On glycemic indexes (GI) and glycemic loads (GL) and on the determination of glycemic carbohydrates in foods (preprint)

Abstract

Objectives:

The aim of this study is to compare the three methods that can be used for the determination of contents of glycemic carbohydrates (GC) in foods. Glycemic carbohydrates (GC) are broken to glucose by the digestive juices. Other carbohydrates (nonglycemic carbohydrates NGC) are resistant to gastric juices and, therefore, they do not increase the concentration of blood glucose.

Methods:

Unlike to the determination of GI and GL, the determination of GC (see eq 12 – 14), does not impose any stiff requirements on the masses of tested food and of the reference glucose.

Results:

Contrary to widespread – and, unfortunately, spurious – belief, theglycemic indexes (GI) are not reliable indicators of glycemic properties of foods. The glycemic loads (GL) are equal to the contents of glycemic carbohydrates in foods but they are charged with errors associatedwith the determinations of glycemic indexes and of the contents of carbohydrates in foods (8).

 

Conclusions:

The determination of the contents of glycemic carbohydrates in foods can be carried out by three methods. It would be useful for diabetics ifthose methods were compared between themselves in different laboratories using the same foods (e. g. raw and cooked carrot, sucrose, rice and other selected foods). Not only healthy people, but also type 2 diabetics can participate in these studies.

 

Key words: glycemic index, glycemic load, glycemic carbohydrates, blood glucose, food products, glycemic carbohydratez, diabetics

Introduction

There are no “good” or “bad” carbohydrates. All carbohydrates can be divided into two types: glycemic carbohydrates (GC) and nongflycemic carbohydrates (NGC). The glycemic carbohydrates under the action of gastric juices are broken down into glucose. Glycemic carbohydrates increase the concentration of glucose blood and, therefore, diabetics should control their daily intake.

Thenongflycemic carbohydrates (fiber and others) are resistant to gastric juices and therefore, they do notincrease glucose blood, they do not provide energy to the body and they are excreted from it.

Glycemic Indexes (GI) and glycemic loads (GL)

Glycemic indexes had been introduced to characterize glycemic properties of food products and to enable diabetics to control their glucose blood levels. A lot of work has been devoted to determine the glycemic indexes of all kinds of foods [1, 3,4 ]. The obtained results have been compiled in International Tables of Glycemic Index and Glycemic Load … [1].

The glycemic index of a food product is defined as the percentage ratio of the glucose response function after ingestion the portion of a food product containing 50 g of carbohydrates to the glucose response function after ingestion by the same person of 50 g of reference glucose:

GI = 100*Ap* / AG* (1)

where:

Ap* is the area of glucose response function after ingestion the portion of a food product containing 50 g of carbohydrates and

AG* is the area of glucose response function after ingestion by the same person of 50 g of reference glucose,

That definition is only a working since it explains how to determine a glycemic index, of any food but it does not say what is the physical meaning of glycemic index.

According to the definition of a glycemic index the mass of a tested food should be calculated from the following relation:

mp* : mpC* = 100 : PC (2)

where:

mp* is the mass of a tested food,

mpG* is the mass of carbohydrates contained in the tested food,

PC is the percentage of carbohydrates in a tested food.

Since only glycemic carbohydrates are broken down to glucose the following relation is valid:

Ap* : AG* = mpGC* : mG* (3)

where:

mpGC* is the mass of glycemic carbohydrates contained in the sample of the tested food.

mG* is the mass of 50 g of pure glucose which is consumed as the reference substance.

Therefore, the glycemic index may be written as follows:

GI = 100 * (mpGC*/mG* ) (4)

Since:

mG* = mpC* = (mp* * PC)/100 (5)

then substituting (5) into (4) the following relationship is obtained:

PGC* = GI * PC)/100 = GL (6)

The glycemic load GL is, therefore, equaled to the percentage of glycemic carbohydrates PGC* contained in a tested food.

Indirect determination of glycemic carbohydrates in foods

The percentage of glycemic carbohydrate PGC may also be calculated by subtracting the percentage of nonglycemic carbohydrates (PNGC = PFIB) from the percentage of total carbohydrates in the food product PC.

PGC = PC – PFIB (7)

It should be noted, however, that carbohydrates – contrary to the fats and other components of foods – are calculated from the rest to one hundred percent.

PC = 100 – PFAT – PPRO – PASH – PWAT (8)

As a consequence the contents of glycemic carbohydrate are calculated according to equation:

PGC = 100 – PFAT – PPRO – PASH – PWAT – PFIB (9)

where:

PFAT, PPRO , PASH , PWAT and PFIB are percentages of fats, proteins, ash, water and fiber in a food, respectively.

It should b noted that the values shown on the left side of Equation (8. 9) are burdened with t errors associated with the determination of the values given on the right side of those equations.

Direct determination of glycemic carbohydrates in foods

The glucose response function after ingestion any mass of glucose, should be equal to the glucose response function after ingestion by the same person any food that contains the same mass of glycemic carbohydrates. As in the case of glycemic indexes the increase in weight of glucose due to hydrolysis of glycemic carbohydrates will not be considered.

