Early dietary experience influences ontogeny of intestine in response to dietary lipid changes in later life

E. Jarocka-Cyrta, N. Perin, M. Keelan, E. Wierzbicki, T. Wierzbicki, M. T. Clandinin, and A. B. R. Thomson

Nutrition and Metabolism Research Group, Division of Gastroenterology, Department of Medicine, Agriculture, Food, and Nutrition Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2C2

    ABSTRACT
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

This study was undertaken to test the hypothesis that a change in the mother's diet at the time of birth and continued during suckling modifies the intestinal transport of nutrients in the suckling offspring. Pregnant rat dams were fed one of four semisynthetic diets during pregnancy [high or low n-6/n-3 diet or a diet enriched with arachidonic acid (AA) or docosahexaenoic acid (DHA)] and were fed the same diet at the time of birth or switched to another diet. The greatest body weight gain was in the suckling rats (15-16 days of age) fed a low n-6/n-3 diet. Switching from this diet caused weight loss, and the observed weight gain with the low n-6/n-3 diet was prevented by previous exposure of the mother to the high n-6/n-3 diet or the AA- or DHA-containing diet. Although continuous feeding of a high n-6/n-3 diet to the mother during pregnancy and lactation was associated with the lowest in vitro rates of fructose uptake, switching the mother to another diet during lactation did not necessarily correct the low absorption. In contrast, continuous feeding of a high n-6/n-3 diet to the mother during pregnancy and lactation is associated with the highest maximal transport rate of glucose uptake into the jejunum and ileum. Jejunal uptake of fatty acids 12:0, 18:0, 18:3(n-3), and cholesterol was less with the low n-6/n-3 diet compared with the high n-6/n-3 diet, whereas the ileal uptake of 18:0 and 18:3(n-3) was higher with the low n-6/n-3 diet. Thus the ontogeny of the intestine is critically influenced by the mother's diet during gestation as well as during the nursing period. Some of the diet-associated changes in nutrient uptake resulting from the mother's diet during pregnancy could be corrected by dietary interventions introduced after birth.

adaptation; amniotic fluid; diet effects; ontogeny

    INTRODUCTION
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

THE INTESTINAL MUCOSA is a dynamic structure that undergoes biochemical, ultrastructural, and morphological changes throughout life (56, 57). The absorptive cells of the small intestine have the capability of adapting their structure and function in response to changes in the intraluminal content of nutrients (24). The lipid components of biological membranes are the main determinants of their physicochemical properties (9, 10, 52, 54). Many of the protein-mediated digestive and transport functions of the intestinal brush-border membrane are influenced by its lipid composition (53, 54). The signals by which dietary fats influence intestinal morphology and function remain unknown. The fatty acid composition of membrane phospholipids responds to changes in the animals' intake of fatty acids and the balance between the n-6 and n-3 fatty acids (15, 27). The phenotypic expression of the intestinal digestive and transport functions may be modified by the lipid content of the diet (55). Isocaloric modifications in the ratio of saturated to polyunsaturated fatty acids in the diet also modify the intestinal uptake of nutrients. For example, glucose uptake in the intestine is upregulated by feeding a low polyunsaturated/saturated fatty acid diet and conversely is downregulated by feeding a high polyunsaturated/saturated fatty acid diet (53). The diabetes-associated changes in intestinal uptake of nutrients are modified by isocaloric alterations in the type of dietary lipids and are associated with variations in the phospholipids and fatty acyl content of the intestinal brush-border membrane (28).

During its life in utero, the fetus is supplied with most of its substrate needs by the placenta (44). The healthy fetus swallows large amounts of fluid, consisting of a mixture of amniotic fluid, lung liquid, and oral/nasal secretions (48). Ingestion of this fluid in utero may be an important regulator of prenatal gastrointestinal tract development (43). For example, in the presence of congenital defects, such as esophageal or intestinal atresia (both of which restrict access of the swallowed fluid to the gastrointestinal tract), there may be impaired function of the intestine. In addition, ligating the fetal sheep esophagus results in morphological changes in the intestine, which partially regress if the initial esophageal obstruction is relieved (59).

