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Department of Animal Nutrition and Physiology and * Department of Agricultural Systems, Danish Institute of Agricultural Sciences, Research Centre Foulum, DK-8830 Tjele, Denmark
3To whom correspondence should be addressed. E-mail: Mette.Hedemann{at}agrsci.dk.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • pigs • weaning • pancreatic secretion • dietary fat
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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).
The digestion of fat is to a large extent dependent on pancreatic enzymes (2
). Studies have shown that pancreatic secretion increases during the period after weaning (3
). In accordance with this observation, it has been shown that the digestibility of fat increases with time postweaning (4
). Furthermore, the dietary fat source influences the digestibility of fat, in relation to fatty acid chain length and degree of saturation (5
).
Enzyme secretion from the exocrine pancreas is dependent on the composition of the diet (6
); when the amount of dietary fat is increased, lipase secretion increases (7
). Considerable controversy exists concerning the effect of the fatty acid composition of dietary fat on the adaptation of pancreatic enzymes, especially lipase. It has been demonstrated that the fat source affects the activity of lipase in pancreatic tissue in pigs (8
) and rats (9
), and inclusion of polyunsaturated fatty acids (PUFA)5
in the diet increased the activity of lipase in pancreatic homogenates (9
,10
). Nevertheless, in other studies, the degree of saturation or chain length of fatty acids did not affect lipase and colipase activities (11
). Using growing pigs surgically fitted for continuous collection of pancreatic juice and adapted to diets containing fish oil, rapeseed oil or coconut oil, an increased output of chymotrypsin was observed when pigs were fed a diet supplemented with coconut oil, and a higher output of carboxylester hydrolase (CEH) was seen in pigs fed a diet containing fish oil (12
). These results, in combination with results showing that the dietary fat source influences the digestibility of fat in weaned pigs (4
,5
), indicate that it cannot be ruled out that the fatty acid composition has an effect on the secretion of enzymes from the exocrine pancreas in newly weaned pigs.
The aim of the present study was to investigate the effect of three different fat sources (fish oil, coconut oil and lard) having different fatty acid profiles on the exocrine pancreatic secretion in piglets during wk 2 and 3 after weaning.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The protocol used in the present experiment complied with the Danish Ministry of Justice concerning animal experimentation and care of experimental animals. The experiment was carried out with a total of 16 crossbred barrows obtained from the Danish Institute of Agricultural Sciences swineherd. The pigs were weaned at 4 wk of age and placed individually in metabolism cages. The average weaning weight was 8.9 ± 1.2 kg. Light from outside was excluded and the room was lit artificially from 0700 to 2230 h. The pigs were assigned to one of three experimental diets. The compositions of the experimental diets are shown in Table 1
. The diets differed with respect to fat source, including fish oil, coconut oil or lard. To minimize the risk of oxidation, an antioxidant (BHT) was added to the diet. Furthermore, the feed was stored cool and was weighed out for only a few meals at a time. The feed was provided in three equal meals at 0800, 1600 and 2200 h. During wk 1 of the experiment, the amount of feed was adjusted to be 
3% of body weight, increasing to 3.75% of body weight during wk 2 of the experiment. The pigs had free access to water.
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The pigs were divided into four blocks separated by time. Within each block, one pig was assigned to each of the experimental diets, and the fourth pig was assigned to one of the experimental diets at random.
Three days after weaning, the pigs underwent surgery. After 15 h of food deprivation, the pigs were premedicated by an intramuscular injection (67 µL/kg body) composed of Zolazepam, litetamine (Zoletil R50, Virbac, France), ketamine (Ketaminol Vet., Veterinaria AG, Switzerland), metadone (Nycomed DAK A/S, Denmark) and xylazine (Rompun Vet., Bayer, Germany) (Grøndahl, C., personal communication). General anesthesia was maintained with a gas mixture of halothane (1.5–2%) (Halothane Laboratories, North Augusta, SC) and as a carrier O2 and N2O in a ratio of 2:3. The pigs were surgically fitted with a pancreatic catheter and a T-shaped duodenal fistula for the continuous collection and return of pancreatic juice (13
,14
). Postsurgically, the pigs were treated with a painkiller (Buprenorphinum, Tempgesic, Reckitt & Colman, Hull, UK) and an antibiotic (Streptocillin Vet., Boehringer Ingelheim, Copenhagen, Denmark, 0.2 mL/kg body) for 3 d.
The pancreatic catheter was checked several times daily throughout the experimental period to ensure continuous flow of pancreatic juice. The catheter was connected to the duodenal fistula when the flow of pancreatic juice was stable (usually 2–3 d postsurgery).
