Influence of several plant and animal proteins on rat pancreas

Twelve different plant and animal proteins were fed to rats for 4 weeks. Trypsin inhibitor (TI) content of the diet was significantly correlated with (although not directly related in all cases to) pancreas weights and with the pancreatic biochemical parameters that indicate hypertrophy. In vitro, but not in vivo, digestibility was correlated with the TI content of the diet. Quantity of DNA per gram pancreas was not found to be related to TI content. A second experiment compared graded levels of TI from raw or heated soy flour and soy protein isolate (SPI). The SPI diet produced higher relative pancreas weights per TIU than did the flour diets. Two commercial SPI's were fed as is or autoclaved in a third experiment. Autoclaving to very low TI values made no improvement in weight gain or pancreatic parameters measured, indicating that there may be a threshold level of TI below which rat pancreata do not respond, and that other factors in SPI are responsible for the slight pancreatic enlargement seen with SPI compared to casein.     

Effects on the monkey, pig and rat pancreas of soy products with varying levels of trypsin inhibitor and comparison with the administration of cholecystokinin

Raw and heated soy flour and casein diets were compared in rats, pigs, and monkeys with respect to growth, pancreatic changes, fecal trypsin, and nitrogen digestibility. Cholecystokinin injection was compared to feeding raw soy flour and casein in rats and casein in pigs. Several soy protein preparations were fed to rats and monkeys. Neither raw soy flour nor any other soy product produced pancreatic enlargement in pigs or monkeys. Casein and heated soy flour performed similarly. By comparison, other effects of raw soy flour were as follows. Growth was depressed 60% in rats and 84% in pigs, but not at all in monkeys. Nitrogen digestibility was depressed 5, 45, and 9% in rats, pigs, and monkeys, respectively. Pancreatic DNA, RNA, and protein levels were unchanged in monkeys fed raw soy flour. In rats, RNA per milligram pancreas was increased 40%, in pigs 20%. Pancreatic protein was decreased 7% in pigs and increased 47% in rats. Changes in pancreatic trypsin, chymotrypsin, lipase, and amylase were dissimilar in the three species. Fecal trypsin was elevated 300-400% in rats, and decreased approximately 50% in pigs and monkeys. Cholecystokinin injections in pigs and rats produced changes both quantitatively and qualitatively different from those seen with raw soy flour. Feeding of heated soy flour or soy protein isolates was comparable to feeding casein in all three species, and produced no deleterious effects.     

The failure of long‐term feeding of raw soy flour,in the presence or absence of Azaserine,to induce carcinogenic changes in the mouse pancreas

The effects of feeding mice raw or heated soy flours or casein in the presence and absence of injected azaserine were investigated over a peiod of 18 months. Although the feeding of raw soy flour (compared with heated soy flour or casein) caused a significant inhibition of growth and an enlargement of the pancreas, there was no macroscopic evidence of pancreatic nodules in any of the six experimental groups. Microscopic examination of the pancreas revealed a somewhat higher (not significant) incidence of atypical acinar cell nodules in all animals injected with azaserine, but this difference was little influenced by the diets themselves. We concluded that raw soy flour itself has no carcinogenic effect on the mouse pancreas and does not enhance the sensitivity of the mouse pancreas to azaserine. Thus, it cannot be assumed that the appearance of pancreatic nodules constitutes an obligatory sequela of pancreatic hypertrophy and/or hyperplasia in all species of animals.     