Limitations in transplacental nutrient uptake may be a major factor influencing fetal growth (44). Inadequate nutrient supply to an otherwise normal fetus (intrauterine growth retardation) is a major cause of perinatal morbidity and mortality (40). Amniotic fluid contains known fetal nutritional substrates, including glucose, protein, and lactate (35). The human fetus swallows up to 750 ml of amniotic fluid each day (47, 48), and fetal intestinal epithelium has the capacity to absorb carbohydrates, protein, and lipids in the third trimester of life (8, 31, 34, 49). Thus amniotic fluid may be a potential source of fetal nutrition. In fetal rabbits, amniotic fluid provides 10-14% of the nutritional requirements of the normal fetus, and amniotic fluid may contain a potent and as yet undefined gastrointestinal trophic factor (43).

In the differentiation of the intestinal mucosa, there are variations among transporters in the time of their first expression within the same species. For example, some transporters are expressed long before birth (6). Fetal swallowing has an important role in small intestinal development, as well as in the maturation of the small intestine (43). Supplementation of the amniotic fluid with solutions containing higher concentrations of nutrients has been proposed as a possible treatment in utero for intrauterine growth retardation caused by maternoplacental insufficiency. Harrison and Villa (21) refer to the continuous infusion of nutrient solutions as transamniotic fetal feeding. The ideal composition of intra-amniotic feedings for optimal growth and development has not been determined. Infusion of galactose into the amniotic fluid of pregnant rabbits stimulates intestinal absorption in the fetus (46). Intra-amniotic galactose infusion causes upregulation of its own mucosal transport and also that of glucose (4). In sheep, fructose in the amniotic fluid may regulate the activity and abundance of the sodium-dependent glucose transporter in the brush-border membrane (SGLT1) (14). Epidermal growth factor (EGF), a polypeptide with extensive proliferative and maturative effects, is a hormonal constituent of amniotic fluid (2, 5). EGF is postulated to have a role in fetal development (5), especially because it modulates lipid transport, triglyceride-rich lipoprotein formation, and secretion, as well as apolipoprotein synthesis during intestinal development (33, 34). Other hormones can be found in amniotic fluid, and they regulate lipid transport, such as insulin (32, 37) and glucocorticosteroids (36).

In addition, the maternal diet is important in the development of the fetus (29, 30). The type of maternal dietary fat consumed during gestation is an influencing factor in the fatty acid composition of lung phospholipids and in fetal growth and development (11). During the postnatal period, the quality and composition of dietary fat supplied to infants have important structural and functional effects on their rapidly growing tissues (25). Several studies (16, 19, 20, 22) have shown that the maternal diet, including the dietary fatty acid content, is reflected in the lipid composition of the milk. The fatty acids in milk have three different origins: the diet of the nursing mother, adipose stores, and the de novo synthesis in mammary glands (7). The dietary intake and metabolism of long-chain polyunsaturated fatty acids have received recent attention, since docosahexaenoic (DHA) and arachidonic acid (AA) serve an important role in perinatal visual and neural development (1, 38, 39). The influence of DHA and AA on intestinal development is unknown.

In a previous study, we showed that the changes of the fatty acid content of the nursing mother's diet are sufficient to alter the ontogeny of intestinal nutrient transport as well as intestinal adaptation in suckling offspring (45). In the present study, we tested the hypothesis that 1) changes in the fatty acid composition in the maternal diet during gestation alter ontogeny of intestinal nutrient transport of the fetal intestine and 2) these changes in intestinal transport are associated with modifications of the lipid content of amniotic fluid swallowed by the fetus during gestation.

