index or glycemic load value may help you manage your weight because many foods that should be included in a well-balanced, low-fat, healthy diet with minimally processed foods — whole-grain products, fruits, vegetables, and low-fat dairy products — have low-GI values.
For some people, a commercial low-GI diet may provide needed direction to help them make better choices for a healthy diet plan. The researchers who maintain the GI database caution, however, that the “glycemic index should not be used in isolation” and that other nutritional factors — calories, fat, fiber, vitamins, and other nutrients — should be considered.
View the Glycemic Index Food List for further information and a list of how common carbohydrate-heavy foods rate.