The failure of long-term feeding of raw soy flour, in the presence or absence of azaserine, to induce carcinogenic changes in the mouse pancreas

The effects of feeding mice raw or heated soy flours or casein in the presence and absence of injected azaserine were investigated over a period of 18 months. Although the feeding of raw soy flour (compared with heated soy flour or casein) caused a significant inhibition of growth and an enlargement of the pancreas, there was no macroscopic evidence of pancreatic nodules in any of the six experimental groups. Microscopic examination of the pancreas revealed a somewhat higher (not significant) incidence of atypical acinar cell nodules in all animals injected with azaserine, but this difference was little influenced by the diets themselves. We concluded that raw soy flour itself has no carcinogenic effect on the mouse pancreas and does not enhance the sensitivity of the mouse pancreas to azaserine. Thus, it cannot be assumed that the appearance of pancreatic nodules constitutes an obligatory sequela of pancreatic hypertrophy and/or hyperplasia in all species of animals.     

Effect of sulfur amino acid supplementation of raw soy flour on the growth and pancreatic weights of rats

Rat feeding studies showed that L-cystine, L-cysteine, N-acetyl-L-cysteine and L-methionine enhance the nutritional quality of raw (unheated) soy flour as measured in rats by the protein efficiency ratio (PER). Pancreas weights of rats fed diets supplemented with sulfur amino acids were higher than those of rats fed unsupplemented diets. In some instances this increase resulting from nutritional improvement of the diet was relatively greater than that in total body weight. Explanations are offered for these effects in terms of the proposed biofeedback mechanism involving the gastrointestinal endocrine system and the hormone cholecystokinin. The observed stimulation of pancreatic growth by added sulfur amino acids may be closely associated with this pancreatic regulatory mechanism when activated in response to dietary trypsin inhibitors present in unheated soy protein. The extent to which pancreatic hypertrophy develops as a result of a given exposure to trypsin inhibitors may be limited when the total amount or quality of the protein in the diet is less than optimal.     

Effect of soy protein, casein and trypsin inhibitor on cholesterol, bile acids and pancreatic enzymes in mice

Dietary vegetable proteins may lower plasma cholesterol compared to animal proteins. We considered the possibility that lower digestibility and the trypsin inhibitor (TI) content of plant proteins could lead to alterations in bile acid metabolism and exocrine pancreatic function mediating some of this change. Mice were fed cholesterolemic diets of different protein source and TI content: casein, soy protein isolate, or casein plus TI for 4 weeks. Plasma and liver cholesterol were measured; pancreata, intestinal contents and mucosal scrapes were collected for bile acid, trypsin, chymotrypsin, amylase, and lipase assays. The soy group had lower plasma cholesterol levels. Intestinal bile acids in this group were higher, suggesting a causal relationship (increased bile acid secretion leading to increased cholesterol catabolism). Conversely, liver cholesterol in this group was raised, reflecting a possible shift in body cholesterol pools. TI addition did not affect lipid metabolism, though it did affect pancreatic function: it led to increased pancreatic weight and depressed intestinal trypsin activity, but elevated chymotrypsin and amylase levels in pancreas and intestine and increased trypsin levels in the pancreas. Therefore, soybean TI does not seem to affect cholesterol metabolism, though it greatly affects pancreatic secretion. On the other hand, soy protein has a marked effect on the bile acid and cholesterol metabolism, which may be a function of protein quality.     

Effects of dietary proteins and trypsin inhibitor on growth and lipid metabolism in hamsters

The purpose of this study was to determine trypsin inhibitor (TI) activity in sweetpotato and soy flour diets and their effects on the growth and lipid metabolism of hamsters. Male Golden Syrian hamsters were fed different types of dietary protein containing casein, soyprotein, transgenic sweetpotato plus soy flour (TSPF+SF), nontransgenic sweetpotato (NTSPF) plus soy flour (NTSPF+SF), transgenic sweetpotato (TSPF) and nontransgenic sweetpotato flour for 28 days. The TI activity was highest in TSPF+SF (19.30 TIU/mg) and NTSPF+SF (17.20 TIU/mg) diets that induced growth retardation in animals, lowest in TSPF (5.80 TIU/mg) and NTSPF (5.50 TIU/mg) diets, which did not affect the growth of the animals, and negligible in casein (<1.00 TIU/mg) and soyprotein (2.00 TIU/mg) diets. Plasma total cholesterol (TC), high density lipoprotein-cholesterol (HDL-C), low density lipoprotein-cholesterol (LDL-C) and triglyceride (TG) concentrations were significantly higher in hamsters fed the casein diet compared to those fed the soy protein, TSPF+SF, NTSPF+SF, TSPF and NTSPF diets. A positive correlation was observed between plasma TC concentrations of hamsters and dietary methionine, lysine, leucine content and methionine/glycine ratios. Liver TC and TG concentrations of hamsters fed casein were significantly higher than those of all other diet groups. The supplementation of sweetpotato flour with soy flour increased both protein and TI activity in the diets and the lipid metabolism of hamsters were unaffected by TI activity.     

