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Department of Nutritional Sciences, Faculty of Medicine, University of Toronto, and Clinical Nutrition and Risk Factor Modification Center, St. Michael's Hospital; Toronto, Ont., Canada

Abstract

The glycemic index concept owes much to the dietary fiber hypothesis that fiber would reduce the rate of nutrient absorption and increase the value of carbohydrate foods in the maintenance of health and treatment of disease. However, properties and components of food other than its fiber content contribute to the glycemic and endocrine responses postprandially. The aim of the glycemic index classification of foods was therefore to assist in the physiological classification of carbohydrate foods which, it was hoped, would be of relevance in the prevention and treatment of chronic diseases such as diabetes. Over the past two decades low glycemic index diets have been reported to improve glycemic control in diabetic subjects, to reduce serum lipids in hyperlipidemic subjects and possibly to aid in weight control. In large cohort studies, low glycemic index or glycemic load diets (glycemic index multiplied by total carbohydrate) have also been associated with higher levels of high-density lipoprotein cholesterol, reduced C-reactive protein concentrations and with a decreased risk of developing diabetes and cardiovascular disease. More recently, some case-control and cohort studies have also found positive associations between the dietary glycemic index and the risk of colon, breast and other cancers. While the glycemic index concept continues to be debated and there remain inconsistencies in the data, sufficient positive findings have emerged to suggest that the glycemic index is an aspect of diet of potential importance in the treatment and prevention of chronic diseases.

Copyright © 2006 Nestec Ltd., Vevey/S. Karger AG, Basell

The glycemic index concept combines the concepts of the dietary fiber hypothesis and the insulin-resistance syndrome in an attempt to find dietary strategies to prevent and manage chronic disease (diabetes, coronary heart disease - CHD). The original dietary fiber hypothesis predicted that different carbohydrate foods would result in different physiological responses, notably, lower postprandial glycemic responses for the same amount of carbohydrate from high fiber, less processed foods. By the mid 1970s, viscous fibers were shown to effectively blunt both postprandial glucose and insulin responses [1]. Shortly thereafter, different low fiber starchy carbohydrate foods were shown to have different effects on the blood glucose response curve in both healthy and diabetic subjects [2]. In the early 1980s, it was considered important to start to systematically document differences between carbohydrate foods to allow a more rational form of carbohydrate exchange and to provide physiological information on foods rather than simply theoretical assessments based on macronutrient and fiber composition. The glycemic index was therefore developed to address these needs [3].

Glycemic Index Methodology

The glycemic index assesses the blood glucose response of a fixed amount of available carbohydrate (generally 50 g) from a test food to the same amount of available carbohydrate from a standard food (glucose or white bread). Both test and standard foods were taken by the same subject in order to account for individual variation. The test food's blood glucose response area was then expressed as a percentage of the standard's. In general, the insulin responses observed for particular foods were found to be similar to the glycemic responses they elicited [4]. It also appeared that the rate of digestion of the food was a major determinant of the observed glycemic response [5]. Thus, when foods were digested in vitro, the rate of liberation of the carbohydrate products of digestion over 3-5 h reflected the blood glucose area in vivo [5]. A range of intrinsic and extrinsic factors which alter the rate of gastrointestinal motility, digestion and absorption of starchy foods was found to result in differences in glycemic index including the nature of the starch, cooking, particle size, the presence of fiber, fat and proteins [6]. Foods with lower glycemic indices were often the starchy staples of traditional cultures, such as pasta, whole grain pumpernickel breads, cracked wheat or barley, rice, dried peas, beans and lentils. In fact it appeared that the traditional use of low glycemic index carbohydrate foods in the diet was particularly true for those cultures which are now experiencing high rates of diabetes, such as the Pima Indians and the Australian Aborigines, and where the change to high glycemic index foods has been a recent phenomenon [7]. Obviously other lifestyle factors, including reduced physical activity and increased obesity also play a major role in increasing diabetes risk. Nevertheless, it was speculated that the desire for sweetness, resulting from rapid carbohydrate breakdown of refined starches in the mouth, has resulted in the selection of rapidly digested and hence high glycemic index foods as cultures become more affluent. Thus high glycemic index foods are potentially a further dietary factor favoring the development of chronic disease.

