What is Glycemic Index GI?
When we eat carbohydrates (including starch and sugars), they are digested and converted into glucose, a simple sugar, by our bodies. Glucose is then absorbed and therefore enters into the blood stream providing energy for our daily activities. Glycemic Index is a standardized system of ranking foods based on their effect on blood glucose levels over 2 - 3 hours compared to a reference food. Foods that are digested and absorbed faster will have a higher glycemic index.
Glycemic Index Range Low 55 or less Intermediate 55 - 70 High 70 or higher | ||||||||
Background
The chemical nature of carbohydrates is not a reliable indicator of their physiological effect. To help clarify the physiological impact of carbohydrates the Glycemic Index (GI) concept was developed and published in 1981 by Jenkins, Wolever et al (1) working at the University of Toronto. It is a means of classifying foods based on their potential to increase blood glucose. The initial objective was to provide improved advice to diabetics regarding their carbohydrate intake. During the last 20 years many clinical studies and other scientific research have shown that the concept can be applied to single foods, mixed meals and whole diets, and to people with or without diabetes. The glucose response to foods inherently varies from person to person and according to the context in which the food is consumed. However, reproducible and consistent GI measurements can be achieved when undertaken in accordance with established methodology. Professor Jennie Brand-Miller and colleagues at the University of Sydney have developed a center of excellence for the measurement of Glycemic Index and Tables have been published ranking over 1000 foods by GI, including pure carbohydrates and commercial products (2). Refer to www.glycemicindex.comfor information on GI, Sydney University Glycemic Index Research Service (SUGiRS) and associated publications.
Clinical significance of GI
Health problems related to being overweight are becoming the major health concern of the industrialized world. The World Health Organization and Food and Agriculture Office of the United Nations (WHO/FAO) have stated that, globally, overweight is a bigger problem than undernourishment and they have recommended that people in industrialized countries should base their diets on low GI foods to prevent most common diseases of affluence (3).
High glycemic foods stimulate a high insulin demand which in turn can lead to postprandial hypoglycemia which is implicated in triggering hunger. Lower glycemic foods usually provoke less insulin demand and less possibility of hypoglycemia. Hence, lower glycemic foods may help consumers to eat fewer calories. It should be noted, however, that the relationship between GI and insulin demand is not always linear and various components of the food can modify response.
The clinical significance of GI remains the subject of intense debate. However, it is clear that the rate of carbohydrate absorption after a meal, as quantified by GI, has significant effects on postprandial hormonal and metabolic responses. High GI meals may promote excessive food intake, beta cell dysfunction, dyslipidemia, protein glycosylation and endothelial dysfunction. Thus, the habitual consumption of high GI foods may increase risk factors associated with obesity, type-2 diabetes, and heart disease. Conversely, the consumption of foods that mediate low glycemic responses may help to reduce such risk factors. Glycemic load (GL) is defined as the product of glycemic index and carbohydrate content. GL may be a better predictor of likely health outcomes than GI per se.
In July 2002, the American Journal of Clinical Nutrition published an excellent review of the history of glycemic index, the current status of our knowledge and recommendations for future studies (4-9). Firstly Ludwig and Eckel introduced the subject (4), Jenkins et althen provided an overview of its implications in health and disease (5) and Willett and coworkers discussed the concept relative to type 2 diabetes (6). Brand-Miller et al then summarized the data on GI and obesity (7), Leeds addressed heart disease (8) and finally Pi-Sunyer provided a critique, arguing that insufficient data exists on which to base dietary advice for the achievement of disease risk reduction (9). On reading through this review, it is apparent that an enormous amount of effort has been applied to understanding the role of dietary carbohydrates in human health and disease risk. While there is no definitive proof that reducing glycemic impact will prevent disease in individuals, epidemiological data together with prospective and intervention studies provide a compelling case that reducing the glycemic effect of the diet could reduce disease risk. There are no contraindications other than that consumers may have difficulty in understanding what action they need to take in order to reduce glycemic challenge.
