The ideal composition of oral rehydration solutions (ORSs) for the treatment of acute diarrhea in infants and young children continues to be debated. It is widely believed that improving the taste of ORS might lead to greater patient acceptance (1), especially for the maintenance phase of rehydration therapy when the salty taste of the higher sodium solutions may reduce intake. Some manufacturers have added flavoring agents to try to overcome this problem. Another approach could be to change the composition of the solution by partially replacing glucose with sucrose and increasing the citrate concentration at the expense of chloride.
In a pilot study, 32 healthy volunteers compared the taste of four test solutions (Table 1); a sucrose-citrate and a sucrose-chloride ORS were found to taste significantly better than a commercially available European ORS (Orisel, Nutricia). However, the implications of such modifications for epithelial salt and water transport were not examined. Therefore, we examined the effects of partially or totally replacing glucose with sucrose and chloride with citrate in animal perfusion models of normal and rotavirus-infected small intestine; rotavirus was chosen because it has become the leading cause of infantile diarrhea throughout world (2).
MATERIALS AND METHODS
Normal Adult Rats
Male Wistar rats (200-250 g) were fasted 18-20 h before each experiment but had free access to water. Rats were anesthetized with i.p. sodium pentobarbitone (60 mg/kg), then the abdominal cavity was opened through a mid-line incision and the intestine was cannulated at the duodenojejunal junction and just proximal to the ileocecal valve. The segment was gently rinsed with warm saline and then returned to the abdominal cavity. The body temperature of the rats was maintained at 37°C by a heating pad and overhead lamp. Single-pass perfusions were performed at a flow rate of 0.5 ml/min (3). The equilibration period was 45 min, and three 10-min collections were done to confirm that a steady state had been established. After perfusion, the animals were killed, and the perfused segment was removed, oven-dried, and weighed.
Rotavirus-Infected Neonatal Rats
Eight-day-old neonatal Wistar rats (12-15 g) of both sexes were used. Animals were infected at 6 days of age by intragastric inoculation with 0.25 ml of rotavirus containing medium consisting of 20 μl of intestinal homogenate containing 107-108/ml group B rat rotavirus particles mixed with 0.23 ml of RPMI-1640 culture medium (Gibco Ltd., Scotland). This method has been demonstrated to induce acute diarrhea and partial villous atrophy in the small intestine by 24 h (4). The rats were then returned to their mothers and allowed to suckle. At 48 h postinfection, they were perfused using a method similar to that described for adult rats except the small intestine was cannulated distal to the gastroduodenal junction and the flow rate was 0.25 ml/min. A minimum of six animals was used to test each solution.
Three hypotonic experimental ORSs (Table 1) and the commercially available European ORS (Euro-ORS) were used in each model. The experimental ORSs were of identical sodium, potassium, and chloride content except that in the Suc/cit-ORS, chloride was removed in favor of increased citrate. In Euro-ORS, potassium and chloride concentrations were higher than in the other ORSs and the citrate concentration was lower. Suc/cit-ORS and Suc/Cl-ORS had glucose partially replaced by sucrose, while Glu-ORS and Euro-ORS contained glucose alone as substrate.
All solutions contained polyethylene glycol (PEG), 4,000, and 14C-PEG, 4 μCi/L, as nonabsorbable volume marker (5).
Analysis and Statistics
The concentrations of glucose, sodium, chloride, and 14C-PEG were determined in each 10-min collection and the results averaged. Free glucose was measured by the glucose-oxidase method with the Beckman Glucose Analyzer 2. The quantity of unhydrolyzed sucrose in the effluent was determined from the differences in the glucose concentration before and after acid hydrolysis. Sodium was measured by flame photometry with an IL 943 flame photometer (Instrumentation Laboratory), chloride with a Corning 925 chloride meter, and 14C-PEG in a LKB 1219 Ultrabeta liquid scintillation counter. Water and solute fluxes were calculated by means of standard formulae from their measured concentrations in the perfusate and effluent (6). All results are expressed as medians and interquartile ranges; differences were considered significant at p < 0.05 by Wilcoxon unpaired-rank test.
Normal Small Intestine
Water absorption (Fig. 1) was similar from Suc/cit-ORS and Suc/Cl-ORS but was greater from Glu-ORS than Suc/cit-ORS or Euro-ORS. Glucose absorption was similar from all ORSs (Table 2). Suc/cit-ORS caused profound chloride secretion (Fig. 2), in contrast to the other ORSs, which all resulted in net chloride absorption. Sodium movement followed a similar pattern, with Glu-ORS and Suc/Cl-ORS performing better than either of the other two ORSs. Sucrose hydrolysis was similar for both sucrose-containing ORSs (Table 3).
