Quantitative determination of glucose in foods after digestion

Private research without any support, Kraków, Poland. E-mail: janik.aleksander@gmail.com

Abstract

Background and aims: Glycemic indices (GI) are believed to be the best indicators of glycemic properties of foods. For that reason, a lot of work has been done for their determination. The aim of this study is to propose a new method for determination of the percentage contents of glucose in digested foods.

Results: Contrary to the widespread belief GI are not the reliable indicators of glycemic properties of foods. Glycemic loads (GL) are in fact equal to the percentage contents of glucose in digested foods. They are, however, biased by errors in determinations of GI and in the determination of total contains of carbohydrates.

Conclusions: The methods that are used for quantitative determination of glucose ought to be compared in different laboratories using the same selected foods such as raw and cooked carrot, sucrose, rice and other comestibles. Both healthy subjects and type 2 diabetics may participate in that research.

Key words: Glycemic index; Glycemic load. Glycemic carbohydrates; Glucose; Glucose response function; Diabetics

Introduction

There are no “good” or “bad” carbohydrates. Taking into account the needs of diabetics all carbohydrates may be divided into two types: glycemic carbohydrate (GC) and nonglycemic carbohydrates (NGC). GC are hydrolyzed and decomposed to glucose during ingfestion in the stomach. Thus, they increase the concentration of glucose in the blood. On the other hand NGC are resistant to digestive juices and therefore they are not broken down to glucose but they are excreted from the body.

GI had been introduced to characterize glycemic properties of food products and to enable diabetics to control their glucose blood levels. A lot of research has been donein order to determineGI of all kinds of foods. The obtained data have been compiled in the International Tables of GI and GL [1].

Glycemic index and Glycemic load of a food

The GI of any food is defined as the percentage ratio of the glucose response functions (GRF) obtained after ingestion of a food containing 50 g of carbohydrates to the GRF that is obtained after ingestion by the same person of 50 g of reference glucose:

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

where:

Ap* is the area of GRF obtained after ingestion the portion of a food containing 50 g of carbohydrates,

AG* is the area of GRF obtained after ingestion by the same person of 50 g of reference glucose.

According to that definition of GI the mass of tested food may be calculated using the relation:

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

where:

mp* is the mass of the ingested food,

mpC* is the mass of all carbohydrates contained in that food,

PC is the percentage content of carbohydrates.

An asterisk * indicates that the mass of a food product, the mass of the reference glucose and the respective GRF are mutually dependent.

GC is the only component of a food that is hydrolyzed and broken down to glucose during digestion. The mass of glucose that appears in the stomach due to hydrolysis of GC is grater than the mass of GC from which it was obtained. Therefore, the relation:

mG = 1,11*mCG (3)

may be used when it is necessary.

The glucose, which is formed in the stomach, is the only variable that determines the area of GRF of the tested food:

Ap* = ApG* (4)

where:

ApG* is the area of GRF of glucose that is formed during digestion of a tested food.

Therefore:

GI = 100 * (mpG*/mG* ) (5)

where:

ApG* is the area of GRF of glucose that is formed during digestion f a tested food,

mpG* is the mass of glucose obtained by decomposition of GC contained in the sample of the ingested food,

mG* is the mass of 50 g of pure glucose that is consumed by the same person as the reference substance.

Thus GI is equal to the percentage ratio of the mass of glucose obtained by digestion of a tested food mpG* to the mass of reference glucose mG* .

Since:

mG* = mpC* = (mp* * PC)/100 (6)

then using (4) and (5) the following relation is obtained:

GL = PG (7)

where:

PG is the percentage content of glucose that is formed by digestion of GC contained in the food sample of mass mp* .

That means that GL is equal to the percentage content of glucose which is formed from GC during digestion of a tested food PG*. Therefore, only GL has clear physical and chemical meaning.

Indirect determination of glycemic carbohydrates in foods

The percentage content of GC (PGC) may be calculated by subtracting PNGC (fiber) from the PC.

PGC = PC – PFIB (8)

where:

PFIB is the the percentage content of fiber in a tested food (PNGC = PFIB).

It should be noted that carbohydrates – unlike fats, proteins and other components of foods – are calculated from the rest to one hundred percent [2]:

PC = 100 – PFAT – PPRO – PASH – PWAT (9)

Thus, the content of glycemic carbohydrates may be calculated according to equation:

PGC = 100 – PFAT – PPRO – PASH – PWAT – PFIB (10)

where:

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

The variables on the left hand side of Equation (9, 10) are biased by errors in determination of the variables indicated on the right hand side of those equations. This is the only reason for which the above basic relations have been reminded.

Quantitative determination of glucose in a food after digestion

GRF after ingestion of any mass of pure glucose should equal to GRF after ingestion by the same person any food that contains the same amount of glucose.

Therefore, the area of GRF obtained after ingestion of reference glucose is proportional to its mass:

 

AG = kG*mG (11)

where:

AG is the area of GRF after ingestion of the reference glucose,
mG is the mass of the ingested reference glucose,

kG is the proportionality factor.

