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Gluten: digestibility

by luciano

Gluten which is a compound formed by gliadin and glutenin which is the basis of baked products (bread and other) is not, as such, assimilable by the intestine but must be reduced to the amino acids components or small series (peptides) of them. The reduction occurs by different enzymes such as trypsin in the stomach, pepsin in the small intestine and other enzymes [1]. In normal health the intestine expels the parts of gluten that are not digested because they are too large to be assimilated. The digestibility of gluten is not only, however, dependent on the “strength of the gluten”, that is on the strength of the different types of bonds that “connect” the proteins of gluten but also on the type of enzymes that hydrolyse “break” the gluten and from the environment in which these processes take place. For example, trypsin in the stomach is activated (ie works), only in an acid environment. Furthermore, all digestive enzymes have the possibility of working better if directly in contact with gluten: something that can only occur in laboratory experiments, since these enzymes will have to “work” on in the stomach and intestines a “complex” of foods and not on gluten [2]. Knowledge of the digestibility of gluten is therefore extremely complex being affected by multiple factors, not least the variability of the conditions of the environment where it occurs (stomach and intestine).

The method of preparation of the finished product should not be overlooked. Indeed the digestibility of gluten, and more specifically, of the finished product is greatly influenced by the preparation method and the ingredients used [3]. Among these a primary role is played by the type of flour and the use of sour dough and / or yeasts. Certainly the use of flours that have little and weak * gluten favor the digestive process but a fundamental role is played by the sourdough (better if associated with very limited quantities of brewer’s yeast). The sourdough with its lactobacilli carries out a strong action of hydrolysis (chopping) of the gluten proteins both directly and by activating the proteases of the flour. Many studies and researches have been devoted to this subject, one in particular:

Protein Digestibility of Cereal Products Iris Joye
Department of Food Science, University of Guelph, Guelph, ON N1G 2W1, Canada; ijoye@uoguelph.ca; Tel.: +1-519-824-4120 (ext. 52470). Published: 8 June 2019
Abstract: Protein digestibility is currently a hot research topic and is of big interest to the food industry. Different scoring methods have been developed to describe protein quality. Cereal protein scores are typically low due to a suboptimal amino acid profile and low protein digestibility. Protein digestibility is a result of both external and internal factors. Examples of external factors are physical inaccessibility due to entrapment in e.g., intact cell structures and the presence of antinutritional factors. The main internal factors are the amino acid sequence of the proteins and protein folding and crosslinking. Processing of food is generally designed to increase the overall digestibility through affecting these external and internal factors. However, with proteins, processing may eventually also lead to a decrease in digestibility. In this review, protein digestion and digestibility are discussed with emphasis on the proteins of (pseudo)cereals.”

Gluten and intestine

by luciano

Digestion of Gluten Peptides in the Large Intestine

It has been shown that removing gluten from the diet affects the composition of the bacterial community in the large bowel, where the undigested food in the small intestine and could be hydrolyzed by microbial metabolism, generating beneficial compounds for the host.

“Alimentary protein digestion followed by amino acid and peptide absorption in the small intestinal epithelium is considered an efficient process. Nevertheless, unabsorbed dietary proteins enter the human large intestine as a complex mixture of protein and peptides.53,63 The incomplete assimilation of some dietary proteins in the small intestine has been previously demonstrated, even with proteins that are known to be easily digested (e.g., egg protein).64,65 The high proline content of wheat gluten and related proteins renders these proteins resistant to complete digestion in the small intestine. As a result, many high molecular weight gluten oligopeptides arrive in the lower gastrointestinal tract.66 While gluten peptides pass through the large intestine, proteolytic bacteria could participate in the hydrolysis of these peptides. A recent study from our group has shown that some of the gluten ingested in the diet is not completely digested while passing through the gastrointestinal tract, and is consequently eliminated in feces.