Collections of pancreatic juice began 4 d postsurgically; the collections were made every other day, on weekdays, from 0800 to 1600 h for 2 wk. The pigs had been adapted to their experimental diets for 1 wk when the collections of pancreatic juice started. During the collections, the pigs were harnessed; the harness was attached to the wall of the cage by a chain, allowing the pigs to move around in the cage. The pancreatic catheter was connected to a tube in an isolated ice-filled bottle affixed to the side of the piglet. The pancreatic juice was collected at hourly intervals; after each hour of collection, the volume of the juice secreted was recorded. Ten minivials were labeled and 0.5 mL of the pancreatic juice was pipetted into each one. The samples were stored at -80°C for further analyses. The remainder of the pancreatic juice was continuously reinfused into the duodenal T-cannula with a peristaltic pump (model 2650, Ole Dich Instrument Makers, Hvidovre, Denmark) during the following hour of collection.
Feces were collected in two 6-d periods during the experiment using plastic bags attached to the rear of the piglets. The plastic bags were changed twice a day and the feces were weighed and frozen at -20°C until analyses. The daily weight gain during the experiment was 207 ± 131 g. At the conclusion of each part of the study, the pigs were killed with an overdose of pentobarbital.
Chemical analyses and enzyme assays.
Diet samples were taken at the beginning of each experimental block. Feces were freeze-dried, pooled within pig and period and ground with a mortar and pestle. The dry matter content of feed and feces was determined by drying at 105°C to a constant weight. Protein (N x 6.25) was determined by the Kjeldahl method using a Kjell-Foss 16200 autoanalyzer and energy by a LECO AC300 automated calorimeter system 789–500 (LECO, St. Joseph, MI). Starch plus free sugars were measured according to procedures previously described (15
). Fat (hydrochloric acid-fat) was extracted with diethyl ether after acid-hydrolysis (16
) and fatty acids were determined in the Stoldt fat extract (17
). The procedure for determining the concentration of protein in pancreatic juice (18
) was modified to be performed in a 96-microwell plate, using bovine serum albumin (A 7638, Sigma Chemical, St. Louis, MO) as a standard.
The activities of the enzymes amylase (EC 3.2.1.1), CEH (EC 3.1.1.1), chymotrypsin (EC 3.4.21.1), lipase (EC 3.1.1.3), colipase and trypsin (EC 3.4.21.4) were determined as previously described (19
). To activate proelastase to elastase (EC 3.4.21.36), pancreatic juice was diluted 50 times in a buffer (100 mmol/L Tris(hydroxymethyl)-aminomethane, 20 mmol/L CaCl2, pH 7.9). Thereafter 30 µL of a trypsin solution [10 mg of bovine trypsin (T 8253, Sigma Chemical) in 10 mL of 154 mmol/L NaCl] was added to 1 mL of diluted pancreatic juice, and the solution was incubated at 37°C for 45 min. N-Succinyl-ala-ala-ala-p-nitroanilide (S 4760, Sigma Chemical) was used as a substrate to determine elastase activity (20
). The methods described by Hedemann et al. (21
) were used to measure the activity of carboxypeptidase A (CPA; EC 3.4.17.1) and carboxypeptidase B (CPB; 3.4.17.2).
Calculations and statistical analyses.
One unit of enzyme activity was defined as the hydrolysis of 1 µmol of substrate in 1 min. The enzyme output per hour was calculated as activity multiplied by the volume. The enzyme output was divided by the metabolic weight (kg0.75) of the pig to minimize differences due to different body weights of the animals (22
).
The experiment can be regarded as a split-plot experiment in which fat source is the whole-plot treatment and pigs are the whole-plot units. The experimental days are the split-plot treatments. Hence, we assumed that the following model could describe the responses:
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in which Yfpd is the dependent variable, µ is the overall mean, 
d is the systematic effect of day of sampling (d = 1, 2, 3, 4, 5, 6), ßf is the systematic effect of fat source (f = fish oil, coconut oil, lard), 
df is the systematic effect of interaction between day and fat source, ufp is the random pig effect (P = 1, 2, 3, 4, 5) and 
fpd is the error term. Note that the random pig effect was imposed to account for repeated measurements being made on the same experimental unit (pig).
The statistical analysis was performed using the Mixed procedure of SAS (23
). The model was verified by plotting the residuals against the predicted values and by use of quantile plots of the residuals. If this indicated that the model did not describe the variable, a logarithmic or quadratic transformation of the data was tested. In this way, a satisfactory fit of the statistical model was obtained for all variables.