    MATERIALS AND METHODS
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

Animals. The guiding principles in the care and use of laboratory animals approved by the Canadian Committee of Animal Care were observed in the conduct of this study. Pregnant Sprague-Dawley rats were housed individually in a well-ventilated room maintained at 21°C with a 12:12-h light-dark cycle. Water and food were supplied ad libitum.

Diets. One-week pregnant Sprague-Dawley rats weighing 220-250 g were fed one of four diets during the last 2 wk of gestation. Each diet was fed to four dams. On delivery of the pups, the nursing dams in each diet group were subdivided into four dietary treatment groups: a high or low n-6/n-3 diet or a diet enriched with AA or DHA. One dam was fed on the same diet as used during pregnancy. To minimize any possible effects of variations due to litter size, the number of pups per litter was reduced to 12 shortly after birth.

All semisynthetic diets contained 20% (wt/wt) fat. The high n-6/n-3 diet, with a ratio of n-6 to n-3 of 7.3:1, was based on the fat composition of an infant formula (Wyeth Ayerst, Radnor, PA). This mixture, referred to as "formula" fat blend, consisted of 35% oleo oil, 25% coconut oil, 23% canola oil, and 17% corn oil. The remaining diets were formulated from this fat blend by addition of other triglycerides. The composition of the diets is given in Table 1. The low n-6/n-3 diet (n-6/n-3, 4:1) was obtained by the addition of linseed oil to the fat blend. The AA diet contained 1% AA, and the DHA diet contained 0.7% DHA. These isocaloric semisynthetic diets were nutritionally adequate, providing all known essential nutritional requirements.

                              
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Table 1.   Fatty acid composition of diets

Probe and marker compounds. [3H]inulin was used as a nonabsorbable marker to correct for the adherent mucosal fluid volume. 14C-labeled probes included lauric acid (12:0), palmitic acid (16:0), stearic acid (18:0), oleic acid (18:1), linoleic acid [18:2(n-6)], linolenic acid [18:3(n-3)], cholesterol, D-glucose, L-glucose, and D-fructose. Unlabeled and 14C-labeled probes were supplied by Sigma Chemical (St. Louis, MO) and New England Nuclear (Boston, MA), respectively. Probes were shown by the manufacturer to be >99% pure by HPLC.

Tissue preparation and determination of uptake rates. The rate of in vitro uptake of sugars and lipids into everted intestinal rings was examined in the suckling pups (15-16 days of age). Animals were anesthetized by intraperitoneal injection of pentobarbital sodium (35 mg/kg body wt Euthanyl). A midline incision was made into the peritoneal cavity. The whole length of the small intestine was rapidly removed. Beginning at the ligament of Treitz, the jejunum was represented by the proximal third and the ileum by the distal third of the remaining intestine. The intestine was everted and cut into small rings of ~3 mm each (45). The rings were immersed immediately in preincubation beakers containing oxygenated Krebs-bicarbonate buffer (pH 7.2) at 37°C and were allowed to equilibrate before commencement of the uptake studies. Uptake was initiated by the timed transfer of the tissue rings to a shaking water bath (37°C) containing 5-ml plastic vials with gassed Krebs buffer, plus [3H]inulin and one of the following 14C-labeled substrates: D-glucose (4, 8, 16, 32, and 64 mM), L-glucose (16 mM), D-fructose (4, 8, 16, 32, and 64 mM), 0.1 mM fatty acids (12:0, 16:0, 18:0, 18:1, 18:2, 18:3), or 0.05 mM cholesterol. The long-chain fatty acids and cholesterol were solubilized in 20 mM taurocholic acid. After incubation for 5 min, the uptake of nutrient was terminated by pouring the vial contents onto filters on an Amicon vacuum filtration manifold maintained under suction, followed by washing the jejunal or ileal rings with ice-cold saline. The tissue was dried overnight to a constant weight. The dry weight of the tissue was determined, and the tissue was saponified with 0.75 N NaOH. Scintillation fluid was added, and radioactivity was determined by means of an external standardization technique to correct for variable quenching of the two isotopes.