Growth, digestibility and enzymatic activities in the pancreas and intestines of hamsters fed raw and heated soy flour

The nutritional effects of feeding raw and heated soy flour to young golden Syrian hamsters were investigated over a period of 32 days. Those animals fed raw soy flour grew much more poorly than those fed heated soy flour, an effect which was reflected in a lower food efficiency as well. Growth retardation of hamsters fed raw soy flour was accompanied by a lower apparent digestibility of the protein (54%) compared to heated soy flour (76%). The pancreases of animals fed raw soy flour were increased in size and had elevated levels of trypsin, chymotrypsin, amylase, and lipase. With the exception of trypsin activity in the small intestine, similar differences in enzyme activities between the raw and heated soy groups were generally found in the small intestine, cecum, large intestine, and feces. There was, however, a progressive decrease in these activities in the lower regions of the intestinal tract and feces. It is concluded that the hamster, in common with several other species of animals, is sensitive to the effects of the trypsin inhibitors in raw soy flour, and may provide a useful model for studying the long-term effects of trypsin inhibitors on the pancreas.     

Role of dietary protein in rat pancreatic enzyme secretory response to a meal

The role of dietary protein in the pancreatic enzyme secretory response to complete test meals was studied in rats fitted with chronic bile-pancreatic fistulas. Pancreatic enzyme secretory response was observed by collecting combined bile-pancreatic juice and continuously returning it to the intestine by a stream-splitter, which diverted 5% for assay. All rats were fed ad libitum a 24% casein diet during the recovery period. Four days postoperative, rats were infused intragastrically with 5 g of isonitrogenous, complete test meals containing casein (24%), alpha-protein (soy protein), amino acids patterned after casein or alpha-protein, casein + lima bean trypsin inhibitor (LBTI), and a nitrogen-free diet. Pancreatic enzyme secretion under basal conditions and in response to the test meals was determined. The pancreatic response to test meals containing intact casein as nitrogen source was the same as the response to nitrogen-free test meals, and was only slightly greater (not statistically significantly) than the response to test meals containing amino acids. Test meals containing alpha-protein, which was devoid of soybean trypsin inhibitor activity, were much more potent than corresponding amino acid test meals or nitrogen-free test meals. Addition of LBTI greatly increased the pancreatic response to casein test meals. The results indicate that intragastric infusion of diets containing casein as nitrogen source do not stimulate greater pancreatic enzyme secretion than test meals of nitrogen-free or amino acid diets under normal physiological conditions, and it is proposed that this is a consequence of the large reserve capacity of the exocrine pancreas.     

The effect of fasting on enzyme levels in the enlarged and involuting rat pancreas