When the systematic effect of interaction between day and fat source was not significant (P > 0.05), this effect was taken out of the model and the analysis was repeated. A further reduction of the model was made if the systematic effect of day of sampling was not significant (P > 0.05). Finally, the effect of fat source was assessed.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. The fish oil diet contained a relatively high amount of long-chain PUFA [20:5 (n-3) and 22:6 (n-3)] and a large amount of 16:0 (22.7 g/100 g fatty acids). Medium-chain fatty acids (8:0, 10:0, 12:0 and 14:0) contributed more than half of total fatty acids in the coconut oil diet, which furthermore was characterized by a very high level of saturated fatty acids. The lard diet contained a large amount of 16:0 (29.2 g/100 g fatty acids) and 18:1 (31.8 g/100 g fatty acids).
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. Pigs fed the fish oil diet secreted a greater volume of pancreatic juice than the pigs fed the coconut oil or lard diets (P < 0.001). Protein output tended (P = 0.09) to be affected by the experimental diets. Piglets fed fish oil or coconut oil had similar protein outputs [12.9 and 11.0 mg/(h · kg0.75), respectively] and piglets fed lard had a lower protein output [6.6 mg/(h · kg0.75)]. The diets did not affect chymotrypsin output. No difference in trypsin activity output was observed between piglets fed fish oil and coconut oil. However, the activity of trypsin was significantly lower in the group fed lard (P < 0.01). Feeding piglets the fish oil diet resulted in more total activity of CPB secreted (P < 0.001) compared with piglets fed the coconut oil or lard diets. Elastase activity was not affected by the fat sources. The dietary fat source tended to affect the output of CPA activity (P = 0.06), whereas amylase activity did not differ among treatment groups. The activities of lipase and its cofactor colipase were influenced by the fat sources included in the diets. Lipase output was higher (P < 0.001) in pigs fed the fish oil diet than in those fed the coconut oil or lard diets, and the activity of colipase was lowest (P < 0.01) in pigs fed the lard diet. The output of CEH did not differ significantly among the groups.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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,24
). The major changes take place during the first 2 wk after weaning; from wk 2 to 3 postweaning, the juice outflow increases, whereas protein output remains constant (3
). The present study was conducted during wk 2 and 3 postweaning. Analyzing the effect of experimental week on pancreatic juice outflow and protein output revealed no significant differences; therefore, only the effect of fat source on exocrine pancreatic secretion is discussed in the following.
The activity or output of enzymes is regulated by the level of substrate for the given enzyme in the diet (6
). Hence the output of lipase is increased with increasing fat content of the diet (7
). In growing pigs, the effect of fatty acid composition on lipase activity was investigated using diets containing 15–21% fat (8
,12
). In diets for piglets, the level of fat is usually 4–8% (4
,25
,26
); however, this level of fat failed to stimulate the pancreatic lipase in some studies (25
,27
), whereas others observed increased lipase activity when including 4% fat in the diet (26
). On the basis of these results and the results obtained with growing pigs, it was decided to include 10% of fat in the experimental diets in an attempt to stimulate pancreatic lipase. Nevertheless, it cannot be excluded that the effect of fat source may have been different with a lower fat level.
Detailed studies on the pancreatic secretory response to fat have shown that it is the products of lipolysis (28
,29
) rather than triglycerides (30
,31
) that stimulate pancreatic secretion. When digesta reach the duodenum, the liberation of free fatty acids has already started due to the action of gastric lipase. Human gastric lipase has been shown to hydrolyze both long- and short-chain triacylglycerols at comparable rates (32
). Adaptation of gastric lipase to the amount of fat in the diet has been shown in pigs (33
), but an effect of triacylglycerol composition could not be demonstrated in rabbits (34
). To what extent the dietary fat sources in the present study are hydrolyzed at different rates by gastric lipase and thereby expose the intestinal mucosa to different fatty acids is unknown.
Feeding piglets the fish oil diet, containing the longest and most PUFA, resulted in the highest volume of pancreatic juice secreted as well as a higher output of some enzymes. The relative potency of fatty acids of different chain length to stimulate pancreatic secretion has been shown to be in the following order: 18:0 > 12:0 > 8:0; however, the effect of chain saturation or unsaturation remains to be determined in detail (35
). Feeding growing pigs diets containing fish oil, rapeseed oil or coconut oil resulted in only minor changes in exocrine pancreatic secretion (12
), whereas sunflower oil, rich in unsaturated fatty acids, increased the lipase content in pancreatic tissue (8
). The significant effect of fat source on the output of trypsin and CPB is difficult to explain. Previous studies with rats and pigs showed no effect of fat source on protease activities (8
,9
). An increased protein digestibility was observed in piglets fed coconut oil compared with corn oil (5
); others, however, did not show an effect of fat source on protein digestibility (4
,36
,37
). Whether differences in protein digestibility reflect altered protease activity is unknown.
