| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Glycemic Response and Health |
1 From the New Zealand Institute for Crop & Food Research Ltd, Palmerston North, New Zealand (JAM), and The Good Carb Food Company Ltd, Llanelli, Wales, United Kingdom (MS)
2 Presented at an ILSI Europe workshop titled "Glycemic Response and Health," held in Nice, France, on 6-8 December 2006. 3 Supported by The New Zealand Foundation for Research Science and Technology capability fund. 4 Address reprint requests to ILSI Europe. E-mail: publications{at}ilsieurope.be. 5 Address correspondence to JA Monro, New Zealand Institute for Crop & Food Research Ltd, Private Bag 11 600, Palmerston North, New Zealand. E-mail: monroj{at}crop.cri.nz.
ABSTRACT
Glycemic impact, defined as "the weight of glucose that would induce a glycemic response equivalent to that induced by a given amount of food" (American Association of Cereal Chemists Glycemic Carbohydrate Definition Committee, 2007), expresses relative glycemic potential in grams of glycemic glucose equivalents (GGEs) per specified amount of food. Therefore, GGE behaves as a food component, and (relative) glycemic impact (RGI) is the GGE intake responsible for a glycemic response. RGI differs from glycemic index (GI) because it refers to food and depends on food intake, whereas GI refers to carbohydrate and is a unitless index value unresponsive to food intake. Glycemic load (GL) is the theoretical cumulative exposure to glycemia over a period of time and is derived from GI as GI x carbohydrate intake. Contracted to a single intake of food, GL approximates RGI but cannot be accurately expressed in terms of glucose equivalents, because GI is measured by using equal carbohydrate intakes with usually unequal responses. RGI, on the other hand, is based on relative food and reference quantities required to give equal glycemic responses and so is accurately expressed as GGE. The properties of GGE allow it to be used as a virtual food component in food labeling and in food-composition databases linked to nutrition management systems to represent the glycemic impact of foods alongside nutrient intakes. GGE can also indicate carbohydrate quality when used to compare foods in equal carbohydrate food groupings.
Key Words: Glycemic impact • glycemic glucose equivalent • glycemic index • glycemic load • virtual food component • glycemic control
INTRODUCTION
It is increasingly accepted that the blood glucose response to a food is not accurately predicted by the content of available carbohydrate in the food, as measured in traditional food analysis. At the same time, the need for food values that will complement available carbohydrate values, to enable dietary control of postprandial glycemic responses, has become recognized (1).
Accordingly, in 2004 the American Association of Cereal Chemists (AACC) established an ad hoc committee on the definition of glycemic carbohydrates charged with the task of providing "a measurable definition that will enable manufacturers to communicate the glycemic response in grams per serving of food." After lengthy and stimulating discussion, the committee proposed several recommendations and definitions, including, "Glycemic impact is the weight of glucose that would induce a glycemic response equivalent to that induced by a given amount of food" (2).
This is a simple but profound definition. It has important implications for the meaning of terminology surrounding the glycemic potency of foods, for the way glycemic potency is measured, for the management of data arising from the measurements, and for ways it can be used in controlling postprandial glycemia. Although the AACC definition of glycemic impact is simple, it requires a change in thinking, from food carbohydrates to entire foods, and from static index values to intake-sensitive values, and from unit-free values to nutrient-like values that represent glycemic effects with weight units (3).
In this article, the concept of glycemic impact is explored as much as space permits. It is explained why, in light of current knowledge, glycemic impact is an advance on the current concepts of glycemic index (GI) and glycemic load (GL) for expressing the glycemic potency of foods.
GLYCEMIC IMPACT, GLYCEMIC INDEX, GLYCEMIC LOAD: DEFINITIONS
Glycemic impact, GI, and GL are all based on the currently accepted but nonetheless crude measurement of glycemic response as the incremental area under the blood glucose response curve (IAUC) for 2–3 h after consuming food, but they each express different aspect of glycemic potency:
The (relative) glycemic impact (RGI) refers to the relative tendency of a given amount of food consumed in a single intake, such as a serving, to induce a postprandial glycemic response. RGI is measured directly by determining the amount of glucose reference required to give the same glycemic response as a relevant amount of food, so the measurement is equiglycemic, and the values produced may be accurately termed glycemic glucose equivalents (GGEs).
 | (1) |
 | (2) |
 | (3) |
The GL refers to the cumulative exposure to postprandial glycemia, as a measure of insulin demand, over a specified period of time (5). It does not take into account the pattern of loading within the specified time, ie, few high–glycemic impact meals versus frequent meals of low glycemic impact. It is calculated indirectly as the product of the average GI of carbohydrate foods consumed and the total carbohydrate intake over a specified time period.
