Glycemic index diet: What's behind the claims

By Mayo Clinic Staff

Glycemic index diet is a general term for weight-loss diets that are based on your blood sugar level. Many popular commercial diets, diet books and diet websites revolve around the glycemic index, including Nutrisystem, the Zone diet and Sugar Busters.

A glycemic index diet uses the glycemic index to guide your eating plan. The glycemic index was originally developed to help improve blood sugar control in diabetes. The glycemic index classifies carbohydrate-containing foods according to their potential to raise your blood sugar level.

The glycemic index diet is not a true low-carbohydrate diet because you don't have to count carbohydrates (carbs). Nor is it a low-fat diet. It also doesn't require you to reduce portion sizes or count calories. But the glycemic index diet does steer you toward certain types of carbs.

Purpose

Diets based on the glycemic index suggest that you eat foods and beverages with low glycemic index rankings to help you keep your blood sugar balanced. Proponents say this will help you lose weight and reduce risk factors for certain chronic diseases.

Why you might follow the glycemic index diet
You might choose to follow the glycemic index diet because you:

  • Want to change blood sugar imbalances related to your current diet
  • Want to change your overall eating habits
  • Don't want to count calories or go low-carb
  • Want a diet that you can stick to for the long term

Check with your doctor or health care provider before starting any weight-loss diet, especially if you have any health conditions, including diabetes.

Diet details

Proponents of the glycemic index diet, sometimes called a low GI diet, say that high blood sugar levels are linked to a variety of health problems, including diabetes, obesity and heart disease. They say that following a diet based on the glycemic index can help you choose foods that will result in weight loss and prevention of chronic diseases. But scientific evidence supporting the role of the glycemic index diet in weight loss remains mixed. And you might be able to achieve the same health benefits by eating a healthy diet, maintaining a healthy weight and getting enough exercise.

Blood sugar basics
Sugar (glucose) is a main source of energy for the cells that make up your muscles and other tissues. Glucose comes from two major sources: carbohydrates in food and extra stores in your liver. Carbohydrates come in the form of sugar, starch and fiber. After you eat or drink something with carbs, your body breaks down each type of carbohydrate in essentially the same way, converting it into sugar. The exception is fiber, which passes through your body undigested. The sugar then enters your bloodstream. From there, it enters individual cells throughout your body to provide energy. Extra sugar is stored in your liver and muscles in a form called glycogen.

Two hormones from your pancreas help regulate the level of blood sugar. The hormone insulin moves sugar from your blood into your cells when your blood sugar level is high. The hormone glucagon helps release the sugar stored in your liver when your blood sugar level is low. This process helps keep your body fueled and ensures a natural balance in blood sugar.

Blood sugar imbalance
Some food is thought to disrupt this natural balance by creating large spikes in your blood sugar level. When your blood sugar and insulin levels stay high, or cycle up and down rapidly, your body has trouble responding and over time this could contribute to insulin resistance. Insulin resistance is associated with a host of health problems, including:

  • Type 2 diabetes
  • Obesity
  • High blood pressure
  • Stroke
  • Heart disease

Glycemic index ranking
The glycemic index ranks foods and beverages based on how they affect your blood sugar level. Foods are scored on a scale of 0 to 100. Only foods and beverages that contain carbs are ranked, since they have the biggest effect on blood sugar. You can find extensive lists online and in books of GI rankings, but many foods and beverages remain unranked. Manufacturers can pay to have their brand-name products ranked by Sydney University Glycemic Index Research Services in Sydney, Australia, which maintains a comprehensive database of glycemic index values for carbohydrate-containing foods.

Foods ranked by the glycemic index are given scores:

  • High: 70 and up. Examples include instant white rice, brown rice, plain white bread, white skinless baked potato, boiled red potatoes with skin and watermelon.
  • Medium: 56 to 69. Examples include sweet corn, bananas, raw pineapple, raisins and certain types of ice cream.
  • Low: 55 and under. Examples include raw carrots, peanuts, raw apple, grapefruit, peas, skim milk, kidney beans and lentils.

With the glycemic index diet, a high glycemic index is undesirable. Proponents say that foods and beverages with high glycemic index scores are rapidly digested by your body. This causes a spike in your blood sugar, which may then be followed by a rapid decline in blood sugar, creating wide fluctuations in your blood sugar level. In contrast, items with low glycemic index rankings are digested more slowly, raising blood sugar in a more regulated and gradual way.

Because low glycemic index foods are absorbed more slowly, they stay in your digestive tract longer. This is why these foods are sometimes called slow carbs. These foods may help control appetite and delay hunger cues, which can help with weight management. Balanced blood sugar also can help reduce the risk of insulin resistance.

Typical menu for a glycemic index diet
Many commercial diets are based on the glycemic index. What you can eat depends on the specific commercial diet you follow. Sydney University's glycemic index website doesn't promote specific commercial weight-loss plans or label carbs as good or bad. Rather, it recommends that you use the glycemic index to help you choose what foods to eat and suggests that you:

  • Focus on breakfast cereals based on oats, barley and bran
  • Choose breads with whole grains, stone-ground flour or sourdough
  • Eat fewer potatoes
  • Eat plenty of fruits and vegetables
  • Avoid oversized portions of rice, pasta and noodles
Aug. 24, 2011 See more In-depth