Effect of dietary protein and methionine on sulfite oxidase activity in rats

Sulfite oxidase catalyzes the oxidation of sulfite to sulfate. To investigate whether or not sulfite oxidase activity (EC 1.8.3.1) is regulated by the amount of sulfur from dietary protein or excess methionine, we fed rats diets containing 5, 10, 20 and 50% casein with or without excess methionine and measured sulfite oxidase activity in liver and intestinal mucosa. Hepatic sulfite oxidase activity was significantly lower in rats fed 5 or 10% casein diets and significantly higher in rats fed 50% casein than in rats fed the control diet containing 20% casein, but activity did not change in response to the addition of methionine at any level of protein. Sulfite oxidase activity in the intestinal mucosa was only 5% of that seen in liver and did not change in response to dietary protein or methionine. Activity did not change in rats fed low iron diets (5 mg Fe/kg diet) at any level of protein tested or in response to glycine. These results show that sulfite oxidase activity can adapt to different levels of dietary protein but is unaffected by the level of methionine, total amino nitrogen or iron in the diet.     

Effect of antibiotics on growth of the immature rat.

The purpose of this study was to quantify the growth promoting effect of a mixture of antibiotics for rats eating diets deficient in protein or an essential amino acid. Male albino weanling rats (70 to 80 g weight, 4 weeks old) were fed (a) a control diet containing all other required nutrients and varying amounts of casein (0 to 27%), or (b) a purified amino acid diet containing all other required nutrients and varying amounts of valine (0 to 70 mumoles/g diet), threonine (0 to 69 mumoles/g diet) or tryptophan (0 to 8.6 mumoles/g diet), with and without an oral antibiotic supplement consisting of neomycin sulfate (10 mg/100 g body weight/day), bacitracin (500 units/100 g body weight/day), and polymyxin B sulfate (1 mg/100 g body weight/day). At suboptimal intake of casein, valine, tryptophan or threonine, rats eating antibiotic-enriched diet showed up to 3 times greater daily body weight gain (deltaBW) than rats eating a similar diet without antibiotics. The growth-promoting effect of antibiotics can be expressed as percent sparing of specified nutrient (casein or individual amino acid), defined as below: (see journal) where nutrient intakeO ab or nutrient intakeab represents that intake of casein or of a particular amino acid which is required to produce a specific deltaBW in antibiotic-free or antibiotic-supplemented group, respectively. The percent sparing was inversely proportional to the dietary content of casein or limiting amino acid. For diets containing 10% to 25%, 25% to 50%, 50% to 75%, and 75% to 100% of the daily requirement of the limiting nitrogenous nutrient, sparing on the average was greater than 80%, 60%, 20%, and less than 10%, respectively. Data on daily food intake of ad libitum fed rats, and data from an experiment with tube-fed rats, showed that the growth-enhancing effect of antibiotics was independent of changes in food intake.     

Pyridoxine requirements of channel catfish.

In 20 and 12 week feeding trials, channel catfish fingerlings were fed purified diets containing five levels (0, 5, 10, 20, and 30 mg/kg) and six levels (0, 1, 2, 5, 10, and 20 mg/kg) of supplemental pyridoxine hydrochloride. Fish fed unsupplemented diets (pyridoxine content of 1.2 mg/kg) were characterized by anoxeria, nervous disorders, tetany, greenish-blue body coloration, and eventual mortality. Anemia, which has been reported in pyridoxine deficient salmonids, was not observed in pyridoxine deficient catfish. However, a microcytic, normochromic anemia was observed in groups fed high dietary levels of pyridoxine (20 mg/kg or greater). The dietary pyridoxine level required for maximal growth was approximately 3 mg/kg of diet. All other deficiency signs were prevented by 2.2 mg/kg of diet.     

Threonine metabolism in vivo: effect of threonine intake and prior induction of threonine dehydratase in rats

The metabolic fate of threonine was investigated in young male rats fed 15% amino acid diets containing from 0.15% to 0.85% of L-threonine. Liver serine-threonine dehydratase (S-TDH) activity did not increase with increasing dietary threonine content. The level of threonine required for maximum weight gain was not greater than 0.55% of the diet (or about 600 mumoles/day). Tissue free threonine content of rats fed the diets with 0.15% or 0.3% of threonine was very low but increased sharply with increasing dietary threonine content above 0.3%. During ad libitum feeding of these diets containing L-[U-14C]threonine, rate of oxidation of threonine was low when intake was in the range of the requirement for maximum growth, but increased, thereafter as threonine intake increased. A 30-fold induction of liver S-TDH, by prior feeding of an 80% casein diet, did not result in increased oxidation of threonine when dietary threonine content was 0.15%. When dietary threonine content was increased to 0.5%, oxidation of threonine increased slightly but significantly. With 3% of threonine in the diet, rats previously fed a 15% casein diet had extremely high tissue threonine concentrations whereas those with high S-TDH activity, due to the previous feeding of the 80% casein diet, oxidized threonine rapidly and tissue threonine concentrations were elevated much less.     