 | (4) |
GL (GI x available carbohydrate) contracted to a single intake of a single food approximates but is not the same as RGI, because the GGE of a food quantity is measured from the amount of glucose reference required to induce an equal glycemic response. In contrast, when GI is measured, the food and reference are consumed to provide an equal available carbohydrate content, but seldom induce equal responses. The inequivalence of food and reference responses in GI determination may lead to large discrepancies between RGI (GGE intake) and GL if GL is calculated from GI. The differences in determination of GI, GL, and GGE are summarized in Figure 1
.
|
The term impact has 2 meanings in English (Concise Oxford Dictionary): 1. The act of impinging (from which the word is derived). 2. A marked effect or influence. In the term glycemic impact, it is used in the first sense, as a food property, which is the stimulus leading to a postprandial change in blood glucose concentration, which is the response. Although the glycemic impact of a food is quantified in the first place by measuring glycemic responses to a given amount of the food and to a glucose reference, it is subsequently treated as a food value that can inform consumers of the relative glycemic potential of the food as glucose equivalents, compared with that of other foods and quantities of them. Importantly, it allows RGP to be measured as a food property independent of the modulating factors that determine highly individual glycemic responses and introduce a high degree of variability to blood response measurements.
The word impact has another essential implication: that the stress or load is rapidly imposed, so it is discretely related to the glycemic response that immediately follows a food intake event. In contrast, the term load, as in GL, is defined as "the daily carbohydrate intake multiplied by the average glycemic index," and refers to a total accumulation. The term impact was intentionally used by the AACC committee to communicate the fact that values refer to a single intake and its associated acute postprandial response.
THE CONCEPT OF GLYCEMIC EQUIVALENCE
Another essential facet of the definition of glycemic impact is that it is based on the concept of equivalence. Saying that glycemic impact is a weight of glucose, equivalent in effect to a given weight of food, means that it may be expressed as glucose equivalents. But because glycemia is the specific context of the equivalence, the term glycemic glucose equivalence is more precise. Thus, if a serving of a food has a glycemic impact equivalent to that of 15 g glucose, one may say that the food serving has a GGE content of 15 g.
There are important implications of expressing the glycemic impact of a given weight of food in terms of its glucose equivalence, related to both the use and the measurement of glycemic impact:
and in Table 1. ). A GGE content embodies all factors that determine the net glycemic effect of a food, expressed as the weight of glucose that would have the same effect as a given weight of the food, its RGI.
|
|
With the enormous emphasis being placed on functional foods, there is need for a family of virtual food components that will inform food manufacturers, health professionals, and consumers of the relative efficacy of foods along several dimensions of efficacy. When the AACC Board asked for a value that would "communicate the glycemic response in grams per serving of food," they were, with great foresight, asking for a virtual food component to express glycemic impact. GGE is a virtual food component that allows glycemic impact to be expressed as grams of glucose per amount of food consumed.
APPLICATIONS OF GLYCEMIC IMPACT
Expressing glycemic impact in the form of a food component enormously extends the range of applications in glycemic control beyond those possible with GI. By being based on a virtual food component (GGE), glycemic impact can be used in the same way as most real food components to show the following:
) or in food tables [Table 2 (8)]
|
|
METABOLIC CONSEQUENCES OF CHOOSING FOODS BY GLYCEMIC IMPACT
The metabolic consequences of using GI and RGI to manage glycemia may differ, because GI is used when foods have already been selected on the basis of their carbohydrate content, whereas a low GGE intake may be achieved by the less desirable practice of replacing carbohydrate with fat (11). The apparent differences between RGI and GI do not, however, have anything to do with an intrinsic property of GI versus GGE; such an argument implies that nutrient intakes are considered when GI, but not GGE, is used. This is certainly not the case; the main purpose of GGE values is to allow concurrent management of food effects and nutrient intakes. GGE values have always been intended for use in multidimensional nutritional management.
When foods have been placed into groups of equal carbohydrate content, GGE values function in exactly the same way as GI to identify alternatives of lesser glycemic impact. Conversely, if GI is used without regard to nutrient intake or carbohydrate content by the consumer in the supermarket, where foods are not necessarily grouped by equal composition and serving size and where there are no constraints on freedom of choice, it may not lead to healthier or even less glycemic food choices.
METHODOLOGIC IMPLICATIONS OF GLYCEMIC IMPACT
Because the AACC definition of glycemic impact is based on glucose equivalents, and the glycemic response to glucose intake is nonlinear like most physiologic responses, important requirements exist for a procedure that will measure RGI. To directly measure true equivalence to a reference, the effects of the food and the reference need to be compared at a point of similar responsiveness on the dose-response curve. Also, the measurement should be made with a usual intake of food to minimize the inaccuracy that inevitably arises when daily food choices differ from the quantity used to measure the GGE content.