Amniotic fluid lipid composition. The amniotic fluid composition of fatty acids was examined by analyzing the amniotic fluid aspirated from anesthetized pregnant dams fed one of the four diets. Lipids were extracted with chloroform-methanol (2:1, vol/vol) containing 0.005% (wt/vol) butylated hydroxytoluene, according to a modification of the methods of Bowyer and King (3) and Folch et al. (17). Lipids were separated on a solvent system composed of a mixture of petroleum ether, diethyl ether, and glacial acetic acid (84:18:1.0) (18). Total phospholipids and triacylglycerol spots were scraped off the plates for transesterification of their fatty acid components using 14% (wt/wt) boron trifluoride-methanol reagent-hexane (1.5:2) for 60 min at 100°C (41). Fatty acid methyl esters were extracted with hexane and analyzed on the Varian model 6000 gas chromatograph interfaced with the Varian Vista 402 data system (Varian Instruments, Georgetown, ON, Canada) equipped with a 20-m fused silica capillary column (25 m × 0.25 mm internal diameter; BP-20, Varian Instruments) and flame ionization detectors (26). The column was an open tubular glass capillary column (18 m × 0.25 mm internal diameter coated with SP1000/grade AA). Helium was used as a carrier gas at a flow rate of 1.8 ml/min and inlet pressure of 80 kPa. The inlet splitter was set at 4:1. Samples were injected at 175°C, and after 3 min in the oven, temperature was increased at a rate of 2°C/min for 20 min. Chromatography was complete after 50 min. Fatty acid methyl ester peaks were identified by injecting authentic standard mixtures of fatty acid methyl esters using the method of equivalent chain length (42).

Expression of results. The rates of uptake were expressed as nanomoles of substrate taken up per 100 mg dry weight of tissue per minute (nmol · 100 mg tissue-1 · min-1). The values obtained from the four dietary groups are reported as means ± SE of results obtained from 12 suckling animals in each group.

Glucose uptake kinetics were determined by nonlinear regression analysis using the Sigma Plot program (Jandel Scientific, San Rafael, CA) and the Systat program for best-fit curves (Evanston, Il). The analysis was performed by fitting the observed data points to the Michealis-Menten equation. As the variance increased with the size of the y-axis variable (the rate of glucose uptake), data points were weighted in proportion to the reciprocal of the within-concentration estimates of variance. Because the relationship between fructose concentration and uptake was linear, the maximal transport rate (Vmax) and apparent Michaelis affinity constant (Km) could not be calculated. Instead, linear regression was used to obtain the value of the slope of this linear relationship.

Data analysis. ANOVA was used to test the significance of the difference between the four dietary groups. Individual differences between diet treatments were determined using a Student-Newman-Keul multiple range test. P <=  0.05 indicated a statistically significant difference between the mean values.

    RESULTS
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

Animal characteristics. The highest body weight gain (g/day) in suckling rats was achieved with feeding dams the low n-6/n-3 diet (Table 2). The lowest weight gain was observed with the DHA diet. When animals receiving the high n-6/n-3 diet were switched to the AA, DHA, or low n-6/n-3 diet, they lost weight. Switching from the AA to the DHA diet caused weight gain, and switching from the AA to the high n-6/n-3 diet caused weight loss. Switching from the DHA to the high n-6/n-3 diet resulted in weight gain. Switching from the low n-6/n-3 diet to the other diets caused weight loss. Thus the greatest weight gain in the suckling rats was with the low n-6/n-3 diet. Switching off this diet caused weight loss, and the expected weight gain with this diet was prevented by previous exposure of the mother to the high n-6/n-3, AA, or DHA diets.

                              
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Table 2.   Characteristics of animals

Fructose. A linear relationship was noted between fructose concentration (4-64 mM) and the rate of fructose uptake (nmol · 100 mg tissue-1 · min-1) (data not shown). In the jejunum, fructose uptake was lowest in animals on the high n-6/n-3 diet, whereas ileal uptake of fructose was highest in this diet group (Table 3).

