Carbohydrate Maldigestion and Intolerance

Abstract

This review summarizes dietary carbohydrate intolerance conditions and recent advances on the possible role of carbohydrate maldigestion and dietary outcomes in patients with functional bowel disease. When malabsorbed carbohydrates reach the colon, they are fermented by colonic bacteria, with the production of short-chain fatty acids and gas lowering colonic pH. The appearance of diarrhoea or symptoms of flatulence depends in part on the balance between the production and elimination of these fermentation products. Different studies have shown that there are no differences in the frequency of sugar malabsorption between patients with irritable bowel disease (IBS) and healthy controls; however, the severity of symptoms after a sugar challenge is higher in patients than in controls. A diet low in 'Fermentable, Oligo-Di- and Monosaccharides and Polyols' (FODMAPs) is an effective treatment for global symptoms and abdominal pain in IBS, but its implementation should be supervised by a trained dietitian. A 'bottom-up' approach to the low-FODMAP diet has been suggested to avoid an alteration of gut microbiota and nutritional status. Two approaches have been suggested in this regard: starting with only certain subgroups of the low-FODMAP diet based on dietary history or with a gluten-free diet.

Keywords: FODMAP; fructose; hydrogen breath test; irritable bowel syndrome; lactose; sorbitol; sucrose; sugar malabsorption.

Figure 1

Short-chain fatty acid and gas (H₂, CO₂, CH₄) production by the colonic microbiota fermentation of dietary unabsorbed carbohydrates. Thirty to fifty grams of soluble dietary fibre, resistant starch, oligosaccharides (fructo-oligosaccharides, galacto-oligosaccharides, raffinose, and stachyose), disaccharides (lactose, sucrose), and monosaccharides (fructose) enter the colon each day and become available for colonic fermentation by the microbiota. Acetate (C2), propionate (C3), and butyrate (C4) are the main short-chain fatty acids that play important roles in gastrointestinal function.

Figure 2

Enterocyte monosaccharide transporters involved in D-glucose, D-galactose, and D-fructose absorption in the small intestine. GLUT2, which is only observed in the apical brush-border membrane at high D-glucose concentrations in intestinal lumen, is indicated in red (see reference [1]). Red dots between enterocytes represent the tight junctions.

Figure 3

‘Top-down’ and ‘bottom-up’ approach to the low-FODMAP diet (adapted with permission from reference [46]; Copyright 2019, John Wiley & Sons, Inc). Reducing a few specific foods/subgroups in the ‘bottom-up’ approach implies an adequate diet history, for example, that in a patient who consumes chewing gum and/or candies, these are withdrawn; if he/she consumes large quantities of fruits and fruit juices, they may benefit from the restriction of only excess fructose and polyols to start with; or if he/she consumes large amounts of wheat, onion, artichokes, and pulses, they may be more likely to benefit from a restriction of fructans. Furthermore, in the ‘bottom-up’ approach, a gluten-free diet has been claimed to be the easiest way to reduce fructan intake (see text).