The blood glucose response has been consistently shown to be an almost quadratic function of the glucose dose (12-17), which reaches a plateau (or maximum) between 60 and 100 g glucose intake (Figure 4). However, in the current standard procedure for measuring the relative glycemic effects of food carbohydrates in GI determination, the glycemic response to a food portion containing 50 g of "available" carbohydrate is compared with the response to a 50-g glucose reference (18), so the comparison is equicarbohydrate, because it involves equal amounts of available carbohydrate. Consequently, if the carbohydrate in a food is significantly less glycemic than is glucose, the glycemic response to the 50-g available carbohydrate food portion may be widely separated from the response to the glucose reference on the GGE dose–glycemic response curve, and is therefore measured at a point of different glycemic responsiveness (Figure 5). Under such circumstances, the effect of the food cannot be directly or accurately expressed as glucose equivalents without making some allowance for the nonlinearity of the dose-response curve.
|
|
It is not possible to exactly position the glycemic effects of a food and a reference without prior knowledge of the food's effect, but many strategies exist that overcome this problem by aligning the food and reference responses closely enough for the effects of nonlinearity to become negligible. For example, one can generate an indicative GL on the basis of an available carbohydrate value adjusted by the GI of its constituent available monosaccharides, particularly if the available carbohydrate content is measured as recommended in the AACC report (2) by in vitro digestion of food as consumed. Second, one can use 2 references, one on each side of the unknown, and measure the equivalent glucose dose by interpolation. Third, one can make a mathematical adjustment on the basis of the equation for the universal dose-response curve, once the separation between responses to the food and to the reference is known. Last, one can read the glucose equivalent off a standard glucose reference curve. This is the most accurate way of measuring GGE (Wallace et al; unpublished observations, 2006).
An advantage of directly measuring glycemic impact as a food property is that there would be no need to use available carbohydrate values, with the additional uncertainty that they introduce.
THE TERMINOLOGY OF GLYCEMIC IMPACT
As a descriptor for the virtual food component data set behind RGI, GGE is a scientific, accurate, and concise term. Descriptors for virtual food components need to make reference to the effect, biomarker, or endpoint that they represent; the reference against which the food effects are measured; and the fact that they do not represent the weight of a reference component per se in a food, but rather the weight of the reference that would have an effect equivalent to that of a given weight of the entire food. Glycemia, glucose, and equivalence are all part of GGE as the minimal descriptor for the data that underpin RGI, as a statement of the relative potential of foods to induce a postprandial glycemic response. The term glycemic impact in turn is more scientifically described as relative glycemic impact, with the acronym RGI to distinguish it from GI and GL.
The term glycemic load is often used without units and sometimes with the unit (g), but little about the term indicates what it means or that it does not represent a weight of glucose but rather a relative effect or an accumulation of relative effects. GI is a greater problem. An index by definition is "a number expressing a physical property in terms of a standard." The GI of a food should therefore express the glycemic potency of a food relative to a glucose standard. However, the GI in current use is the imputed GI of available carbohydrate in a food, not a GI of food per se. The misattribution of GI to foods rather than to food carbohydrates has caused confusion among both professional and lay users of GI.
COMMUNICATING GLYCEMIC IMPACT
Because glycemic impact is based on the virtual food component GGE, it may be easily communicated in exactly the same way as any other food component as grams per serving and as grams per 100 g of food. Glycemic impact would be measured per serving or per other relevant quantity, and extrapolation to 100 g would allow food comparisons on an equal weight basis.
In a food label, several options are available for communicating glycemic impact. Two alternatives are shown in Table 1. In the top half of the table, GGE as a virtual food component is included with other food components in the nutrient information panel, which supports glycemic impact as a consumer signpost external to the nutrient information panel. In the bottom half of the table, glycemic impact is included in the nutrient information along with its supporting GGE units.
Which format is the most correct depends on the purpose of a nutrient information panel. If the aim is to present an accurate and unambiguous summary of major components in a food, glycemic glucose equivalents (g) should appear, because the terms glycemic load and glycemic impact themselves give no information about what the load or impact consists of. However, if the main purpose of a nutrient information panel is communication with consumers, glycemic impact, when clearly defined as the glucose equivalent of a specified amount of food, a serving and 100 g, may be appropriate.
The GL calculated from the GI is being increasingly used in the same way as it is intended that glycemic impact be used and is often presented without units. The idea of a load has intrinsic meaning, but as a food value it requires units. Because it is currently calculated from the GI, it contains the nonlinearity error due to changing glycemic responsiveness with dose, as discussed above. If GL is to be used to communicate glycemic impact, it should be measured equiglycemically if it is to represent the glucose equivalent of a food quantity and would then bear GGE or at least glucose equivalent units. GGE may also serve as the acronym for "grams of glucose equivalents" once the glycemia context of the value has been defined.