                              
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Table 3.   Effect of dietary fatty acid composition on intestinal uptake of D-fructose

Switching from the AA to the high n-6/n-3 diet increased jejunal and ileal fructose, whereas switching to the DHA diet decreased jejunal yet increased ileal uptake. Switching from the DHA to the high n-6/n-3 diet reduced jejunal and ileal uptake of fructose, whereas switching from the low n-6/n-3 to the AA diet increased jejunal and ileal uptake of fructose. Thus, although continuous feeding of a high n-6/n-3 diet to the mother during pregnancy and lactation is associated with the lowest rate of fructose uptake, switching the mother to another diet during lactation did not necessarily correct the low absorption. In fact, depending on the previous dietary history of the pregnant mother, consuming a high n-6/n-3 diet during lactation could even result in enhanced uptake.

Glucose. A curvilinear relationship was noted between D-glucose concentration (4-64 mM) and uptake into jejunum and ileum (nmol · 100 mg tissue-1 · min-1; data not shown). Vmax for the ileal uptake of glucose was highest in animals fed the high n-6/n-3 diet. There were no significant differences in the values of Km in the four diet groups (data not shown).

Switching from the high n-6/n-3 diet or from the AA diet to the low n-6/n-3 diet increased the Vmax in the jejunum and ileum (Table 4). Switching from the DHA to the AA diet increased ileal but not jejunal Vmax, whereas switching to the other diets from the low n-6/n-3 diet failed to have any effect on glucose uptake. Thus feeding of the low n-6/n-3 diet to the mother during pregnancy and lactation is usually associated with the highest rate of glucose uptake.

                              
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Table 4.   Effect of dietary fatty acid composition on Vmax of intestinal uptake of D-glucose

The rate of uptake of L-glucose was similar in the four diet groups (Table 5). Switching from the AA to the DHA diet increased the jejunal and ileal uptake of L-glucose, whereas switching from the DHA or the low n-6/n-3 diet to the other diets had no effect. Compared with rats that switched from the AA to the high n-6/n-3 or low n-6/n-3 diet, there was higher jejunal and ileal uptake of L-glucose with switching from the AA to the DHA diet. Switching from the high n-6/n-3 to the DHA diet increased the jejunal uptake of L-glucose, and switching from the high n-6/n-3 to the low n-6/n-3 diet increased both jejunal and ileal uptake of L-glucose. Thus the passive component of glucose uptake in the suckling rat is influenced by the diet of the mother during lactation, but these changes may be prevented by feeding a DHA or low n-6/n-3 diet during pregnancy.

                              
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Table 5.   Effect of dietary fatty acid composition on rate of uptake of L-glucose

Lipids. The intestinal uptake of fatty acids was influenced by differences in maternal dietary lipids. The jejunal uptake of 12:0, 16:0, 18:0, 18:1, 18:2, and 18:3 was lower in animals fed the low n-6/n-3 diet compared with those fed the DHA diet. The uptake of 12:0, 18:0, and cholesterol in animals fed the low n-6/n-3 diet was decreased compared with those fed the high n-6/n-3 diet. Also, the jejunal and ileal uptake of 18:0, 18:2, and 18:3 in animals fed the DHA diet was higher than in animals fed the AA diet. There were no differences in the rates of fatty acid uptake into the jejunum between the high n-6/n-3 and the AA diet (Table 6). In the ileum, the uptake of 18:0 and 18:2 was also lower in those fed the low n-6/n-3 diet than in animals fed the DHA diet. The ileal uptake of 16:0 and 18:3 in those fed the low n-6/n-3 diet was higher than in those fed the high n-6/n-3 diet.

