Gastric emptying and lactase deficiency Chronic lactose intake modifies the gastric emptying of monosaccharides but not of disaccharides in weanling rats

Ninety-six weanling male Wistar rats were fed for four weeks one of two different chows: a normal rat chow containing 55.5% (w/w) starch (control group, N = 48) or a rat chow in which starch was partially replaced by lactose, in such a way that the experimental group (N = 48) received 35.5% (w/w) starch and 20% (w/w) lactose. The gastric emptying of fluid was then studied by measuring the gastric retention of four test meals containing lactose (5% or 10%, w/v) or glucose + galactose (5% or 10%, w/v). Homogenates of the small intestine were assayed for lactase activity. The gastric retention values were obtained 15 min after orogastric infusion of the liquid meals. The median values for gastric retention of the 5% lactose solutions were 37.7% for the control group and 37.0% for the experimental group (P>0.02). For the 10% lactose solution the median values were 51.2% and 47.9% (P>0.02) for the control and experimental groups, respectively. However, for the 2.5% glucose + 2.5% galactose meal the median gastric retention was lower (P<0.02) in the group fed a lactose-enriched chow (38.5%) than in the control group (41.6%). For the 5% glucose + 5% galactose solution the median values were not statistically different between groups, 65.0% for the control group and 58.8% for the experimental group. The median values of the specific lactase activity in the small intestine homogenate was 0.74 U/g in the control group and 0.91 U/g in the experimental group. These values were not statistically different (P>0.05). These results suggest that the prolonged ingestion of lactose by young adult rats changes the gastric emptying of a solution containing 5% monosaccharides. This adaptation may reflect the desensitization of intestinal nutrient receptors, possibly by an osmotic effect of lactose present in the chow. Correspondence E.A.L. da-Costa-Pinto Departamento de Pediatria Faculdade de Ciências Médicas Universidade Estadual de Campinas 13081-970 Campinas, SP

