The Metabolic Effects of Two Meals with The Same Glycaemic Index But Different Slowly Available Glucose Parameters Determined In Vitro: a Pilot Study

Original Research Article

Ann Nutr Disord & Ther. 2014;1(1): 1001.

The Metabolic Effects of Two Meals with The Same Glycaemic Index But Different Slowly Available Glucose Parameters Determined In Vitro: a Pilot Study

Al-Mssallem MQ1*, Frost GS2 and Brown JE3

1Department of Food & Nutritional Sciences, King Faisal University, Saudi Arabia

2Department of Medicine, Division of Integrative Science, University of London, UK

3Department of Nutritional Sciences, University of Surrey, UK

*Corresponding author: Al-Mssallem MQ, Department of Food and Nutritional Sciences, King Faisal University, Al-Hssa 31982, P.O. Box 420, Saudi Arabia

Received: July 14, 2014; Accepted: July 30, 2014; Published: July 31, 2014


Some foods with a similar Glycaemic Index (GI) are known to have different metabolic impacts in terms of blood glucose and insulin responses. This difference could be explained by the difference in the rapidly and slowly Available Glucose (RAG and SAG) values for the foods determined in vitro. This study was set up to investigate the metabolic impact of two meals with essentially the same GI and macronutrient content but with different SAG values. Twelve healthy male subjects were recruited from the University of Surrey postgraduate student population. RAG and SAG values were measured for each meal based on enzymatic hydrolysis of carbohydrate content. The postprandial glucose, insulin, Triacylglycerol (TAG) and Non-Esterified Fatty Acid (NEFA) responses to high and low SAG meals were determined in two randomised occasions. The incremental Area Under the Curve (iAUC) for glucose and insulin was 106.3 ± 14.46 and 26550 ± 3266, respectively for the low SAG meal. The high SAG meal produced a lower glucose and insulin response with an iAUC of 88.1 ± 10.67 and 23701 ± 3065, respectively; although this difference did not reach statistical significance (p = 0.21 and 0.33 respectively). This work has demonstrated that small differences can occur in the metabolic response for low and high SAG meals in terms of the glucose and insulin levels. This highlights that RAG and SAG values may be an important adjunct to the GI of foods in determining metabolic response.

Keywords: Glucose; Insulin; Available glucose; Glycaemic index


GI: Glycaemic Index; CHO: Carbohydrate; RAG: Rapidly Available Glucose; SAG: Slowly Available Glucose; TAG: Triacylglycerol; NEFA: Non-Esterified Fatty Acid; iAUC: Incremental Area Under The Curve; II: Insulinaemic Index; ELISA: Enzyme Linked Immunosorbant Assay; BMI: Body Mass Index; g: Gram; KJ: Kilojoule; UK: United Kingdom; CV: Coefficient of Variation


Carbohydrates (CHOs) are important sources of stored energy providing between 40 and 55 % of the energy supplied by typical Western diets. In some population groups, however, particularly those that exist in the tropics, CHOs contribute up to 85 % of dietary energy [1,2]. CHOs exist in a wide range of forms which are for the most part heterogeneous in terms of their physical and chemical characteristics [3]. Nevertheless, in order to understand further the dietary impact of this important macronutrient it is necessary to classify CHOs in a manner appropriate to their effects. It has been suggested that CHOs should be classified, on the basis of their molecular size, into three groupings, namely i) monosaccharides and disaccharides, ii) oligosaccharides and iii) polysaccharides [4]. However, classifying CHOs based simply on their chemical structure is not a reliable indicator of their physiological effects [5,6]. As such, the approach of classifying foods according to their physiological effects can be considered a more useful method in terms of understanding the health effects of diets containing CHOs. Indeed, a concept, known as the Glycaemic Index (GI), has been introduced which allows foods to be ranked according to their effects on blood glucose levels [7-9].

Alternatively, the in vitro method for starch hydrolysis has been proposed as a faster and more cost effective method for predicting the GI of starchy foods [5,6]. As such, the terms Rapidly Available Glucose (RAG) and Slowly Available Glucose (SAG) have been devised relatively recently and are used as a direct measurement of absolute CHO which is available for absorption in the human small intestine [10-13].

