07 Apr 2016

In Vitro Digestion Study of Factors Affecting Sugar Availability from Fruits and Vegetables

Hoerudin, 2012
The University of Queensland

Abstract
Over the past several decades, the increasing prevalence of hyperglycemia-related diseases has led to growing interest in controlling the uptake of dietary sugars, including those from fruits and vegetables, without a major reduction in the uptake of other beneficial phytochemicals. The present study was aimed at investigating factors controlling bioaccessibility and bioavailability of sugars from fruits and vegetables.

Considering relevant sugar release properties, apple, rockmelon, raw carrot and steamed beetroot were selected as the food models. Appropriate techniques for the determination of sugars in the test foods were firstly established using selective extraction methods, followed by HPLC-ELSD. The effect of food particle size and surface area on the rate and extent of release of sugars was examined using two approaches. Firstly, using a kinetics model, the rate and extent of release of sugars from cut samples with well-defined sizes were determined following in vitro digestion that mimicked the human upper gastrointestinal digestion. Secondly, a similar in vitro digestion study was carried out using human ‘chew and spit’ samples. Based on the two studies, the relationship between the sugar release rate constant and surface area was established and then used to predict the apparent surface area of human chewed particles. In addition, the direct relationship between food structures, mastication, and the resulting characteristics of chewed samples, the viscosity of in vitro digesta analogues, and the bioaccessibility of sugars were established. To assess the potential inhibitory effect of polyphenols on the absorption of released sugars from the small intestine, in vitro pancreatic digesta of apple and a mixture of pure sugars were simultaneously applied to Caco-2 cell monolayers, and the in vitro sugar absorption profiles were determined. To elucidate the underlying mechanisms, the polyphenolic profiles of apple extract and pancreatic digesta were determined by LC-MS analysis.

Fructose, glucose, and sucrose were identified in all test foods. Additionally, sorbitol was found in apple. The developed extraction methods, using either pure water or water-methanol-dichloromethane (3:1:2), followed by reversed phase solid phase extraction, were able to extract 98% of total sugars present in each test food.

The study found that the swallowing threshold increased with food hardness, and that there was an inverse relationship between food hardness and particle size of the masticated food. Plant cells and cell walls were ruptured to a large extent as a result of mastication. In vitro gastric viscosity increased with a reduced swallowing threshold. In addition, the sugar release behaviour of fructose, glucose, sucrose, and sorbitol was similar during in vitro digestion. The sugar release profile was significantly influenced (P<0.05) by the degree of habitual mastication and the characteristics of the non-chewed fruits and vegetables.

For cut and chewed samples, sugar release profiles fitted well to modified first-order kinetics. For the same surface area, the lowest release rate of sugars were found in raw carrot. The sugar release rate was found to be a linear function of specific surface area. Based on the linear relationship between the reciprocal of rate coefficient and the square of the particle size for all foods studied, the apparent diffusion coefficients were 1.15–5.23 x 10-4cm2s-1 and varied with the food type. The diffusion of sugars from raw carrot was about four times slower than from the other test foods, which could be attributed to differences in the cell diameters. The present study demonstrates that the apparent surface area of chewed samples could be deduced from the experimental sugar release kinetics when compared with calibration using tissue cubes. Thus, a novel method for estimating the true (average) surface area of particles after mastication has been developed.

Polyphenols present in apple were incompletely released, and the composition changed following complete in vitro digestion. In addition to unknown compounds, seven monomers and dimers of polyphenols were identified in apple pancreatic digesta. Despite acceptable TEER values (quality of Caco-2 cell monolayers), no sugars were detected in the basolateral solution. Fructose, glucose, and sucrose were absorbed by Caco-2 cells without selectivity. The profile of sugar disappearance from the apple digesta in the apical solution was not significantly different (P>0.05) from that of the mixture of pure sugars.

The present study suggests that mastication may modulate the bioaccessibility of sugars by affecting the intactness of plant cell walls, the food-digestive solution contact area, and gastric viscosity. In the presence of cellular barriers, the movement of digestive solution across cell walls, or across the plasma membrane is expected to be the rate-limiting step for sugar release. The degree of habitual mastication may be a significant contributing factor to inter-individual variability of GI of fruits and vegetables, and may be useful in glycaemic control. Sugars are absorbed and presumably metabolised for energy by Caco-2 cells. The released polyphenols in apple digesta may not be a significant inhibitor of sugar absorption.

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