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 |
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 |
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 |
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.
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 |
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.
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).
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.
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.
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.
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.
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).
 |
DISCUSSION |
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.
 |
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