                              
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Table 6.   Effect of dietary fatty acids on rate of uptake of fatty acids and cholesterol

Switching from the high n-6/n-3 diet to the AA or DHA diet had no effect on jejunal fatty acid uptake, whereas switching to the low n-6/n-3 diet increased the jejunal uptake of 18:0 and 18:3 and increased ileal uptake of 16:0 and 18:3 (Table 7). Switching from the AA diet to the other diets increased the jejunal uptake of 18:3 without any changes in the uptake of the other fatty acids (Table 8). The ileal uptake of 18:0 and 18:2 was increased by switching to the high n-6/n-3 diet. The ileal uptake of 18:0 and 18:2 was increased, but 16:0 was decreased, in switching from the AA to the DHA diet, and the ileal uptake of 18:0, 18:1, 18:2, 18:3, and cholesterol was decreased by switching to the low n-6/n-3 diet. Switching from the DHA to the AA diet or the low n-6/n-3 diet did not change jejunal fatty acid uptake, whereas switching from the DHA to the high n-6/n-3 diet decreased the jejunal uptake of 12:0, 16:0, 18:0, 18:2, and 18:3 and decreased the ileal uptake of 12:0, 16:0, 18:0, 18:1, and 18:3 (Table 9). Switching from the low n-6/n-3 diet to the other diets resulted in an increase of the rate of uptake of some fatty acids (except linoleic acid when switched to the high n-6/n-3 diet, lauric acid when switched to the AA diet, and lauric and linolenic acid when switched to the DHA diet) (Table 10). Thus changing the lipid composition of the mother's diet between pregnancy and lactation resulted in variable responses in the jejunal and ileal uptake of lipids.

                              
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Table 7.   Effect of diet switch on rate of uptake of fatty acids and cholesterol

                              
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Table 8.   Effect of diet switch on rate of uptake of fatty acids and cholesterol

                              
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Table 9.   Effect of diet switch on rate of uptake of fatty acids and cholesterol

                              
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Table 10.   Effect of diet switch on rate of uptake of fatty acids and cholesterol

Lipid composition of amniotic fluid. The lipid composition of the fatty acids in the triglycerides of the amniotic fluid was not influenced by the lipid composition of the diet of the pregnant dam. The 20:3(n-6) content of the phospholipids in amniotic fluid was higher (P < 0.05) in the AA diet than in the high n-6/n-3 diet, and 22:6(n-3) was higher (P < 0.05) in the DHA diet than in the high n-6/n-3 diet, but the magnitude of these changes was small (Table 11).

                              
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Table 11.   Fatty acid composition of amniotic fluid total phospholipids

    DISCUSSION
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

The mother's diet during the gestation period is important for normal fetal development. The lipid content in the diet of pregnant dams influences the fatty acid composition of fetal tissue (11). The mother's diet also influences amniotic fluid composition (29). In a previous study (45), we showed that when nursing dams were fed diets with high or low n-6/n-3 fatty acids, 1% AA, or 0.7% DHA, the suckling rat intestine adapted its uptake of sugars and lipids. In the present study, there are only minor changes in the fatty acids in the phospholipids of amniotic fluid (Table 11), and no changes in the fatty acids in the triglycerides (data not shown). This suggests that it is not the lipids in the amniotic fluid that are responsible for the variations in the adaptability of the intestine depending on the lipid content of the pregnant mother's diet. Further studies examining the fatty acid composition of individual fatty acids may be important, because the fatty acid composition of the total phospholipids may mask changes in the fatty acid composition of individual phospholipids.

We did not measure any hormones such as EGF, insulin, or glucocorticosteroids in the mother, amniotic fluid, or the offspring, so it remains unclear what the signal might be for these diet-associated changes.