The European perspective
There is growing awareness in Europe of the concept of GI among healthcare professionals and more enlightened consumers. The objective is to improve understanding of glycemic control. Media citations have become frequent, particularly in the UK, Germany and Scandinavia. The following extract from the UK ‘Daily Mail’ 4 November, 2003, Page 33 is typical:
Slimmer start to the day
... ... ...Experts at Oxford Brookes University calculated the glycemic index (GI) of foods by comparing the rise in blood glucose after eating different types. They studied children aged nine to 12 and found those who had a low-GI Breakfast ate more moderately for the rest of the day. But those given high GI breakfasts were more likely to be hungry by Lunchtime.
To date there is no harmonized legislation in Europe relating to nutrition and health claims on food products. However, the European Commission proposal for a regulation on nutrition and health claims –COM (2003) 424 final, has been adopted by the Commission and is currently under consideration by the European Parliament and Council of Ministers. This proposal does not currently include mention of GI as a specified nutrition claim. There is a provision for including new claims at a later date but it is considered important that glycemic labeling be adopted at the outset in order to enable consumers to derive maximum benefit. A proposal relating to glycemic labeling has already been developed and is being forwarded to Member States. The recommendation is to include a nutrition claim as follows:
“Reduced glycemic: A claim that a food is reduced glycemic, and any claim likely to have the same meaning for the consumer, may only be made where the glycemic effect is at least 30% reduced compared to conventional foods of the same type without increasing the fat content, with an indication of the characteristic(s) which makes the food reduced in its glycemic effect.”
In the meantime the National Food Administration of Sweden has formally endorsed labeling of foods based on their GI and their potential to slow glucose uptake. The first commercial products making this claim are now on the market in Sweden. Elsewhere in Europe glycemic labeling has become popular despite lack of a clear legal framework. The primary objectives of food labeling are to inform and not mislead consumers. In this spirit major companies have introduced products with GI labeling and information.
The USA perspective
Health professionals in the USA have generally been slower to endorse the benefits of GI reduction. This could be seen as a paradox given the increasing epidemic of obesity, diabetes and other lifestyle related diseases in the USA. Reasons cited include lack of definitive proof and the dangers of diabetics changing from traditional carbohydrate exchange routines. The debate will certainly continue and there are signs of growing acceptance of GI. For example, the Food and Nutrition Board of the Institute of Medicine of the U.S. National Academy of Sciences recently concluded:
"There is a significant body of data suggesting that more slowly absorbed starchy foods which are less processed, or have been processed in traditional ways, may have health advantages over those which are rapidly digested and absorbed. These foods have been classified as having a low GI and reduce the glycemic load of the diet. Not all studies of low GI or low glycemic load diets have resulted in beneficial effects, however, none have shown negative effects. There are also theoretical reasons at a time when populations are increasingly obese, inactive and prone to insulin resistance that dietary interventions that reduce insulin demand may have advantages. In this section of the population, it is likely that more slowly absorbed carbohydrate foods and low glycemic load diets will have the greatest advantage. . . . However, the principle of slowing carbohydrate absorption, which may underpin the positive findings made in relation to GI, is a potentially important principle with respect to the beneficial health effects of carbohydrates. Further research in this area is needed." (10)
The United States Department of Agriculture (USDA) which is responsible for meat and poultry products has recently stated that it will not object to the use of statements that do not expressly state or imply a specific level of carbohydrate in a product (e.g., "Carb Conscious" and "Carb Wise") nor will it object to terms such as "Net Carbs," "Effective Carbs" and "Net Impact Carbs" when used in a truthful and not misleading manner. The document can be found at:
http://www.fsis.usda.gov/oppde/larc/policies/carblabel.htm.
Several petitions relating to carbohydrate labeling have been submitted to the US Food and Drug Administration (FDA) and are currently pending.
GI acceptance elsewhere
Australia is leading the way with a GI logo used on products which have been accredited by the University of Sydney (see www.glycemicindex.com) referred to above. South Africa has also formally adopted GI labeling. In Japan there is growing awareness among healthcare professionals and food manufacturers and glucose attenuating properties of fiber-based products are being promoted to consumers.
