Role of Short-Chain Carbohydrates in GI Tract Disorders

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(Last Updated On: April 23, 2017)
Gastrointestinal discomfort due to short-chain carbohydrates

Gastrointestinal discomfort due to short-chain carbohydrates

In this article, you will find the role of short-chain carbohydrates in Gastrointestinal tract disorders. How they affect and how they interact within the GI tract?

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Short-Chain Carbohydrates are Oligosaccharides

Carbohydrates occur in a range of foods which are regularly consumed. These foods are wheat, ray, rice, maize, vegetables, fruits and legume. Short-chain carbohydrates with 10 or lesser monosaccharide units are called as oligosaccharides and they vary in their digestibility and subsequent absorption in the gastrointestinal tract. Those oligosaccharides that are poorly absorbed exert osmotic effects in the intestinal lumen increasing water retention in the gastrointestinal tract.

These short chain carbohydrates are fermented rapidly by bacteria (residing in the intestine) producing gas as a consequence. These two effects alone may cause different gastrointestinal symptoms after being ingested in the moderate amounts via luminal distension in visceral hypersensitive patients.

This type of effect is the basis of lactose-free diet in those individuals suffering from lactose malabsorption and fructose reduced diet for people with fructose malabsorption. As all dietary poorly absorbed short-chain carbohydrates have similar and additive effect in the intestinal tract, a concept has been developed to assign them as FODMAPs which is a short form of Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols and to evaluate a dietary approach that restricts these short-chain carbohydrates.

Absorption of Short-Chain Carbohydrates in Small Intestine

Monosaccharides like glucose, fructose, galactose can be absorbed across the small intestinal epithelium. Thus, all the disaccharides and oligosaccharides must be hydrolyzed to their constituent monosaccharides prior to absorption. The main dietary disaccharides include sucrose, lactose, maltose, isomaltose, and trehalose which are hydrolyzed by their respective enzymes (hydrolases) expressed by intestinal epithelium to yield monosaccharide units.

However, among the enzyme digesting disaccharides, lactose is only one which is deficient in its activity with the subsequent malabsorption and thus lactose is delivered to distal small and proximal large intestine in undigested form for fermentation by bacteria.

In the case of oligosaccharides, only maltooligosaccharides which are also called as α-glucans are hydrolyzed (digested) in the small intestine and other oligosaccharides are thought to be nonhydrolyzable form. These may include non-α-glucans like galactooligosaccharides, xylooligosaccharides, fructooligosaccharides because mammalian intestine does not synthesize enzymes that can hydrolyze them.

Structure of Raffinose, a disaccharide

The structure of Raffinose, a disaccharide. Image: via Common Wikimedia

Different Absorption Rates of Monosaccharides in Intestine

Dietary monosaccharides vary in their absorption. As, for example, glucose and galactose have specific transporter-mediated uptake mechanism in the small intestine where they are absorbed rapidly (complete absorption). However, fructose has two absorption pathway; a) low glucose independent facilitated transport via GLUP and b) high capacity glucose-dependent fructose cotransport via GLUT2.

This dual system implies that fructose is absorbed with different efficiencies depending upon the current presence of glucose.

Thus, in the presence with a low concentration of glucose, and a higher concentration of fructose, fructose is slowly absorbed and, therefore, fructose malabsorption may lead to a physiological condition. Two other dietary hexoses showing similar behavior are xylose and arabinose, both of which are passively absorbed. Polyols including sorbitol, mannitol, lactitol, xylitol are also believed to be absorbed by passive diffusion.

The ability of an individual person to absorb an individual carbohydrate can be tested by breath hydrogen testing, the principle of which is that hydrogen gas will be produced and a rise is detected in the breath if the tested carbohydrate reaches the fermenting bacteria before it can be digested/absorbed.

The Mechanism by which poorly absorbed Short-Chain Carbohydrates Induce Gut Symptoms

As small carbohydrates are osmotically active in the intestinal lumen, poor absorption or digestion of such carbohydrate leads to increase of intestinal luminal water volume. They are rapidly fermented producing hydrogen gas, carbon dioxide, methane etc. poorly absorbed short-chain carbohydrates also have other physiological effects like an alternation of esophageal motility, increased gastroesophageal reflux.

Reason Why Poorly Absorbed Short-Chain Carbohydrates Are Restricted in FGID

Lactose: lactose is digested by the enzyme lactase. The secretion of which decreases along with the increase in the age of a child and therefore in adult people lactose is mal-digested or mal-absorbed causing different intestinal discomfort. This is called as lactose intolerance. Some people can digest and absorb lactose and they are called as lactose tolerance.

Fructose: patients with FGID are also found to intolerance with fructose. They are unable to digest and absorb the fructose completely and can show different intestinal discomfort including diarrhea.

Thus, based on the concept the short-chain carbohydrates induce the same effect in the intestine when ingested and their effects are additive. Therefore, all class of poorly absorbed short-chain carbohydrates are grouped as FODMAPs and are restricted in the foods.

Altering The carbohydrate Intake May Lead to Prevent Getting This Problem

The accumulation of FODMAPs in gastrointestinal tract triggers the gastrointestinal syndrome. Altering carbohydrate intake can be used as a treatment for functional gastrointestinal disorders. Carbohydrates are the major target of dietary modifications for functional gut symptoms. It is found that patients with irritable bowel syndrome (IBS) are taking a large content of wheat and other grains show symptoms.

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