Glycemic Index Diets

Glycemic index diets rank carbohydrates based on their ability to affect blood glucose (sugar) levels. These diets generally consider foods high in carbohydrates, such as bread, sugar, and pasta, as “bad,” and low carbohydrate foods, such as meat, fish, and dairy products, as “good.”

Glycemic index values of common food items

SOURCE: Harvard Medical School. “Glycemic Index and Glycemic Load for 100+ Foods.” Original data from Atkinson, Fiona S., Kaye Foster-Powell, and Jennie C. Brand-Miller. “International Tables of Glycemic Index and Glycemic Load Values: 2008.” Diabetes Care 31, no. 12 (December 2008): 2281–83.

Low-glycemic diet concepts were first developed in the 1960s and were originally designed for individuals with diabetes. At that time, the prevailing medical attitude was that a diet emphasizing well-balanced foods while paying special attention to carbohydrates (carbs) and avoiding carbohydrate-rich foods helped to control blood sugar and insulin levels. This came after a number of medical studies linked eating foods high in carbohydrates with elevated blood glucose levels in people with diabetes. In the 1980s, researchers developed the glycemic index (GI).

Since 1981, dozens of low-carb diets and diet books using the glycemic index have come out. Among the more popular glycemic index–inspired diets are the Sugar Busters diet, Zone diet, Protein Power diet, Suzanne Somers diet, and South Beach diet.

In 1997, epidemiologist and nutritionist Walter Willett of the Harvard School of Public Health developed the glycemic load as a more accurate way of rating carbohydrates than the glycemic index. This is because the glycemic load factors in the amount of a food eaten, whereas the glycemic index does not. The glycemic load of a particular food is determined by multiplying the amount of net carbohydrates in a serving by the glycemic index and dividing that number by 100. Net carbohydrates are determined by taking the amount of total carbohydrates and subtracting the amount of dietary fiber. For example, popcorn has a glycemic index of 72, which is considered high. However, a serving of two cups has 10 net carbs because of its high fiber content, for a glycemic load of seven, which is considered low.

Glycemic index (GI) diets vary in specifics, but most have one simple rule: people can eat as much food as they want providing the foods have a low glycemic index ranking. Most foods that are rated high on the glycemic index contain high levels of carbohydrates. Some people with diabetes use the GI as a guide in selecting foods and planning meals. The GI ranks foods based on their effect on elevating blood sugar levels. Foods with a high GI tend to increase blood glucose levels higher and faster than foods with a low GI value. The GI is not a measure of a food's calorie content or nutritional value.

The GI is a ranking of carbohydrate foods that individuals with diabetes can use to manage their disease. The ranking is based on the rate at which carbohydrates affect blood glucose levels relative to pure glucose or white bread. Generally, the glycemic index is calculated by measuring blood glucose levels following the ingestion of a carbohydrate. This blood glucose value is compared to the blood glucose value acquired following an equal carbohydrate dose of glucose or white bread. Glucose is absorbed into the bloodstream faster than any other carbohydrate, and is thus given the value of 100. Other carbohydrates are given a number relative to glucose. The lower the GI of a food, the slower the rate with which it is absorbed into the bloodstream.

A number of factors influence the digestion and absorption rate of food, including ripeness, particle size, the nature of the starch, the degree of processing and preparation, the commercial brand, and the characteristics of the individual consuming the food. These factors naturally affect each food's glycemic index rank. In addition, differences exist in various glycemic indexes of foods due to the choice of reference food, the timing of blood sampling, or the computational method used to calculate the glycemic index.

The following is the GI for a few common foods:

The GI is not a straightforward formula when it comes to reducing blood sugar levels. Various factors affect the GI value of a specific food, such as how the food is prepared (boiled, baked, sautéed, or fried, for example) and what other foods are consumed with it.

The following foods are acceptable on a low glycemic index diet:

Glycemic index diets have two separate functions. The first is to help individuals with diabetes or insulin resistance syndrome maintain normal and steady blood glucose levels. The second is to aid in weight loss.

The objectives of insulin management in diabetic patients are to reduce hyperglycemia, prevent hypoglycemic episodes, and reduce the risk of complications. For people with diabetes, the glycemic index is a useful tool for planning meals to achieve and maintain control of blood glucose. Foods with a low glycemic index release sugar gradually into the bloodstream, producing minimal fluctuations in blood glucose. High-value GI foods, however, are absorbed quickly into the bloodstream, causing an escalation in blood glucose levels and increasing the possibility of hyperglycemia. The body compensates for the rise in blood sugar levels with an accompanying increase in insulin, which within a few hours can cause hypoglycemia. As a result, awareness of the glycemic index values of food assists in preventing large variances in blood glucose levels.

Athletes may also use GI diets to prepare for athletic competitions or to recover from training. Low GI is often favored before an event, while higher GI aids in the replenishment of glycogen stores.

There is conflicting scientific research on the benefits of a low glycemic index diet for both people with diabetes and people trying to lose weight. Glycemic index diets may help people with diabetes maintain constant levels of blood glucose. By consuming more fruits and vegetables and whole grains rather than processed foods, low-glycemic diets encourage higher fiber consumption.

Experts disagree regarding the use of the glycemic index for athletes' diets and exercise performance. Research published in the January 2010 issue of Sports Medicine found that eating a low-glycemic meal prior to prolonged exercise may have some merit, though this effect may be minimized if carbohydrates are consumed during the activity. Regardless, a low-GI pre-event meal may be beneficial for athletes who respond negatively to carbohydrate-rich foods prior to exercise or who cannot consume carbohydrates during competition. Athletes are advised to consume carbohydrates of moderate to high-GI foods during prolonged exercise to maximize performance, approximately one gram per minute of exercise. Following exercise, moderate to high-GI foods enhance glycogen storage.

If an individual has health concerns, a low glycemic index diet should be undertaken only under the supervision of a doctor. Doctor supervision of the GI diet is not necessary when the individual is healthy and disease free. People with diabetes should consult an endocrinologist, who may recommend discussing the diet with a diabetes dietitian.

Eating a diet based solely on the glycemic index of foods can lead to overeating and a weight gain rather than loss. No emphasis is placed on total calorie intake or on the amount of saturated fat content. By basing one's diet on glycemic index alone, it is still possible to eat excess calories and to gain weight.

There is mixed acceptance of glycemic index diets by the medical community. Some studies have shown GI diets can be effective in controlling blood sugar levels in people with diabetes and in helping people lose weight. Other studies have contradicted these findings. No major studies or research have shown that GI diets are harmful to a person's health. The American Diabetes Association has adopted the position that there is not enough conclusive evidence to recommend the general use of a low-GI diet for people with diabetes. Not all physicians and endocrinologists (medical specialists who treat disorders of the glands, including diabetes) subscribe to the association's position.

See also Acne diet ; Bodybuilding diet ; Carbohydrates ; Diabetic diet ; High-protein diet ; Low-sugar diets ; South Beach diet ; Whole grains ; Zone diet .