Glycemic index diet: What's behind the claims
Medically reviewed on August 1, 2017
A glycemic index 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.
Many popular commercial diets, diet books and diet websites are based on the glycemic index, including the Zone Diet, Sugar Busters and the Slow-Carb Diet.
The purpose of a glycemic index (GI) 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.
Why you might follow the GI diet
You might choose to follow the GI diet because you:
- Want to lose weight or maintain a healthy weight
- Need help planning and eating healthier meals
- Need help maintaining blood sugar levels as part of a diabetes treatment plan
Studies suggest that a GI diet can help achieve these goals. 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.
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.
A GI diet prescribes meals primarily of foods that have low values. Examples of foods with low, middle and high GI values include the following:
- Low GI: Green vegetables, most fruits, raw carrots, kidney beans, chickpeas, lentils and bran breakfast cereals
- Medium GI: Sweet corn, bananas, raw pineapple, raisins, oat breakfast cereals, and multigrain, oat bran or rye bread
- High GI: White rice, white bread and potatoes
Commercial GI diets may describe foods as having slow carbs or fast carbs. In general, foods with a low GI value are digested and absorbed relatively slowly, and those with high values are absorbed quickly.
Commercial GI diets have varying recommendations for portion size, as well as protein and fat consumption.
Depending on your health goals, studies of the benefits of GI diets have produced mixed results.
Results of a 16-year study that tracked the diets of 120,000 men and women were published in 2015. Researchers found that diets with a high GL from eating refined grains, starches and sugars were associated with more weight gain.
Other studies show 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
Studies show that the total amount of carbohydrate in food is generally a stronger predictor of blood glucose response than the GI. Based on the research, for most people with diabetes, the best tool for managing blood glucose is carbohydrate counting.
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
In order for you to maintain your current weight, you need to burn as many calories as you consume. To lose weight, you need to burn more calories than you consume. Weight loss is best done with a combination of reducing calories in your diet and increasing your physical activity and exercise.
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.