1. The effect on pancreatic digestive enzyme levels of fasting and changes from a diet containing trypsin inhibitor (raw soya-bean flour, RSF) to diets free of trypsin inhibitor (heated soya-bean flour, HSF, or commercial rat chow) was studied in rats for up to 7 d. 2. In RSF-fed rats killed without fasting, enzyme levels were low, but after fasting for 24 h before killing there was a marked increase in all enzyme levels. Histological studies showed that pancreatic acinar cells from RSF-fed rats killed without fasting were devoid of zymogen granules, but following a 24 h fast there was a marked accumulation of zymogen granules which extend into the basal cytoplasm. Fasting either produced no change or a fall in enzyme levels in rats fasted after feeding HSF or chow continuously. 3. If animals fed on RSF were changed to HSF and either fed or fasted for 24 h up to the time of killing there was an increase in amylase (EC 3.2.1.1), trypsin (EC 3.4.21.4), lipase (triacylglycerol lipase; EC 3.1.1.3) and protein 1 d after the change, followed by a fall over the next 6 d to levels similar to those seen in rats fed on HSF continuously. 4. Animals changed from RSF to chow showed similar effects as far as trypsin, lipase and protein were concerned, but amylase rose, to reach the level seen in rats fed on chow continuously (about ten times that seen in soya-bean-fed rats), after 2 d. 5. These results suggest that in the rats fed on RSF, pancreatic enzyme synthesis is rapid but secretion is equally rapid and intracellular enzyme levels are low. When these animals are fasted or changed to a diet free of trypsin inhibitor the rate of secretion falls but the high rate of synthesis continues for at least 24 h and enzymes accumulate in the pancreas. In studies of pancreatic enzyme levels in rats fed on trypsin inhibitor the extent of fasting before killing the animal is therefore an important variable. Such animals should probably not be fasted before study.     

Rapid changes in pancreatic DNA, RNA and protein in the rat during pancreatic enlargement and involution

Pancreatic weight, nucleic acid content and protein content were measured on the involuting rat pancreas after its initial enlargement from a raw soy bean flour (RSF) diet. Rats were fed an RSF diet for four weeks and then changed to an inhibitor-free diet of either heated soy bean flour (HSF) or rat cubes for 1, 2, 7 or 14 days. Control rats were fed cubes continuously from weaning or cubes followed by HSF for two weeks. There was no significant difference in the body weights of rats in any of the test or control groups. After four weeks on RSF the wet weight of the pancreas was double that of the control animals and the protein content and nucleic acid content were also significantly greater. After RSF-fed animals were changed to cube or HSF diets, pancreatic parameters rapidly reverted to normal. The most striking change was in pancreatic DNA which returned to control values within 2 days of changing to cubes or HSF. Pancreatic weight, RNA and protein reverted to control values within 7 days of the change of diet. The rapid increase and decrease in the size of the rat pancreas after diet alterations suggest that pancreatic growth may be more labile than previously thought. If rapid changes in pancreatic cell number occur in rats fed alternating cube and RSF diets, then these animals may be more susceptible to pancreatic carcinogens.     

The effects of long-term soy protein and milk protein feeding on the pancreas of Cebus albifrons monkeys

Twenty-seven 2- to 4-yr-old cebus monkeys (Cebus albifrons) were fed from infancy purified diets containing lactalbumin, soy isolate, casein or soy concentrate as the sole protein source. Hematologic and clinical chemistry values were similar for all groups. Head and tail portions of each pancreas were surgically removed for histopathologic evaluation and determination of protein, RNA and DNA content, and for trypsin and chymotrypsin activity. Hematoxylin and eosin-stained sections from 26 of 27 monkeys showed normal pancreatic tissue with occasional acinar vacuolation in all diet groups. The remaining animal, one of only two fed soy concentrate, had diffuse interstitial fibrosis of the pancreas associated with mild to moderate atrophy of acinar tissue. Biochemical analyses of the pancreatic biopsies indicated no group differences among animals fed lactalbumin, soy isolate or casein. One of two monkeys in the soy concentrate group showed decreased pancreatic protein, RNA and trypsin concentrations; this was probably due to the fibrosis in this animal. No evidence of pancreatic hypertrophy or hyperplasia, as measured by RNA/DNA and protein/DNA ratios, respectively, was seen in any diet group.     