Euro-ORS promoted the least water absorption, while Glu-ORS produced more water absorption than either of the sucrose ORSs (Fig. 3). As with the adult rat model, glucose absorption was not significantly different between any of the ORSs (Table 2). Chloride secretion was observed with all ORSs (Fig. 4) and was greater with Suc/cit-ORS than Suc/Cl or Glu-ORS. Sodium movement paralleled the chloride movement pattern. Sucrose hydrolysis (Table 3) was similar for both sucrose-containing ORSs but was substantially reduced compared with hydrolysis in normal intestine (p < 0.01).
There are a number of possible ways in which the taste of ORS may be improved. Some manufacturers have added flavoring agents, although doubts have been raised as to the advisability of this approach, particularly for neonates and young infants. An alternative approach might be to change the relative composition of the solutes. An unpublished pilot study in adult volunteers revealed that sucrose-containing solutions tasted better than a standard glucose-containing Euro-ORS. Sucrose-based solutions have been used in experimental animal models and in clinical trials, but the results are conflicting (7-11).
Using our test solutions in normal intestine, water absorption from Suc/Cl-ORS was not different from that from Glu-ORS, which otherwise had a similar composition. However, the chloride-free Suc/cit-ORS induced less water absorption than Glu-ORS; the probable explanation for this finding is the complete absence of chloride, since chloride secretion results in less net water flux (12). The higher osmolality of Euro-ORS appeared to impose a clear penalty for water absorption versus the other sucrose-free solution (Glu-ORS), as seen previous observations (13). In neonatal rats infected with rotavirus, net water absorption followed a pattern similar to that seen in the adult rat model although the differences between ORSs were more evident.
The chloride-free Suc/cit-ORS promoted profound chloride secretion in contrast with the net absorption seen with all other ORSs in normal intestine. It is of note that the high-chloride Euro-ORS did not alter net chloride secretion in the rotavirus-infected intestine.
Positive sodium balance was achieved in normal intestine with all solutions tested except for the chloride-free Suc/cit-ORS, probably due to the decreased chloride availability for Na-Cl cotransport. A previous study showed that a sucrose-ORS led to increased sodium absorption than seen with an equimolar glucose-ORS in a similar model (14). We were not able to demonstrate this phenomenon, possibly because of the relatively low concentrations of sucrose in the solutions. In the rotavirus model, net sodium secretion was observed with all solutions; it was significantly increased with Suc/cit-ORS and Euro-ORS.
In the process of sucrose absorption, hydrolysis at the brush border releases glucose and fructose, the first being more readily absorbed than the second (15,16). The explanation for this finding lies in the mechanism of absorption, which is active for glucose but passive for fructose (17), allowing much more fructose than glucose to return to the lumen and increasing the osmolality of the bulk phase. Patra et al. have shown, using an animal model, that the osmotic penalty imposed by intraluminal fructose accumulation reduces water absorption from ORSs (14). The consensus of the last few years has been that glucose rather than sucrose should be the preferred substrate for ORS. However, to our knowledge, no study has used partial replacement of sucrose for glucose as a means of improving taste while trying to avoid intestinal overload with fructose.
Glucose absorption was similar from all ORSs, suggesting that the small amount of substituted sucrose allowed efficient hydrolysis and did not detract from overall glucose absorption. The extent of sucrose hydrolysis (Table 3) did not differ between the sucrose-containing solutions in either model. In the rotavirus model, sucrose was hydrolyzed to a lesser extent than in normal adult rat small intestine, but comparing data from these models in this regard may not be wholly appropriate. The low amount of sucrase present in normal neonatal rat intestine probably precludes complete sucrose hydrolysis, but this is not the case in human infant intestine in which full maturation of sucrase is present at birth. However, it has been shown that precocious maturation of sucrase in neonatal rats may be induced by increasing plasma corticosterone and rotavirus infection (18,19). These observations indicate that this rotavirus model in neonatal rats is valid for testing sucrose-containing ORSs. The glucose absorption from each of the sucrose-ORSs was not different from that from Glu-ORS, presumably because of the low sucrose content of the solutions. Previous work has shown that a glucose concentration of 90 mM in an ORS is adequate (20), and the 77 mM in the two sucrose-based solutions was probably enough to obtain an effect similar to that of the glucose-ORS tested.
Citrate has been shown to have a proabsorptive effect on water in normal rat and human intestine (21), but our findings suggest that increasing it at the expense of chloride failed to induce more water absorption in both normal and diseased intestine.
While it appears that increasing the sucrose and citrate content of ORSs improves palatability, our results in these models suggest that such modifications would incur a significant penalty in terms of salt and water absorption. One cannot, however, extrapolate from these animal models to humans; however, concern about reducing chloride concentrations in ORS has been raised previously (22), and our study supports the view that there is a need for caution before attempting to improve ORS taste by altering composition.
Acknowledgment: This work was supported by a grant from Cow & Gate/Nutricia. M.J.G.F. gratefully acknowledges financial support from the Wellcome Trust.
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Keywords:© Lippincott-Raven Publishers
Oral rehydration solutions; Sucrose; Citrate; Animal models; Rotavirus