The impact of glycemic index (GI) and glycemic load (GL) on food intake, blood lipids, the development of insulin resistance, and other metabolic effects has been the subject of numerous studies. These concepts have also been popularized in several best selling diet books like those of Atkins, Sears (The Zone Diet) and Agatston (South Beach Diet). However, nearly all of the studies that have examined the effectiveness of focusing on the GI or GL of the diet have suffered because of either their short-term and/or the failure to control other dietary variables known to impact satiety and ad libitum calorie intake (eat at will or free feeding).
Dietary factors such as calorie density, fiber content, the ratio of macro-nutrients, variety, and the palatability of the foods recommended are all known or suspected to impact satiety and ad libitum energy intake. In addition to the failure to control all of these variables, getting subjects to comply with the dietary restrictions imposed on them by the researchers in longer term studies has made interpretation of the results of most of these studies difficult. Two recent studies, which were longer than most and one of which that did a better job than others at controlling most of the other dietary variables that can impact calorie intake were recently published.
The first of the two studies looked at the impact of high vs. low GL diets on a group of obese young (18-35y) adults. In this study, researchers failed to control calorie density, palatability, and the lower GL diet had 15% more fat calories and 15% fewer carbohydrate calories than the higher GL diet. Nevertheless, the researchers found no significant difference between weight loss after 1.5 years on the high and low GL diets. However, before the start of the study the researchers examined insulin response to an oral glucose load. Those with a greater insulin rise were likely to be insulin resistant (IR). In these IR subjects the researchers observed a somewhat greater weight loss than in those who had better insulin sensitivity despite being obese.1
The second study looked at the impact of high and low GI and GL diets on weight loss of overweight subjects with an average age of 35. In this study calorie density, fiber, palatability, and variety were comparable on the high and low GI & GL diets. The two diets did differ in their macro-nutrient content with the high GL diet being 60% carbohydrate and 20% fat and protein while the low GL diet had only 40% carbohydrate and 30% calories from fat and protein. For the first several months of the study food on both diets was supplied to the subjects by the researchers. For the final 6 months the subjects were instructed on how to continue on the high and low GL diets on their own. The loss of weight and body fat on the two diets was not significantly different after 6 months or 1 year on the high and low GL diets. Importantly, during the first 12 weeks of the study when dietary adherence to the high and low GL diets was best controlled there was no significant difference in hunger ratings of subjects on the two diets.2
Neither of these studies lends much credence to those who claim diets with less carbohydrate, a lower GI and/or a lower GL are more effective at promoting weight loss or reducing hunger. However, the results of the first study suggest that people prone to develop IR might lose a little more weight by limiting processed carbohydrates with a higher GI. Results of the second study could be interpreted to suggest that increasing protein at the expense of carbohydrate could modestly increase satiety. Better to focus on reducing calorie density, increasing fiber, and reducing calories from beverages to promote weight loss without hunger than focus on GI or GL.
By: James J. Kenney, PhD, RD, FACN
1. JAMA 2007;297:2092-102
2. Am J Clin Nutr 2007;85:1023-30
Glycemic Index Facts:
What is glycemic index?
The glycemic index (GI) assigns a numerical value to a food indicating how much and how rapidly 50 grams of its carbohydrate content will raise blood-sugar levels, compared to 50 grams of a reference food (glucose or white bread). The reference food is given a value of 100, and the GI value of a particular food is expressed as a percentage of that value.
What is glycemic load?
Glycemic load is the amount of carbohydrate in a serving of food multiplied by that food's Glycemic Index. Thus, a ½ cup serving of carrots(which has 8 grams of carbohydrate) has a glycemic load of about 10 (8 x 131%,or 1.31 = 10.48).
What is best?
2 recent studies showed that GI?and GL have little or no effect on losing weight. If you want to lose weight you should try to eat foods that are higher in fiber and lower in calorie density. This includes fruits, vegetables and cooked whole grains like brown rice, oatmeal and whole grain pasta. It means eating less fat and sugar and more fiber. FMI see:
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