Glucose. In the small intestine, the absorption of glucose is mediated by the action of SGLT1 in the brush-border membrane (13). The regulation of SGLT1 expression involves a complex set of cellular and molecular control mechanisms (23, 56, 57). The transcriptional and posttranscriptional events are the major determinants of the cell-specific expression of SGLT1 in physiological states, as well as in models of intestinal adaptation. The intestinal uptake of glucose by SGLT1 is influenced by the dietary fatty acid composition (53). In most dietary groups the prenatal exposure to high or low n-6/n-3 fatty acids and to 0.7% DHA prevented any changes in jejunal glucose transport in response to dietary lipids fed to the mother during the suckling period. This inhibitory effect was not seen in the jejunum or in the ileum when switching from the AA diet to the low n-6/n-3 diet; in fact, switching from the DHA to the AA diet resulted in enhanced ileal, but not jejunal, glucose uptake. There were also opposite modes of intestinal response: when mothers consumed the low n-6/n-3 diet during pregnancy and were switched to the AA diet during lactation, the ileal glucose uptake in suckling rats was reduced. The adaptation in glucose transport resulted from alterations in the value of Vmax, suggesting variations in SGLT1 activity. Continuous feeding of high n-6/n-3 to the mother during pregnancy and lactation is associated with the highest Vmax of glucose uptake, but switching the mother to this diet during lactation does not necessarily increase glucose uptake. The responsiveness of the intestine to changes in glucose uptake resulting from alterations in the mother's diet during lactation depends on the composition of her diet during pregnancy. Thus the ontogeny of the intestine is influenced by the mother's diet during gestation as well as during the nursing period. Importantly, some of the diet-associated changes in nutrient uptake resulting from the mother's diet during pregnancy cannot be corrected by dietary intervention introduced after birth.

Fructose. The absorption of fructose across the enterocyte brush-border membrane is mediated by a sodium-independent facilitative transporter, GLUT5. Both SGLT1 and GLUT5 are restricted to the brush-border membrane domain of the villus, but there were different patterns of fructose uptake compared with glucose uptake. Also, maternal dietary fatty acid consumption during gestation had different effects on the adaptability of jejunal and ileal fructose transport. For example, although continuous feeding of the high n-6/n-3 diet to the mother was associated with higher rates of glucose uptake, the rates of fructose uptake were lower. The differences observed in the adaptation of the intestinal glucose and fructose transport in response to the various dietary lipids as a consequence of exposure to different fatty acids during gestation suggests that SGLT1 and GLUT5 are influenced in a different manner by various dietary fatty acids. Western and Northern blot analyses were not performed in this study, so it is unknown whether the diet-associated alterations in glucose uptake were associated with changes in the abundance of SGLT1 and GLUT5, or in their respective mRNAs. Nonetheless, the ontogeny of the intestinal transport of fructose is influenced by the composition of the mother's diet during pregnancy and lactation, and switching the mother to another diet during lactation did not necessarily correct the low absorption of fructose.

Lipids. Lipid uptake occurs largely by the process of passive diffusion, but there also are protein-mediated steps. For example, membrane fatty acid-binding proteins may be important for the uptake of long-chain fatty acids (50). The sodium/hydrogen exchanger also plays a role in the uptake of fatty acids (51). In the present study, the intestinal uptake of long-chain fatty acids in suckling rats was influenced by variations in dietary lipids in the pregnant dams' diet, but the pattern of changes was different for various fatty acids and for the jejunum vs. the ileum. For example, early exposure to high n-6/n-3 and AA prevented changes in the jejunal uptake of most fatty acids after switching to other diets. In animals whose mothers consumed DHA during pregnancy and were switched to the high n-6/n-3 diet during lactation, the uptake of fatty acids was decreased in the jejunum and ileum. The lower jejunal uptake of fatty acids seen in the offspring of mothers fed the low n-6/n-3 diet was upregulated by switching to the other diets, whereas this same switching resulted in only a few changes in the ileum. Thus various dietary fatty acids consumed during gestation have a different effect on the intestinal transport of lipids, and this effect becomes more marked after exposure to changed dietary lipid composition during the suckling period.