• Gastric emptying Gastric emptying is regulated by the activation of receptors situated in the stomach and in the small intestine (1).In the small intestine, equivalent amounts in calories of lipids, proteins and carbohydrates delay gastric emptying to the same extent as a saline solution (2).
To control the gastric emptying of disaccharides and polysaccharides, the above receptors are only activated following the hydrolysis of carbohydrates (3).Individuals with incomplete hydrolysis of lactose have a lower gastric retention of solutions containing this disaccharide, per unit time, than those with normal hydrolysis (4).The same phenomenon has been observed in rats with ontogenic lactase deficiency.In these animals, the gastric retention of a lactose solution up to 10 min after administration was 20% lower than that of maltose which is completely hydrolyzed (5).
In adult rats, the prolonged intake of lactose induces a significant increase in lactase activity, although the levels reached are not as high as those observed during lactation (6).At the same time, the large intestine is enlarged, with an increase in wet weight (7).From a functional point of view, the mechanisms of adaptation to prolonged lactose intake lead to a recuperation of unabsorbed liquids and caloric substrate in the colon (8,9).Since there is little information on the gastric responses to this condition, the present study was conducted to investigate the effects of a prolonged intake of lactose on the gastric emptying of this disaccharide and its constituent monosaccharides in young adult rats.
Ninety-six 4-week old male Wistar rats were studied.The animals were divided into two groups (control and experimental) and were matched for weight and age.
The control group (N = 48) was fed a standard chow containing 55.5% starch, 20% casein and 11% vegetable oil, as well as a mixture of vitamins, mineral salts and codliver fat, for four weeks (10).The experimental group (N = 48) was fed a chow in which starch was partially replaced by monohydrated lactose, so that this group received a chow containing 35.5% (w/w) starch and 20% (w/w) lactose.Each group was housed in cages containing a maximum of 12 animals each.Tap water and the respective chows were provided ad libitum throughout the four weeks.
The gastric emptying of fluid was studied on the 28th day following a 20-h fast.The control and experimental groups were divided into four subgroups containing 12 animals each, which were then fed either an aqueous lactose solution or an aqueous solution containing equal amounts of glucose + galactose.Two final concentrations (5 and 10 g/dl) were tested for each of the solution.The 5% glucose + galactose solution consisted of 2.5% glucose and 2.5% galactose, and the 10% solution consisted of 5% of each monosaccharide.Phenol red (6 mg/dl) was used as a marker (11).
The meals were infused via an orogastric tube in a volume of 2 ml/100 g body weight.After 15 min, the residue remaining in the stomach was aspirated.The orogastric intubation and the recovery of gastric residue were performed as previously described (11,12).Gastric retention was determined by the method of Gupta and Brans (13) and the marker was detected using a Klett photocolorimeter equipped with a green filter.The gastric retention values were compared by the Kruskal-Wallis test at a significance level of 0.10 (14).When the values were significant, the multiple comparisons test was applied, with a P value ≤0.02 indicating a significant difference (15).
Following the gastric emptying test, the small intestine of all animals was separated and homogenized on ice, using a Potter-Elvejehem homogenizer.Lactase activity was measured in the homogenates using the assay described by Dahlqvist (16).
The median valve of specific lactase activity (U/g small intestine wet weight) was higher in the group fed a lactose-enriched chow (0.91 U/g) than in the one fed a normal chow (0.74 U/g), but the difference was not statistically significant (P>0.05,Student ttest) (17).
Figure 1 shows the gastric retention of test meals containing 5% lactose or 2.5% glucose + 2.5% galactose solutions.In the control group, the median retention value for the monosaccharide solution was significantly higher than the median value for the lactose-containing test meal (41.6% vs 37.7%).These results are consistent with data in the literature (18) and show that, in young adult rats with ontogenic lactase deficiency, the gastric emptying of lactose-containing solutions is faster than that of a mixture of monosaccharides.In the experimental group, the median retention value of 2.5% glucose + 2.5% galactose solution was not different from the lactose solution (38.5% vs 37.0%).Likewise, there was no difference in the median retention value for the lactose-containing meal between the control and experimental groups (37.7% vs 37.0%).However, the median retention value for the glucose + galactose meal was significantly lower in the experimental group (38.5%) than in the control group (41.6%).These results indicate that the intake of the lactose-enriched chow did not interfere with the emptying of the disaccharide-containing solution but did change the emptying pattern of the monosaccharide meal.This alteration probably reflects a down-regulation of intestinal glucose receptors, and may be mediated by the osmotic effect of the chow, a phenomenon which has been reported in humans following the intake of high quantities of glucose (19).
The retention values for the 10% lactose and 5% glucose + 5% galactose solutions are presented in Figure 1.Emptying was similar to that observed with the 5% solutions; the lactose retention values were lower than those for the glucose + galactose solution and were comparable between the control and experimental groups.Unlike the results for the 5% concentrations, the retention values for the 10% monosaccharide solution did not differ between groups, suggesting that the desensitization effect of a lactose-enriched chow observed with less concentrated meals may have been supplanted by the effect of test meal osmolality.
In conclusion, the present results show that, in adult rats with ontogenic lactase deficiency, the process of adaptation to chronic lactose intake does not involve an alteration in the gastric emptying of the disaccharide but changes the emptying pattern of the 5% monosaccharide test meal.

Control group
Experimental group Figure1-Gastric retention (%) 15 min after the infusion of test meals containing 2.5% glucose + 2.5% galactose (5% glu + gal), 5% lactose, 5% glucose + 5% galactose (10% glu + gal) or 10% lactose.The rats were fed normal chow (control) or chow with 20% (w/w) lactose (experimental) for four weeks after which time the gastric retention was measured (N = 12 per subgroup).The data are presented as box plots, where the intermediate, lower and upper horizontal lines indicate the median, first and third quartiles of the gastric retention values, respectively, and error bars indicate the maximum and minimum gastric retention values observed.Significant differences between subgroups tested by the Kruskal-Wallis test (P<0.10)followed by the multiple comparisons test (P<0.02)are indicated in the figure.