Moreover, it has been shown that the amount of gluten peptides present in feces is proportional to the amount of gluten consumed in the diet. Therefore, several gluten peptides are resistant to both human and bacterial proteases in the gastrointestinal tract.66,67

The large intestine is the natural habitat for a large and dynamic bacterial community. Although the small intestine contains a significant density of living bacteria, the density in the large intestine is much higher. The large intestine has as many as 1011–1012 cells per gram of luminal content that belong to thousands of bacterial taxa. Furthermore, the large intestinal microbiota is extremely complex and performs specific tasks that are beneficial to the host.68–71 Among the important functions that the intestinal microbiota performs for the host are several metabolic functions.72 In contrast to the rapid passage of dietetic components through the small intestine, the transit of the luminal material through the large intestine is considerably slower. The longer transit time in the large intestine has been associated with important bacterial metabolic activity.53 Therefore, undigested food in the upper gut could be hydrolyzed by microbial metabolism in the large intestine, generating beneficial compounds for the host.

The resistance of gluten peptides to pancreatic and brush border enzymes allows large amounts of high molecular weight peptides to enter the lower gastrointestinal tract. Therefore, gluten peptides are available for microbial metabolism in the large intestine and could be important to the composition of the intestinal microbiota. It has been shown that removing gluten from the diet affects the composition of the bacterial community in the large bowel.78,79 De Palma et al.78 observed that healthy subjects who followed a gluten-free diet for 1 month had reduced fecal populations of Lactobacillus and Bifidobacterium, but the population of Enterobacteriae such as E. coli appeared to increase. Similar results were obtained in studies with CD patients. Treated CD patients also showed a reduction in the diversity of Lactobacillus and Bifidobacterium species.80,81Gluten Metabolism in Humans. Alberto Caminero, … Javier Casqueiro, in Wheat and Rice in Disease Prevention and Health, 2014”


Gluten and “toxic” fractions (part I)

by luciano

– the structure of the gliadin and the toxicity of some fraction –
Gluten which is a protein compound formed by the prolamine, known as gliadin in wheat and responsible for the main phenomena of adverse reactions, and glutenin present mainly in the endosperm of cereal caryopsis such as wheat, spelled, rye and barley. Gluten is formed when water, flour and yeast are mixed: gliadin and glutenin combine to form a mixture characterized by viscosity, elasticity and cohesion. Therefore the quantity and integrity of the proteins that make up the gluten present in a flour are an important index to evaluate the quality and aptitude for baking.
Gliadin and glutenin, therefore, have been the subject of numerous research both in relation to the properties concerning the rheological characteristics of the doughs and to the adverse reactions that activate the immune system. Studies have been carried out on celiac disease that have discovered who and how this pathology is caused: they are some peptides (a set of amino acids) present, especially in the gliadin that contain sequences that are toxic, ie they activate the adverse reaction in genetically predisposed subjects of the immune system. The gliadin, in turn, is composed of several sub-units and these contain the “toxic” fractions in different quantities and qualities. Not only has William Hekkins’ research highlighted how the shape and location of gliadin molecules also influence not only chemical and physical properties but also toxicity.
“The gliadin proteins are heterogenous in different regions of the molecule and consequently differ in phisical and chemical properties. About 35% of the gliadin molecule is the alfa helix form, whereas 35% are beta turns(5). The latter are concentrated in the N terminal and C terminal more apolar parts of the gliadin. The remaining part has a random structure. These form have conseguences for the immunogenecity of the different regions in the molecule. Especially beta-turns are immunogenic.” The Toxicity of wheat prolamins William TH. J. M. Hekkens Annales Nestlé 1995 n. 51.
The study also analyzed the mechanism underlying the toxicity by detecting how “the passage of undigested gliadin fragments (fragments longer than 8 amino acids) or a lower tolerance to gliadin causes the immune system to react”. It is not enough, therefore, to know how much gliadin is present in a grain, but it is necessary to have the complete screening of its sub-units (quality, quantity, and, according to the study mentioned also form and position). The study on the “structure of gliadin” could partly explain why some ancient grains (for example, the monococcus), despite having a quantity of gliadin (and in particular alpha gliadin) not inferior to modern grains, have almost zero toxic levels.
The Toxicity of wheat prolamins