According to that assumption the area of glucose response function is proportional to the mass of ingested glucose:

 

AG = kG*mG (10)

where:

AG is the areas of the glucose response function after ingestion of the reference glucose,
mG is the mass of the ingested reference glucose,

kG is the coefficient of proportionality.

 

The area of glucose response function after consumption by the same person the sample of tested food is proportional to the mass of glycemic carbohydrates contained in it:

 

 

ApGC = kG*mpGC (11)

 

where:

ApGC is the area of the glucose response functions after ingestion of the sample of tested food,
mpGC is the mass of glycemic carbohydrates contained in the sample of tested food.

 

The mass of glycemic carbohydrates contained in the portion of tested food can be, therefore, obtained from the relations:

 

mpG = (mG/AG)* ApG (12)

Thus, the percentage of glycemic carbohydrates contained in the sample of a tested food is given by the following relations:

 

PpGC = (mpG*100)/mp = 100*(mG/AG)*ApGC /mp (113)

or

 

PpGC = 100*(mG/AG)/ (mp/ApG) (14)

 

where:

PpGC is the percentage of glycemic carbohydrates contained in the sample of tested food,

mp js the mass of tested food.

 

In equations (10) and (11) it is assumed that the proportionality coefficient kG is yhe constant for a given person and, therefore, the areas of glucose response functions are linear functions of ingested glycemic carbohydrates. Such assumption is reasonable for healthy subjects. For diabetics it is valid only if the maximum height of glucose response function does not exceed the renal threshold. Above the renal threshold a nonlinear model should be applied with additional constant kKG which takes into account kidneys function in removing glucose from the body. To avoid those complications the masses of reference glucose and foods should be adjusted so that the respective glucose response functions were below the renal threshold.

Discussion

The values shown in Table 1 indicate that in order to determine the glycemic indexes of foods containing small amounts of carbohydrates (especially. of vegetables and fruits) a healthy person should consume the portions so large as 500 or even 1000 grams. Such large portions of vegetables or fruits differ significantly from those that are normally consumed.

Product PC mp*
A 5 1000
B 10 500
C 25 200
D 50 100
E 75 67
F 90 56

 

Table 1. The masses of food products (mp*) that ought to be consumed in order to determine their glycemic indexes depending on the contents of carbohydrates (PC).

 

Errors of the determination of glycemic indexes are very large, for example: 101.0 ± 30; 70.3 ± 37.98 (SD) [3], and 54.0 ± 6.1 (SEM) [4]. Thus, the errors of determination of glycemic loads (and of glycemic carbohydrates) must also be very large. Glycemic loads accumulate in themselves all errors associated with the determination of glycemic indexes (1) and of the contents of carbohydrates in foods (8).

 

The direct determination of glycemic carbohydrates (DDGC) in foods (13, 14) does not impose any restrictions on the weight of consumed glucose and on the weight of a food product. However, in order to archive the maximum accuracy, these quantities ought to be adjusted so that their glucose response functions were of the similar size and did not exceeded the renal threshold. Those requirements may be fulfilled not only by healthy people but also by type 2 diabetics so that they may determine glycemic carbohydrate in their own meals.

The ratio of the mass of glucose sample to the area of respective glucose response function of ((mG/AG )) is a calibration factor. in the equations (13, 14). Those ratios may be used also for diagnostic purposesl. The highest values of that ratio will be determined for healthy people, and the smallest for diabetics with developed diabetes.

Results and conclusions:
The determination of the contents of glycemic carbohydrates in foods can be carried out by three methods:

  1. 1. calculating the glycemic loads GL using glycemic indexes GI and the total contents of carbohydrates (6)
    2. calculating the difference between the total content of carbohydrates and the content of nonglicemic carbohydrates (fiber) (9);
    3 or by the direct determination of glycemic carbohydrates (13, 14).

It seems to be useful to diabetics if the accuracy of those three methods of determination of glycemic carbohydrates were compared in different laboratories using the same selected foods (e. g. raw and cooked carrot, table sugar, rice and other foods).

References:

[1]FS Atkinson, K Foster-Powell, JC Brand-Miller – Diabetes care, 2008, International Tables of Glycemic Index and Glycemic Load Values: 2008, http://care.diabetesjournals.org/content/31/12/2281.short

[2] H. Kunachowicz, I. Nadolna, K. Iwanow, B. Przygoda, Wartość odżywcza wybranych produktów spożywczych i typowych potraw PZWL. Wydanie IV Warszawa 2005: p. 11

[3] R. Chlup, J. Bartek, M. Reznícková, J. Zapletalová, B. Doubravová, L. Chlupováet al, Determination of the glycaemic index of selected foods (white bread and cereal bars) in healthy persons. Biomed Papers 148(1) 17 – 25 (2004)

[4] Alkaabi et al. Glycemic indices of five varieties of dates in healthy and diabetic subjects, Nutrition Journal 2011 10 – 59, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3112406/