Lipids are an integral component of all membranes, and variations in the lipid composition alter the transport properties of the brush-border membrane. Dietary fats appear to influence the cell function at two levels: 1) the long-term adaptive modulation of the fatty acids of membrane phospholipids and phospholipid fatty acids, leading to changes in membrane-protein function, and 2) the rapid and direct fatty acid regulation of gene transcription. Also, dietary n-6 and n-3 polyunsaturated fatty acids suppress transcription of several lipogenic and glycolytic genes (12). It is unclear what the molecular basis is for these changes in lipid uptake occurring in response to dietary modification. Further studies must be performed to establish if the changes in the composition of the brush-border membrane or the alterations in the activity of the enterocyte lipid metabolizing enzymes may be associated with these variations in lipid uptake, or if there are alterations in the membrane or cytosolic fatty acid-binding proteins that could potentially explain alterations in lipid uptake.

Amniotic fluid lipid concentrations. The sugar composition of amniotic fluid alters the intestinal transport of sugars (4, 14). To our knowledge, this is the first report of the influence of dietary lipid changes on the fatty acids in the phospholipids in amniotic fluid (Table 11). The dietary lipid modifications used here modify the lipid content of mother's milk (unpublished observations). It remains to be determined whether dietary changes in the pregnant mother are sufficient to modify the normal development of the intestine that occurs in utero and whether brain and visual tissue development can be modified by the lipid content of the mother's diet.

Major findings and possible clinical benefits. The greatest weight gain in the suckling rats was with the low n-6/n-3 diet. Switching off this diet caused weight loss, and the expected weight gain with this diet was prevented by previous exposure of the mother to the high n-6/n-3, AA, or DHA diet. Continuing the low n-6/n-3 diet in the suckling period assured the uptake of fructose, but this could be increased further by switching to the AA supplemental diet during nursing. Glucose uptake in the sucklings was less responsive to changes in the lipid content of the mother's diet. The lower jejunal uptake of 16:0 in suckling rats fed the low n-6/n-3 diet during pregnancy and lactation was offset by the higher ileal uptake, and adding AA to the diet of nursing mothers greatly enhanced the jejunal uptake of long-chain fatty acids. We speculate that pregnant and suckling mothers may be fed a low n-6/n-3 diet to ensure accelerated weight gain in their offspring.

In some dietary treatment groups, there were persistent clinical benefits associated with enhanced nutrient uptake. The animals fed the high n-6/n-3 diet had the highest rate of glucose uptake and the highest body weight gain compared with animals switched from the high n-6/n-3 diet to the other diets. Some changes in the intestinal transport were not likely to be of nutritional significance, since body weight gain was lowest in those animals fed the DHA diet compared with rats fed the high n-6/n-3, AA, or low n-6/n-3 diet, despite the fact that in these animals the uptake of fructose and long-chain fatty acids was high. These observations are supported by the results of Clarke and colleagues (11), who showed the tendency for growth retardation in growing rats fed fish oil. In the present study, the decrease in body weight gain was prevented by switching from the DHA to the high n-6/n-3 diet. Interestingly, this switch resulted in a reduction of the uptake of fatty acids and fructose. Thus there may be other mechanisms that compromise growth during this critical period of development. Clearly, the diet of the mother may influence the intestinal function of the offspring, and it is important to establish what dietary constituents must be used prudently during pregnancy.

    ACKNOWLEDGEMENTS

We thank Kim Doring and Mika Wierzbicki for technical assistance. The fats used in this study were kindly provided as a gift by Wyeth-Ayerst Research. The word-processing assistance of Rachel S. Jacobs is acknowledged with thanks and appreciation.

    FOOTNOTES

This study was partly suppported by the Medical Research Council of Canada and the Natural Sciences and Engineering Research Council of Canada. N. Perin was supported in part by the Rotary Foundation.

Address for reprint requests: A. B. R. Thomson, Division of Gastroenterology, Dept. of Medicine, Univ. of Alberta, Edmonton, Canada T6G 2C2.

Received 24 December 1997; accepted in final form 22 April 1998.

    REFERENCES
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References

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