Effect of dietary raw soybean and soybean trypsin inhibitor on trypsin and chymotrypsin activities in the pancreas and in small intestinal juice of growing swine

Sixty-eight growing gilts with a 12 kg average initial weight were used in seven trials to study the effect of dietary raw soybean (Harosov) and SBTI (Kunitz soybean trypsin inhibitor) on pancreatic and small intestinal trypsin and chymotrypsin activities. A solvent-extracted, heated soybean meal (SBM) was used, cause reduced growth. Both a single-meal and continuous feeding of the raw soybean diet caused a decrease in the pancreatic trypsin and chymotrypsin activities. In contrast, to the rat and the chick, the pancreas of the pigs did not enlarge subsequent to consumption of the raw soybean or SBTI diets. Raw soybean feeding also resulted in an inhibition of the intestinal trypsin and chymotrypsin activities. This inhibiting effect was greater than that of the SBTI, especially the chymotrypsin-inhibiting effect. This suggested that soybean constituents other than the SBTI, such as the Bowman-Birk inhibitor, caused inhibition. In the pig the inhibition of the intestinal proteolysis may be a major cause of reduced growth when raw soybean is fed.     

Dietary soy protein attenuates renal disease progression after 1 and 3 weeks in Han:SPRD-cy weanling rats

Compared with casein, dietary soy protein slows disease progression in animal models of chronic renal injury. To determine whether dietary soy protein feeding can alter early disease progression, male Han:SPRD-cy rats (n = 87) in a very early stage of chronic kidney disease were fed soy protein compared with casein-based diets for 1 or 3 wk. Kidneys were assessed for fibrosis, cyst growth, fatty acid composition and prostaglandin E(2) (PGE(2)) production. Soy protein feeding significantly reduced renal fibrosis by 22% (P = 0.0347) and 38% (P = 0.0102) after 1 and 3 wk of diet, and cyst growth was 34% lower after 3 wk (P < 0.0001). Kidney 18:2(n-6) levels were reduced in normal and diseased rats after as little as 1 wk of consuming the soy protein diet. Dietary soy protein also partially ameliorated the suppression of PGE(2) production observed in diseased kidneys. Compared with diseased kidneys from casein-fed rats, ex vivo PGE(2) release was 31-32% higher after 1 (P = 0.0281) and 3 (P = 0.0189) wk of dietary soy protein consumption. Hence, the first signs of a beneficial soy protein effect were observed after 1 wk of feeding, with further improvements evident after 3 wk. These data demonstrate that dietary soy protein compared with casein delays disease progression in an early stage of chronic kidney disease.     

Hepatic cysteine dioxygenase activity and sulfur amino acid metabolism in rats: possible indicators in the evaluation of protein quality

Experiments were carried out to examine the possibility that the sulfur amino acid metabolism of rats may be an indicator of the nutritional value of dietary protein. Rats were fed diets containing 8, 16 or 24% of gluten, soy protein or casein for 3 wk. Hepatic cysteine dioxygenase activity, hepatic concentration of glutathione, cysteine and taurine and urinary taurine were examined. In addition, the sulfur amino acid metabolism of rats fed these diets fortified with the appropriate first limiting amino acid for 7 d was also examined. High urinary taurine excretion was observed in the three gluten groups, whereas very low urinary taurine excretion was observed with up to 24% soy protein or up to 16% casein. The hepatic hepatic cysteine dioxygenase activities of the gluten diet groups were higher than those of corresponding soy protein or casein diet groups, except that of rats fed the 24% casein diet. The hepatic concentrations of both glutathione and cysteine in gluten diet groups were also higher than those of corresponding soy protein or casein diet groups, except 24% soy protein and 16 and 24% casein diet groups. In rats fed the casein or soy protein diets urinary taurine excretion and hepatic cysteine dioxygenase activity increased with increasing methionine supplementation, the first limiting amino acid. Conversely, in rats fed the gluten diet both urinary taurine excretion and hepatic cysteine dioxygenase activity decreased with increasing lysine supplementation, the first limiting amino acid. These findings suggest that urinary taurine excretion and hepatic cysteine dioxygenase activity may be useful as sensitive indicators of the nutritional value of dietary protein.     

