Low-glycemic index diet: What's behind the claims?

A low-glycemic index (low-GI) diet is an eating plan based on how foods affect blood sugar level, also called blood glucose level.

The glycemic index ranks food on a scale from 0 to 100. The low end of the scale has foods that have little effect on blood sugar levels. The high end of the scale has foods with a big effect on blood sugar levels.

A low-GI diet uses the glycemic index as the main guide for meal planning. People also may use the glycemic index as one of many tools for making choices about foods and meals.

Purpose

The purpose of a low-GI diet is to choose foods less likely to raise blood sugar levels.

Why you might follow a low-GI diet

You might choose to follow a low-GI diet because you:

Want to lose weight or keep a healthy weight
Need help planning and eating healthier meals
Need help keeping blood sugar levels from going too high or low as part of a diabetes treatment plan
Want to lower risk of diabetes or diseases of the heart or blood vessels

The glycemic index

The glycemic index is designed to be a food-choice guide for people living with diabetes. An international database is run by the Sydney University Glycemic Index Research Service in Sydney, Australia. The database shows the results of food studies from around the world.

A basic overview of carbohydrates and blood sugar is helpful for understanding low-GI diets.

Carbohydrates

Carbohydrates, also called carbs, are a type of nutrient in foods. The three basic forms are sugars, starches and fiber. Your body breaks down the sugars and starches from carbs. They end up as a type of sugar called glucose. This sugar passes into the bloodstream and is the main source of energy for cells in your body. Fiber passes through your body undigested.

Two main hormones from the pancreas help control glucose in the bloodstream. The hormone insulin moves glucose from the blood into the cells. The hormone glucagon helps release glucose stored in the liver when blood sugar levels are low. This process helps keep the body fueled and blood sugar in balance.

Many different things about food affect how quickly glucose enters the bloodstream.

Understanding GI numbers

The glycemic index ranks the effect food has on blood sugar levels. A low-GI diet suggests foods that have low GI values. The categories are:

Low GI: 1 to 55
Medium GI: 56 to 69
High GI: 70 and higher

In order to assign a rank, also called a GI value, researchers usually compare the effect of eating a food with the effect of eating sugar on blood sugar levels. Sometimes the comparison is made with eating white bread. For example, to test the GI value of cantaloupe, 10 or more healthy people eat enough cantaloupe to digest 50 grams of total carbohydrates. That is about one medium cantaloupe for each person. Over the next two hours, their blood sugar levels are tested several times. On another day, the same 10 people eat or drink 50 grams (12 teaspoons) of sugar. Again, their blood sugar levels are tested several times over two hours.

The researchers compare the results of eating sugar with eating cantaloupe to rank the effect of eating cantaloupe. The GI value for cantaloupe is 65 to 70.

Limits of GI values

The glycemic index doesn't consider how much of a food you are likely to eat during a meal. For example, you likely wouldn't eat a whole medium-sized cantaloupe at once.

To focus on this problem, researchers developed the idea of glycemic load (GL). This number shows the effect on blood sugar levels when you eat a common portion of the food. For example, you might eat one-third of a medium-sized cantaloupe during one meal. The GL value for that much cantaloupe is around 11 or lower.

Sydney University's table of GI values also includes GL values. The GL values are divided into:

Low GL: 1 to 10
Medium GL: 11 to 19
High GL: 20 or more

Other issues

A GI value tells you nothing about other nutritional information. For example, cantaloupe has a medium to high GI score and a medium GL score. But it is a good source of vitamin C, beta carotene and other important nutrients. Whole milk has a low GI value and a low GL value. But it's high in fats and calories. So it may not a good choice for losing or controlling weight.

The published GI database is not a complete list of foods. Instead, it's a list of foods that have been studied. Many nutritious foods with low GI values may not be in the database. The list also includes highly processed foods which may be less nutritious than unprocessed foods. And some foods with low GI values may not be good sources of nutrients.

The GI value of any food item depends on many factors. It matters how the food is prepared and how it is processed. Also, there can be a range in GI values for the same foods. So the values may not be reliable for all food choices.

Diet details

If you follow a low-GI diet, your foods with carbs are mostly limited to choices with low values. You usually will avoid foods with high values. Examples of foods with low, middle and high GI values are:

Low GI: Green vegetables, most fruits, raw carrots, kidney beans, chickpeas and lentils.
Medium GI: Sweet corn, bananas, raw pineapple, raisins, cherries, oat breakfast cereals, and multigrain, whole-grain wheat or rye bread
High GI: White rice, white bread and potatoes

Commercial low-GI diets may refer to foods as having slow carbs or fast carbs. This is because foods with a low GI value are digested and absorbed over a longer time. Foods with high values are absorbed over a shorter time.

Results

Studies of low-GI diets have shown varied results. In general, they have shown a low-GI diet may be helpful for:

Losing weight
Lowering blood pressure
Lowering total cholesterol levels
Improving diabetes management
Lowering the risk of diabetes and heart and blood vessel diseases

Researchers have noted the benefit of the diet may be linked to the nutrient-rich foods and high-fiber foods in the studies. The overall nutritional quality of the food may be more important than the GI value of each food item.

The bottom line

Following a low-GI diet may help you lose weight or keep a healthy weight. It may help you manage a diabetes plan. It may lower your risk of diabetes and heart and blood vessel diseases.

The glycemic index also could be one tool, rather than the main tool, to help you make healthier food choices. The Dietary Guidelines for Americans recommends a focus on healthy dietary patterns and nutrient-rich foods.

A healthy dietary pattern means making consistently healthy choices over time. Foods that fit in that pattern vary. They include a variety of fruits and vegetables that provide vitamins, minerals and fiber. A healthy dietary pattern also includes whole-grain foods that are high in fiber and other nutrients. Beans, legumes, fish, low-fat dairy and lean meats are also good choices.