The role of specialty carbohydrates
Sugar alcohols (polyols) such as lactitol, xylitol, isomalt and maltitol have a low glycemic effect, as do fructose, polydextrose, a randomly constructed polymer of glucose and some resistant starches. These ingredients are used extensively to completely or partially replace sucrose, glucose and high GI polysaccharides such as starch and maltodextrin in a wide range of processed foods such as dairy products, baked goods and confectionery. Specialty carbohydrates can have a useful role in reducing the overall glycemic challenge of the diet and in so doing may help to reduce the risk of a variety of “lifestyle” related diseases. In the case of Litesse® polydextrose, the 1 kcal/gram specialty carbohydrate can beused to boost fiber and replace the functionality of sugar in dairy and bakery products. Definitive studies addressing specifically the role of low glycemic carbohydrates are few. Livesey (11) has recently published a detailed review of the health potential of polyols as sugar replacers, with emphasis on low glycemic properties. Livesey (12) has also undertaken a meta-analysis of relevant studies, focusing on the impact of GI on glycated proteins, particularly HbA1C. He concluded that a reduction in glycemic carbohydrate equivalent to 30 grams of glucose per day could correlate with a 30% reduction in cardiovascular disease risk. This reduction should be readily achievable by most consumers and specialty carbohydrates would have an important role in improving the nutritional profile of processed foods.
Methodology, terminology and the scope for confusion
In time, the scientific community may endorse overall insulin demand as a determinant of disease risk. This in turn correlates well with glycemic challenge and effect. There are exceptions but these do not negate the value of measuring glycemic effect per se. It remains a useful marker of physiological challenge. Traditionally glycemic index (GI) has been used to compare only 'available' carbohydrates where 'available' means ‘absorbed into the bloodstream in the upper gastrointestinal tract.’ Glucose or white bread has been used as standards with current preference for glucose as it is easier to define. It is also important to communicate to consumers the reduced glycemic properties of foods based on carbohydrates which are metabolized in the colon such as the polyols, polydextrose and resistant starch. Currently GI is the parameter which is increasingly being recorded on food labels and promoted to consumers. If the traditional definition of GI prevails, consumers would not gain a full impression of how to reduce the glycemic impact of their diet. Certainly it is not realistic for consumers to understand the difference between glycemic index and other indicators of the glycemic properties of foods such as glycemic load, glycemic challenge, glycemic effect, glycemic impact and glycemic response. Given that only one term is likely to reach the public's perception it important that this term is defined in such a way that it has most relevance for consumer health. Traditionalists argue that this term should not be ‘glycemic index’ but something else. Referring to ‘glycemic impact’ may be a useful alternative but an unambiguous definition would need to be developed, adopted by the scientific community and communicated to consumers. Monro (13) recently discussed this point and argued for the extension of the GI concept to foods. This may be a way forward which could benefit consumers.
Future challenges and opportunities
Several popular books are helping to raise consumer awareness of GI as a strategy for improved nutrition. Of these, ‘The Glucose Revolution’ by Brand-Miller et al is currently in its 3rd edition (14). Gallup (2003) recently published ‘The GI Diet’ (15). Although overshadowed by other currently popular diet books, Gallup’s offering is a valiant attempt to communicate the GI concept to consumers. However, GI should not be described as a diet, it is more a strategy for improved nutrition which should become a lifestyle choice. Consumers will only derive maximum benefit from GI if food labels can be devised which accurately convey the relevant information. In turn, legislators will only modify labeling if scientific substantiation has been achieved. All those involved acknowledge the need for additional studies to fully elucidate the impact of glycemic reduction on disease risk. The next step should be to undertake definitive intervention studies to improve our knowledge. Only large groups such as the European Union DG Research or the World Health Organization are likely to have the necessary resources to undertake such studies. The intense discussion continues and it is clear that the definitive proof of the role of GI is some way off.
There is a strong case that in the meantime consumers should be given the necessary information and food choices to enable them to reduce their glycemic challenge. This approach offers only benefits and in this way consumers would generate their own data.
It is hoped that reducing the glycemic effect of the ‘western’ diet would lead to a reversal in the trend towards ‘lifestyle’ related diseases. It has been estimated that the glycemic challenge of our diet today is typically 10 times as high as it was pre industrial revolution. It is easy to imagine that our bodies have found it difficult to adapt.
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