Fructose water absorption

Sucrase is associated with the brush border of the intestinal mucosal cells. It catalyzed the hydrolysis of sucrose to D-glucose and D-fructose. Its absence means that sucrose passes through the small intestine where it enters the large intestine. There it interferes with water absorption because it is osmotically active and leads to the retention of water in the bowel lumen. This gives rise to abdominal cramps and watery diarrhea. Treatment involves limiting sucrose in the diet and more recently the ingesting of a proprietary enzyme, Sucraid. 

Figure 19.4 shows the water absorption characteristics of several sweeteners. Sorbitol and fructose are very hygroscopic, while other sugars absorb water only at higher relative humidities. Chemical reactions of sugars were covered in detail in Chapter 4. Only those reactions important from a technological viewpoint will be emphasized here. 

FOS and oligofuctose are fructose oligomers that are either produced by enzymic conversion of sugar or extracted from chicory, as inulin, and then hydrolysed. These products behave as soluble fibres and prebiotics. In acid conditions, they can hydrolyse, but are usually sufficiently stable for short-shelf-life juices, near-water products with low acid levels or powdered soft drinks. Prebiotic activity varies with preparation and required daily dose can be as low as 2.5-5.0 g/day for shorter chain FOS preparations (DP 2 1). Some positive effects on magnesium absorption and calcium absorption (in some populations) have also been shown (Beghin Meiji, 2001). 

In our investigation on the effect of Maillard reaction products on the absorption of tryptophan (36), the kinetics of the absorption of tryptophan in the presence of Maillard reaction products formed in the glucose-tryptophan system was studied by bott vitro everted gut sac method and vivo catherization of the portal vein. Fructose-L-tryptophan (Amadori compound) appeared to be the major fraction of the reaction products when fractionated using a cellulose column eluted by water-saturated n-butanol. The absorption of L-tryptophan was partially inhibited vitro and vivo by fructose-L-tryptophan in a competitive manner with an inhibitor constant (Ki) of l.lmM. The relative absorption rate of L-tryptophan was significantly lower in the presence of the Maillard reaction products than in the presence of fructose-L-tryptophan indicating the presence of other inhibitory factors in the reaction products. The in vivo absorption of fructose-L-tryptophan was almost negligible compared to that of tryptophan. The inhibited absorption by Maillard reaction products, may have contributed in part to an incomplete recovery in the growth of the rats when fed a supplemented browned synthetic amino acid diet. 

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