MANAGING GGE VALUES UNDERPINNING GLYCEMIC IMPACT
With the development of modern food-composition database systems and their ability to be interfaced with electronic health care systems, such as blood glucose monitors and insulin pumps, there is increased opportunity for glycemic impact to be integrated into nutritional management that is responsive to an individual's physiologic or health status. The interrelationship of GI or GL with GGE and available carbohydrate values in a food-composition database and their potential connection to nutritional management are summarized in Figure 6.
|
, it is necessary that they be comprehensive enough to include most currently consumed foods. At present, although there already exists an urgent need for data to support healthier food choices, the available number of virtual food component values is too small to constitute a practical working database. The problem of lack of virtual food component data can be partly overcome by generating interim data to fill existing gaps, in the same way that food-composition databases have relied on carefully selected values from other databases, until these interim values can be replaced by indigenous analytic values. Such values would represent different degrees of improvement over available carbohydrate alone.
Although highest quality GGE data are obtained by measuring the effect of a food directly in humans, clinical determinations are too expensive and slow to keep pace with the rate of appearance of new food products. However, additional surrogate GGE values may be obtained in several ways. Such GGE estimates may be attached to "confidence codes" in the database, which identify the source of the GGE values and are linked to other key information, such as their precision and accuracy.
Approximate GGE values may be obtained from existing GI x available carbohydrate, and GL values, but require a correction for the nonlinearity effects due to separation of the reference and food responses in GI determination, as discussed above. They are also subject to error imported from currently available carbohydrate values. A cheap and practical alternative is to obtain measurements from in vitro digestion methods, which are highly precise because they avoid subject variation and show reasonable accuracy as predictors of glycemic response (19-21). Indeed, if the AACC recommendation (2) that available carbohydrate be measured by digestive analysis of foods as consumed were adopted, available carbohydrate would become a reasonable approximation for GGE for foods in which the effects of food structure and carbohydrate composition are major determinants of the glycemic response.
FUTURE DEVELOPMENTS IN GLYCEMIC IMPACT
In the definition of glycemic impact, the underlying glucose equivalence is based on an equivalent glycemic response. But what exactly constitutes a glycemic response is not specified more precisely than "a change in blood glucose concentration." At present, response is taken to mean the IAUC for 2 h after consuming food, without taking into account the response pattern in that time, so it is a crude measure.
As our understanding of which features of the postprandial change in blood glucose concentration are critical to health outcomes improves, the perceived importance of various characteristics of the response, such as slope and amplitude, may change. However, as long as glucose is used as the reference against which responses to foods are standardized, and there is agreement about which characteristics of the postprandial glycemic response are most closely linked to health outcomes, the terms glycemic impact and GGE will remain valid, although the parameters of the glycemic equivalence on which they are based may be refined.
CONCLUSION
Glycemic impact is a term for the total grams of glycemic glucose equivalents consumed in a food. It is well suited to be a guide to food choices for glycemic control, because it behaves as if it were a food component and may be expressed in the same format as nutrients, as grams per serving, per 100 g, or per a given amount of food. It therefore satisfies the demand for a food value that expresses potential, relative glycemic effect, as "grams per serving of food" (2). Accurate measurement of glycemic impact requires a change in methodology to provide a comparison of food quantities giving an equivalent glycemic response, in contrast with the current approach of comparing glycemic responses at an equal available carbohydrate intake, as used in GI determination.
GI and GGE represent only the glycemia dimension in nutritional management and are not intended for use in isolation. The metabolic effects of using either GI or GGE depend on the nutritional management that accompanies their application. When foods have been classified into groups of similar nutrient composition, GI and GGE provide similar rankings of foods, because the GI is approximately the GGE content of 50 g carbohydrate expressed as a percentage.
The main benefit of GGE values will be in their acting as a virtual food component that underpins glycemic impact as a food property, which can be used alongside other nutrient information and which responds to changes in food composition and intake that determine glycemic effect. GGE values therefore have a role to play in food-composition databases and nutrition management systems that combine the nutrient and functional attributes of foods, including their glycemic impact.
ACKNOWLEDGMENTS
The contributions of the authors were as follows—JAM: wrote the article; MS: had the idea of combining normalized results from several studies to provide a universal glucose dose–glycemic response curve, as shown in Figure 4. JAM had no conflicts of interest. MS owns The Good Carb Food Co Ltd, Llanelli, Wales, which uses glycemic load and glycemic glucose equivalents in nutrient information panels on some products.
REFERENCES
This article has been cited by other articles:
|
|
 |
|
  J. Howlett and M. Ashwell Glycemic response and health: summary of a workshop Am. J. Clinical Nutrition, January 1, 2008; 87(1): 212S - 216S. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
50 x 0.2 = 10), the response would be overestimated by 