Criticisms of the Glycemic Index Concept

It has been argued that the glycemic index lacks clinical utility because differences in glycemic indices between foods are lost once these foods are taken in a mixed meal [8]. This observation may in part be explained by the fact that when a mixed meal consists of several carbohydrate sources, the effect of the lower glycemic index component is diluted in proportion to the amount of carbohydrate from the other foods. It is therefore essential that the mixed meal glycemic index be properly calculated [9]. It has also been said that the addition of small amounts of fat to the meal greatly alter the glycemic response. However studies where 8-24 g fat was fed in mixed meals containing 38-104 g carbohydrate had little effect on the predicted glycemic response [10]. Furthermore while large deviations in the dietary macronutrient profile may occur from time to time in the lives of individuals, these differences are likely to average out over time.

It has also been argued that the glycemic index is a difficult concept to explain to the public and that it will add further needless complication and potential dietary restriction in management and prevention of diseases and that perhaps the modest gains that may be achieved are therefore not justified [11]. For the public, however, the glycemic index may simply be used as a tool for selecting better quality starchy foods. Over time it is hoped that the development of new low glycemic index foods will expand the range of choices to be selected not simply for the glycemic index but also for the fact that the foods in question may have a range of other health advantages. Obviously a certain amount of dietary understanding is required. Thus, carrots with a high glycemic index are not to be excluded from the diet. It is realized that there are other considerations relevant to the consumption of carrots and that the glycemic index is not significant in a low calorie food which contains high levels of other desirable factors (i.e. fiber, vitamins, minerals, etc.).

Metabolic Effects of Low Glycemic Index Foods

It has been hypothesized that the health benefits of low glycemic index foods are due to their metabolic effects, specifically their ability to slow the rate at which glucose is absorbed from the small intestine (fig. 1). Studies in healthy men have demonstrated some of the metabolic effects of reducing the rate of absorption, for example, when glucose solution was sipped at an

Fig. 1. Spreading the nutrient load. Hypothetical effect on of a low glycemic index meal (a) versus a high glycemic index meal (b) on gastrointestinal absorption of carbohydrate. Reproduced with permission by the American Journal of Clinical Nutrition. © Am Clin Nutr. American Society for Nutrition.

even rate over 180 min (sipping), as opposed to being taken as a bolus [12]. A marked economy in insulin secretion with sipping was seen along with lower serum free fatty acid (FFA) levels compared to the bolus (fig. 2). In part, this improvement, also observed after feeding low glycemic index meals, may be the result of sustained tissue insulinization, suppression of FFA release and the absence of a counter-regulatory endocrine response [12]. Other studies using low glycemic index meals have demonstrated an improved second meal effect reminiscent of the Staub-Traugott effect (where the first meal improves the glucose tolerance of the second meal) and related the improved postprandial glycemia of the second standard meal to prolonged suppression of FFA levels [13].

Increased food frequency has also been shown to reduce glycemic and insulinemic responses over the day in diabetic subjects and in longer term studies has been associated with reduced fasting blood lipid concentrations despite consumption of the same foods at the same 24-hour caloric intake [14]. However, spreading the nutrient load does not appear to increase the thermogenic effect of diets, which would favor weight reduction.

It is therefore possible that some of the advantages in glycemia and fasting blood lipids seen after prolonging absorption by reducing the rate of absorption such as by sipping or by increasing the dietary meal frequency, may relate simply to reduced fluxes in nutrient uptake and less perturbation of the endocrine environment.

0 60 120 180 240
Fig. 2. Mean ± SE blood glucose, serum free fatty acid (FFA), insulin, and C-peptide; and plasma gastric inhibitory polypeptide (GIP) after taking glucose solution (50 g in 700 ml water) as a bolus over 5min at time 0 (■) or sipping the same solution over 0-3.5 h at an even rate (□).
Table 1. Clinical trials assessing the effect of low glycemic index (GI) diets on glycosylated proteins in type-1 and 2 diabetes mellitus

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