Glycemic Index and Fat Loss

For many years we were told the keys to losing weight and staying healthy are to exercise and eat a low-fat/high-carbohydrate diet. The public evidently heeded this advice as epidemiological studies have shown that dietary practices of Americans have shifted accordingly. Yet, despite this shift, the incidence of obesity and diabetes in America has reached epidemic proportions, particularly in children and adolescents. This phenomenon has been explained as a result of an increasingly sedentary lifestyle and consumption of nutrient-dense food.

By Richard B. Kreider. Photographs by Gregory James

While these factors have certainly contributed to these problems, studies investigating the efficacy of low-fat/high-carbohydrate diets compared to other dietary approaches have not been too impressive. The findings have led a number of obesity researchers to conclude that there may be more to fat loss than simply reducing fat intake by replacing it with carbohydrates. This discussion addresses how the glycemic index (GI) of carbohydrates affects insulin, fat metabolism, appetite, obesity and weight loss.

WHAT IS GLYCEMIC INDEX?

Dietary carbohydrates are converted to glucose and released into the bloodstream. As a result, insulin is released from the pancreas to facilitate storage of the glucose in the liver and muscle. The faster carbohydrates are digested, the more rapidly glucose and insulin levels increase in the blood. The magnitude that a specific carbohydrate increases blood glucose and insulin can be quantified by measuring the glycemic index (GI).

The GI was developed in 1981 as a potentially better way to classify carbohydrates than referring to them as “simple” and “complex.” The GI of a carbohydrate is determined by measuring the blood glucose response from ingesting a standard amount of carbohydrate (i.e., 50 grams) in a fasted individual. The magnitude of change observed is then compared to the blood glucose response to ingesting a standard type of carbohydrate (white bread or glucose), which is given a score of 100. The GI of a food is influenced by a number of factors including: type of carbohydrate; fiber content; methods of food processing and cooking; ripeness of food; food storage methods; types of preservatives used; and types of macronutrients and micronutrients ingested with the carbohydrate.3 The higher the GI, the greater the increase in glucose and insulin levels in the blood. Table 1 presents the GI of some common foods in comparison to a white bread standard. Foods with GIs of less than 70 are typically considered low to moderate GI foods, while foods with GIs of 70-160 are typically considered high GI foods.

ROLE OF GI ON HEALTH

There are several ways ingesting high GI diets may affect obesity and health. First, when you consume a high GI food, blood glucose and insulin levels increase, promoting storage of glucose into the liver and muscle. Insulin also serves to increase carbohydrate oxidation while suppressing fat utilization. Conversely, blood glucose and insulin levels are increased to a lesser degree when ingesting a carbohydrate with a low GI that allows fat to be used as a metabolic fuel to a greater degree than carbohydrate.6