 

Thus, the area of GRF after ingestion of the sample of a tested food by the same person should be proportional to the mass of glucose that is formed from GC contained in it:

ApG = kG*mpG (12)

where:

ApG is the area of GRF after ingestion the sample of a tested food,
mpG is the mass of glucose that is produced in the stomach after ingestion of the sample of a tested food.

Therefore,the mass of glucose contained in the portion of the tested food may be calculated using the relation:

mpG = (mG/AG)* ApG (13)

The percentage content of glucose in a tested food may be calculated by one of the following relations:

PG = 100*(mG/AG)*(ApGC /mp ) (14)

or

PG = 100*(mG/AG)/ (mp/ApG) (15)

where:

PG is the percentage of glucose that is formed after ingestion of a tested food.

mp js the mass of a tested food.

The area of GRF is a linear function of the mass of ingested glucose (11, 12), provided that the maximum of GRF does not exceed the renal threshold. When the concentration of blood glucose is higher than the renal threshold then the kidneys begin to participate in removing of glucose from the blood. In such case an nonlinear model should be applied instead the linear one. To avoid those complications the masses of the reference glucose and of the tested food should be small enough so that the respective GRF to be smaller than the renal threshold.

Discussion

The values in Table 1 have been calculated using equation (2). Those values indicate that a healthy subject in order to determine GI of vegetables or fruits should consume them in quantities so large as 500 or even 1000 grams. Such large quantities of vegetables or fruits differ substantially from those that are normally eaten by people.

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

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

The errors in determination of GI are relatively great, for example: 101.0 ± 30.78 ; 70.3 ± 37.98 (SD) [3], and 54.0 ± 6.1 (SEM) [4]. This is due to the fact that GI and GL are biased by errors in determination of the areas of respective GRS and by errors in determination of the total contents of carbohydrates in foods (9).

Determination of glucose in digested foods (DGDF) is based on relations (14) and (12), which do not impose any restrictions on the masses of ingested glucose or of the tested food (2). However, in order to achieve the highest accuracy, the masses of ingested glucose and of the tested food should be so selected that the respective GRF to be of similar sizes and they to not exceed the renal threshold. Those requirements can be meet both by healthy subjects and by type 2 diabetics. Diabetics may, therefore, determine the percentage contents of glucose in their foods and meals.

The percentage contents of glucose in meals may be determined in the same way as for others comestibles (14, 15). However, they may be also calculated using previously obtained data for the ingredients used to prepare those meals:

PmG = (m1 * PG1 + … mi * PGi … + mN * PGN ) / (m1 + … mi … + mN ) (16)

where:

PG\mG are the percentage contents of glucose in an igested meal,

PG1 , PGi ,PGN are the percentage contents of GC in ingredients 1, i and N, respectively,

m1 , mi ,and mN are the masses of ingredients 1, i and N, retrospectively.

The area of GRF is the main but not the only parameter that can provide information on metabolism of glucose. There are also other parameters of GRF which do not seem to be exploited [5]. Food samples that contain 50 g of carbohydrates give GRF which in their form [3, 4] are very similar to each other. However, GRS of some foods that contain about or less than 25 g of carbohydrates may differ substantially in their forms [6] The research on those phenomenas is yet at the beginning stage.

The ratio of the mass of reference glucose to the area of GRF ((mG/AG) is the calibration factor. in relations (14, 15). However, the values of those ratios are specific for each individual and therefore they may be also used for diagnostic purposes. The highest values of those ratios ought to be observed for healthy individuals and the smallest for type 2 diabetics with advanced diabetes.

Conclusions:
Determination of the percentage contents of glucose in foods may be performed in various ways:

  1. 1. by calculating GL using GI and the total content of carbohydrates in a food (7)
    2. by calculating the difference between the total content of carbohydrates and the content of nonglycemic carbohydrates (fiber) (10);
    3 or by the proposed DGDF (14, 15).

It would be useful if the accuracy of all methods that can be used for determination of the percentage contents of glucose in digested foods were compared in different laboratories using the same selected foods such as raw and cooked carrot, table sugar, rice and other foods.

Funding: The research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

References:

[1]Atkinson FS, Foster-Powell K, Brand-Miller JC. Diabetes care, 2008, International Tables of GI and GL Values: 2008, http://care.diabetesjournals.org/content/31/12/2281.short [accessed 16.04.26].

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

[3] Chlup R, Bartek J, Reznícková M, Zapletalová J, Doubravová B, Chlupová L et 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/

[5] Janik A. https://cukrzycadieta.wordpress.com/2014/07/20/impulsy-glukozowe-glikemiczne-czym-sa-w-czym-i-jak-moga-pomoc/; [accessed 16.04.26].

[6] A. Janik, https://cukrzycadieta.wordpress.com/nieoczekiwane-piki-w-impulsach-glukozowych-surowej-marchwi-oraz-czystej-glukozy-3/; [accessed 16.04.26].

Aleksander Janik

Kraków 05/05/16