Oligo-L-methionine and resistant protein promote cecal butyrate production in rats fed resistant starch and fructooligosaccharide.

We examined the role of resistant protein and peptides in promoting cecal butyrate production in rats fed rapidly fermentable carbohydrates. Rats were fed diets containing raw potato starch (RPS, 200 g/kg diet) or fructooligosaccharide (FOS, 60 g/kg diet) with casein, soy or rice protein (250 g/kg diet) for 13 d. In rats fed RPS with casein, the major cecal organic acid was acetate (441 micromol), but lactate and succinate were also found in considerable amounts (324 micromol). Succinate was the major cecal organic acid (235 micromol) in rats fed FOS with casein. When rice protein was fed with RPS, the contribution of lactate was significantly lower and that of propionate tended to be higher (P < 0.1) than in rats fed casein. In rats fed rice protein with FOS, cecal butyrate and acetate were greater and cecal succinate was lower than in rats fed casein with FOS (P < 0.05). Despite the similar amounts of undigested protein in rice and soy proteins, soy protein did not similarly affect cecal butyrate in rats fed FOS or RPS. In another experiment, rats were fed diets containing high amylose cornstarch (HAS, 200 g/kg diet) with casein, casein + oligo-L-methionine (OM, 3 g/kg diet), soy protein, soy protein + OM (3 g/kg diet) or rice protein (250 g/kg diet) for 10 d. OM (digestibility, 31%) was substituted for the same amount of casein. Rats fed rice protein had greater cecal butyrate than rats fed casein (P < 0.05). OM supplementation to casein or soy protein increased cecal butyrate compared with rats fed casein or soy protein alone (P < 0.05). These data support our hypothesis that resistant protein and peptides promote cecal butyrate production and suggest that the differing potency of rice and soy proteins in promoting cecal butyrate production might be explained in part by the different amino acid composition of resistant protein.     

Nutritional evaluation of the trypsin (EC 3.4.21.4) inhibitor from cowpea (Vigna unguiculata Walp.).

The effect of feeding rats purified cowpea (Vigna unguiculata Walp.) trypsin (EC 3.4.21.4) inhibitor in a semi-synthetic high-quality diet based on lactalbumin (10 g inhibitor/kg) for 10 d was a moderate reduction in the weight gain of rats in comparison with controls, despite an identical food intake in the two groups. The reduction in the growth rate was about 20% on a live weight basis. However, the corresponding value calculated from the weight of dry carcasses was less, only about 7%, probably because the water content of the body of the two groups of rats was different. Although most of the cowpea trypsin inhibitor (CpTI) was rapidly broken down in the digestive tract, its inclusion in the diet led to a slight, though significant, increase in the nitrogen content of faeces but not of urine. Accordingly, the net protein utilization of rats fed on inhibitor-containing diets was also slightly depressed while their energy expenditure was elevated. In agreement with results obtained for the protease inhibitors of soya bean, the slight anti-nutritional effects of CpTI were probably due mainly to the stimulation of the growth and metabolism of the pancreas. Thus, the nutritional penalty for increased insect-resistance after the transfer of the cowpea trypsin inhibitor gene into food plants is slight in the short-term.     