RAG and SAG measurements are based solely on the hydrolysis of dietary CHOs in vitro using a mixture of enzymes resembling those present in the GI. The measurements of RAG give values for glucose that are likely to be absorbed in the human small intestine and, thus likely to influence blood glucose and insulin responses [12,13]. The relationship between the food RAG values and GI has been studied and it has been found that there is a significant positive correlation between these values for starchy foods (r = 0.76, p < 0.001). It is therefore apparent that using the RAG but also the SAG measurements of CHO containing foods may be used as a supplement to the GI approach and may provide further information which could be useful in understanding the impact of different foods containing CHO on blood glucose and insulin levels [11].

As highlighted, RAG and SAG measurements may provide a useful index to help understand the metabolic impact of CHOs in vivo. However, before this can be put into place greater attention needs to be paid in terms of understanding the metabolic impact of CHOs on the basis of their RAG and SAG values in combination with the GI approach. As such, we hypothesised that studying the effect of two CHO rich meals one with a high value of SAG and the other with a low SAG could help us to understand the differences in the metabolic effects of different dietary CHOs on blood glucose and insulin levels versus GI values. Therefore, this study examined the metabolic impact of two meals with similar GI and macronutrient content but different (high and low) SAG values.

Subjects and Methods


Twelve healthy subjects were recruited from the postgraduate student population of the University of Surrey, UK, by the distribution of both e-mails and posters between July and September 2009. Inclusion criteria included: male gender, non-smoking status and age between 20 and 65 y. Subjects also needed to have a normal weight in relation to their height, normal resting blood pressure and normal fasting plasma glucose levels. Subjects who were either overweight (BMI > 25), or had abnormal blood pressure or abnormal fasting plasma glucose were excluded from the study. All subjects gave informed written consent. Weight, height, blood pressure, fat mass and fasting plasma glucose were measured at baseline.

Study design

A randomised controlled crossover trial of a single meal (either low SAG meal followed by high SAG meal or the reverse) was employed. Each subject was randomly assigned to receive one of these two meals first. The study design received a favourable ethical opinion from the University of Surrey Ethics Committee (EC/2007/78/FHMS) and approved as being in accordance with the Helsinki II declaration. Subjects were asked to consume the meal at their breakfast time (at 0830 h).

In vitro measurement of RAG and SAG

The in vitro procedure was based on the enzymatic hydrolysis of the food CHO using the method of Englyst [14]. Portions of the food samples (3 g) were weighed into 50 ml centrifuge tubes (polypropylene tubes from Corning Inc, NY 14831) to the nearest ± 1 mg and incubated with a mixture of hydrolytic enzymes (amyloglucosidase from Englyst Carbohydrate Services Ltd (Southampton, UK), amylase (heat-stable) and pancreatin from Sigma Chemical Co. Ltd., Poole UK) under controlled conditions of temperature (37 °C), pH (5.2) and viscosity. Subsamples were collected from the incubation mixture at specific time points (20 and 120 min) and measured for glucose which was then used to calculate the RAG and SAG values, respectively. The released glucose from each of the samples was determined colorimetrically using Glucose Oxidase/Peroxidase Reagent (Sigma Chemical Co. Ltd.).

Two reference samples, namely potato starch (Sigma Chemical Co. Ltd) and Cornflakes® (Kellogg’s, UK) were included in every batch analysed and the inter-assay CVs were calculated to be less than 10 % for reference 1 and 5 % for reference 2.

Meal design

In order to test the hypothesis, the two near-identical meals were designed to achieve as large a difference as possible in the content of SAG, but with no significant differences between the two meals regarding the overall GI, energy content, absolute available CHO, fat or protein levels. All the RAG and SAG values for the two meals were obtained using the above method [14]. The low SAG meal consisted of Hassawi rice (previously characterised by Al-Mssallem et al. [15], a plain yoghurt drink and Arabic dates (also previously characterised by Al-Mssallem et al. [16]. This meal had a total SAG of 11.8 g and an overall GI of 47 (Table 1). The high SAG meal contained Uncle Ben’s rice, a plain yoghurt drink and Arabic dates; with a total SAG of 20.2 g and an overall GI of 46. The GI values were based on glucose as the standard reference and taken from previous studies [15,16], and were calculated for the two meals from the individual foods [17]. All foods were prepared in the kitchen unit of the Clinical Investigation Unit at the University of Surrey, UK.