Coconut oil has been suggested to be a good fat source for piglets due to its high content of medium-chain fatty acids because these are absorbed at a faster rate from the intestinal lumen than long-chain fatty acids (5
). The present study did not show any stimulation of the pancreatic secretion of lipase by including coconut oil in the diet; however, a high digestibility of fat in the coconut oil diet was still observed. The digestibility of fish oil was similar to that of coconut oil even though piglets fed the fish oil diet had higher output of digestive enzymes from the pancreas than piglets fed the coconut oil diet. The exocrine pancreatic secretion was comparable in piglets fed the coconut oil and lard diets. The present study suggests that the high digestibility of coconut oil in diets for piglets observed in previous experiments (4
,5
) is due to the rapid and easy absorption of the fatty acids rather than to a stimulation of the exocrine pancreas. The high digestibility of fish oil in young pigs may be due to the stimulation of exocrine pancreatic secretion.
The use of fish oil in diets is accompanied by the risk of rancidity of the feed during storage due to oxidation of the PUFA. It was demonstrated that oxidized canola oil stimulated the secretion of lipase from the exocrine pancreas (38
). This could be an explanation for the greater secretion observed in the fish oil–fed piglets although, in the previous study (38
), fat quality did not influence the volume or other enzyme activities. In the present experiment, care was taken to avoid fatty acid oxidation; the vitamin-mineral premix contained an antioxidant (BHT, 100 mg/kg feed), the feed was stored in a refrigerated room and only a few meals were weighed out at a time.
The volume of pancreatic juice secreted during the 8-h collection period is within the range observed in previous studies using piglets of the same age (39
,40
). The output of protein is lower than that observed in our previous study (39
); the variance may be a result of dietary differences, but this remains speculative. In a study in which the pouch technique was used for continuous collection of pancreatic juice, a higher protein output than in the present investigation was observed (40
). This may have been due to the different surgical procedures used in the experiments. It has been shown that pancreatic secretions from pigs fitted for continuous collection of pancreatic juice using the pouch technique contain more protein (19
). The output of trypsin was 90% lower in the present experiment compared with our previous results (39
). The animals in these two experiments were fed diets with a comparable level of protein and the analyses were carried out in the same laboratory using the same technique, which makes the difference hard to explain. However, as stated previously (41
,42
), studies of this kind are characterized by a high level of within and between study variation, which makes comparisons difficult.
It is noteworthy that the digestibilities in the present study were determined at the fecal level, and there may have been effects of the differences in digestive enzyme activity that were masked by the modifying and equalizing effects of the microflora in the hindgut (43
). Furthermore, there may have been an effect of surgical intervention on fecal digestibilities (44
). A digestibility trial with piglets fitted with a cannula at the terminal ileum using the same diets as in this experiment should be performed to evaluate the effect of the stimulation of exocrine pancreatic secretion by fatty acid composition on ileal digestibility of nutrients.
In conclusion, the present experiment showed that piglets fed a diet containing 10% fish oil secreted a larger volume of pancreatic juice and had a higher output of enzymes in the pancreatic juice than piglets fed diets containing 10% coconut oil or 10% lard. The higher output of lipase did not result in a greater apparent digestibility of fat in the fish oil diet. Further studies are required to elucidate whether the increased pancreatic secretion is beneficial to piglets because it may contribute to an optimal intestinal environment during the critical period immediately after weaning.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Supported by the Danish Agricultural and Veterinary Research Council.
4 Current address: Department of Biostatistics, University of Aarhus, DK-8000 Aarhus C, Denmark.
5 Abbreviations used: CEH, carboxylester hydrolase; CPA, carboxypeptidase A; CPB, carboxypeptidase B; PUFA, polyunsaturated fatty acids.
Manuscript received May 23, 2001. Initial review completed June 26, 2001. Revision accepted September 5, 2001.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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  Y.-H. Lee, M. A. Magnuson, V. Muppala, and S.-S. Chen Liver-Specific Reactivation of the Inactivated Hnf-1{alpha} Gene: Elimination of Liver Dysfunction To Establish a Mouse MODY3 Model Mol. Cell. Biol., February 1, 2003; 23(3): 923 - 932. [Abstract] [Full Text]
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