Preventive effect of soybean resistant proteins against experimental tumorigenesis in rat colon

The insoluble 'high-molecular-weight' fraction (HMF) centrifugally separable after digestion of soy protein isolate with a microbial protease of the exo-type, of which about a quarter is regarded as an indigestible 'resistant protein,' was examined for its preventive effect against colonic tumorigenesis in a model system with male F-344 rats. The rats were intraperitoneally injected with azoxymethane (15 mg/kg BW) once a week for 3 wk and were fed a 20.6% HMF diet (+0.4% DL-Met) or 14.7% casein diet (+0.3% DL-Met) supplemented with 0.2% sodium deoxycholate (DCA) or without supplementation. Twelve wk later, 5 rats of each group were inspected for formation of tumors but no tumors were visible to the naked eye. The DCA-fed casein group was conspicuous for a low count of aberrant crypt foci. At 39 wk, 6 rats of the DCA-fed casein group (n = 10) and 3 rats of the DCA-fed HMF group (n = 9) had a total of 18 tumors with a major axis of 4.0 +/- 0.4 mm and 3 tumors with an axis of 2.0 +/- 0.1 mm, respectively, in contrast to only a single tumor for the DCA-unfed casein group (nil for the DCA-unfed HMF group). The difference in tumor number and size was considered significant between these DCA-fed casein and HMF groups; that is to say, HMF feeding retarded tumor development despite the frequent occurrence of pre-neoplastic lesions. In addition, fecal bile acid excretion was much more elevated by HMF feeding than by casein feeding. It can be assumed from these observations that the antitumorigenicity of HMF is due to the inhibitory effect of soybean resistant proteins on reabsorption as well as the mucosal contact of bile acids in the intestine.     

Morphological and biochemical changes in the pancreas of the copper-deficient female rat

Earlier histological studies have demonstrated that copper deficiency results in a selective and progressive atrophy of pancreatic acinar tissue. The present study examined both biochemical and morphological changes of the exocrine pancreas in nutritional copper deficiency. Groups of mature female rats were fed a purified diet either deficient (less than 0.5 micrograms/g) or sufficient (6.2 micrograms/g) in copper for 6 wk. Copper deficiency resulted in distinct ultrastructural changes in acinar cells, including marked variability in zymogen granule content, autophagic vacuoles and dilation of acinar lumen. Pancreatic weight and total DNA, RNA and protein content of the pancreas were similar in both groups of rats, whereas pancreatic amylase, trypsin and chymotrypsin activity was significantly lower in the copper-deficient group. In addition, secretagogue-induced release of these enzymes from dispersed acini isolated from copper-deficient rats was significantly reduced in comparison to enzyme secretion from normal controls. Pancreatic Cu-Zn and Mn superoxide dismutase activity was also found to be significantly lower in the copper-deficient rats than in normal controls. We conclude that nutritional copper deficiency in adult female rats reduces the responsiveness of the pancreas to secretagogues and may increase the susceptibility of the pancreas to oxidative damage.     

Effects of feeding rats low protein diets containing casein or soy protein isolate supplemented with methionine or oligo-L-methionine

The effects of supplementing 8% casein or 10% soy protein isolate (SPI) diets with graded levels of oligo-L-methionine (a mixture of hexa- and heptapeptides, OM) or L-methionine (Met) were studied in rats to determine the reason for the difference in nutritional quality between proteins and corresponding amino acid mixtures. As the OM concentration of the casein-based diet was increased from 0.02% to 0.6%, maximum weight gain was attained at 0.2%, and the growth-promoting activity of OM was comparable to Met at all the corresponding levels tested. Liver fat began to accumulate when supplemental Met reached a level of 0.08% of the casein diet, but OM addition did not produce a fatty liver at dietary levels of less than 0.3%. When SPI was used as the dietary protein source, the effect of supplemental OM was significantly less than that of Met. Digestibility of OM (assessed by incremental portal plasma Met concentration) was measured 30 min after feeding the casein or SPI diet supplemented with 3% OM using rats fasted for 24 h. Plasma Met concentration was greatly increased in rats fed the casein plus OM diet compared with that of rats fed the SPI + OM diet. Similarly, the 30-min portal Met concentration significantly increased in response to the casein + OM diet compared with the SPI + OM diet regardless of the prefed proteins (25% casein and 25% SPI for 2 wk).(ABSTRACT TRUNCATED AT 250 WORDS)