1. Unlike standard glucose-electrolyte oral rehydration solutions, solutions containing polymeric glucose as substrate can significantly reduce stool output, duration of diarrhoea and total oral rehydration solution requirements. However, neither the underlying mechanisms nor the optimal size and concentration of glucose polymer has been defined.

2. We have used a model of rotavirus diarrhoea in neonatal rats to compare the effects on water and solute absorption of varying the concentration of a glucose polymer (mean chain length five glucose residues) in experimental oral rehydration solutions. Three polymer (P) solutions were compared with solutions of identical electrolyte content (mmol/l: sodium, 60; potassium, 20; chloride, 60; citrate, 10) containing equivalent amounts of free glucose (G) as substrate by perfusion of the entire small intestine in situ. The polymer (9, 18, 36 mmol/l; 159, 168, 186 mosmol/kg, respectively) and the monomer (45, 90, 180 mmol/l; 195, 240 320 mosmol/kg) solutions were perfused in normal and rotavirus-infected neonatal rats.

3. In normal intestine polymer solutions promoted greater water absorption [P9, mean 291.4 (SEM 16.4); P18, 331.9(13.1); P36, 284.3 (11.8) μl mi−1 g−1] than their equivalent monomer solutions [G45, 220.8 (8.4); G90, 240 (21); G180,79.4 (145) μl min g−1; P < 0.02]. In rotavirus-infected intestine, water absorption from all solutions declined, but the fall was much less pronounced from the polymer solutions [P9, 232.8 (6); P18, 277.2 (20.5); P36, 166 (18.2) μl min−1 g−1] than from their monomeric counterparts [G45, 116.7 (25.5); G90, 68.7 (12.4); G180, 21 (11.6) μl min−1 g−1; P < 0.005].

4. In both the normal net absorptive state and the net secretory state induced by rotavirus infection, there was a striking inverse correlation between net water absorption and perfusate osmolality (r = −0.94 and r = −0.88, respectively; P < 0.05). In rotavirus-infected intestine, increasing the polymer concentration from 18 to 36 mmol/l resulted in a relative fall in water absorption (P < 0.01). The hypertonic solution G180 was associated with the lowest water absorption (P < 0.01). None of the solutions was able to reverse rotavirus-induced net secretion of sodium, which was similar from all solutions, whether polymer- or monomer-based.

5. These results (i) emphasize the pre-eminence of hypotonicity among the factors promoting water absorption from polymer-based oral rehydration solutions in acute diarrhoea, (ii) confirm the adverse consequence of raising substrate concentration (whether polymer or monomer) beyond certain limits and (iii) indicate that the concentration of this glucose polymer yielding the optimum compromise between substrate availability and low osmolality may be approximately 9–18 mmol/l.

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