Dietary soy protein isolate modifies hepatic retinoic acid receptor-beta proteins and inhibits their DNA binding activity in rats

Retinoic acid receptors (RAR) belong to the same nuclear receptor superfamily as thyroid hormone receptors (TR) that were previously shown to be modulated by dietary soy protein isolate (SPI). This study has examined the effect of dietary SPI and isoflavones (ISF) on hepatic RAR gene expression and DNA binding activity. In Expt. 1, Sprague-Dawley rats were fed diets containing 20% casein or 20% alcohol-washed SPI in the absence or presence of increasing amounts of ISF (5-1250 mg/kg diet) for 70, 190, or 310 d. In Expt. 2, weanling Sprague-Dawley rats were fed diets containing 20% casein with or without supplemental ISF (50 mg/kg diet) or increasing amounts of alcohol-washed SPI (5, 10, and 20%) for 90 d. Intake of soy proteins significantly elevated hepatic RARbeta2 protein content dose-dependently compared with a casein diet, whereas supplemental ISF had no consistent effect. Neither RARbeta protein in the other tissues measured nor the other RAR (RARalpha and RARgamma) in the liver were affected by dietary SPI, indicating a tissue and isoform-specific effect of SPI. RARbeta2 mRNA abundances were not different between dietary groups except that its expression was markedly suppressed in male rats fed SPI for 310 d. DNA binding activity of nuclear RARbeta was significantly attenuated and the isoelectric points of RARbeta2 were shifted by dietary SPI. Overall, these results show for the first time, to our knowledge, that dietary soy proteins affect hepatic RARbeta2 protein content and RARbeta DNA binding activity, which may contribute to the suppression of retinoid-induced hypertriglyceridemia by SPI as reported.     

Maternal protein deficiency causes hypermethylation of DNA in the livers of rat fetuses

Maternal protein deficiency during pregnancy is associated with changes in glucose tolerance and hypertension in the offspring of rats. In this study the growth of rat fetuses was examined when the dams were fed diets containing 18% casein, 9% casein or 8% casein supplemented with threonine. The extra threonine was added to reverse the decrease in circulating threonine concentrations that occurs when pregnant rats are fed protein-deficient diets. The fetuses of the group fed the low protein diet supplemented with threonine were significantly smaller than those of the control group and not significantly different from those fed low protein. Homogenates prepared from the livers of dams fed the diet containing 9% casein oxidized threonine at approximately twice the rate of homogenates prepared from dams fed the diet containing 18% casein. We conclude that circulating levels of threonine fall as a consequence of an increase in the activity of the pathway that metabolizes homocysteine produced by the transulfuration of methionine. Serum homocysteine was unaffected in the dams fed low protein diets compared with controls, but was significantly greater in dams fed the low protein diet supplemented with threonine. Elevated levels of homocysteine are associated with changes in the methylation of DNA. The endogenous methylation of DNA was greater than that of controls in the livers of fetuses from dams fed the 9% protein diets and increased further when the diet was supplemented with threonine. Our results suggest that changes in methionine metabolism increase homocysteine production, which leads to changes in DNA methylation in the fetus. An increase in maternal homocysteine may compromise fetal development, leading to the onset of glucose intolerance and hypertension in adult life.     

Beneficial effect of methionine and threonine supplements on tyrosine toxicity in rats.

The growth retardation and external pathological lesions that occur in rats fed a 10% casein diet containing 5% tyrosine could be alleviated by the supplementation of extra protein (20% casein) or 0.66% methionine plus 0.90% threonine (equivalent to the contents in a 20% casein, respectively). Liver tyrosine aminotransferase activity is elevated by ingestion of excess tyrosine, but lowered by the supplementation of extra casein of methionine plus threonine. In rats fed a high tyrosine diet supplemented with methionine plus threonine, liver p-hydroxyphenylpyruvate hydroxylase activity was not higher than that of the 10% casein group, but liver homogentisate oxidase activity increased significantly. When excess tyrosine was included in the 10% casein diet, free tyrosine concentrations in plasma, liver, muscle and brain were extremely elevated, but when the high tyrosine diet was supplemented with extra casein or methionine plus threonine, their plasma and tissues tyrosine concentrations lowered significantly. A large increase in total phenols, p-hydroxyphenylpyruvate and free tyrosine excretions in urine was produced in animals fed the high tyrosine-low protein diet, but these were lowered by the supplementation of methionine and threonine to the diet.     

Effect of dietary protein and energy on protein synthesis during lactation in rats

A series of experiments was carried out in which effects of dietary protein and energy on protein synthesis during lactation were evaluated. Purified diets were fed to female rats from conception until d 15 of lactation. Protein supplied by wheat gluten, gluten + lysine + threonine or casein + methionine was fed at 20 or 25% in the diet. Protein synthesis in mammary gland and liver was measured with a large dose of L-[4-3H]phenylalanine. As lactation proceeded, protein absolute synthesis rates (ASRs) in casein-fed dams increased from 1056 to 2246 mg/d in mammary gland but remained unchanged in liver. In contrast, ASRs in wheat gluten-fed dams remained unchanged in both tissues. Feeding a protein-free diet or starving dams for 3 d reduced ASR in mammary gland 59 and 72%, respectively, when measured on d 15 of lactation. The corresponding reductions in liver ASR were 41 and 46%. A 20% restriction of dietary energy at a constant protein intake reduced protein synthesis in mammary glands 34, 55 and 28% in dams fed gluten, gluten + lysine + threonine or casein, respectively. In summary, protein synthesis during lactation was reduced less in liver than in mammary gland in response to dietary restriction of protein or energy.     

Effects of dietary limitations of histidine,methionine, or threonine on the susceptibility of mice to infection with salmonellatyphimurium and listeriamonocytogenes

The effects of dietary limitations of histidine, methionine, or threonine on the susceptibility of mice to Salmonella Pultyphimurium and Listeriamonocytogenes were investigated. Weanling CF1 or SW mice fed diets limited in 75% histidine or threonine but not methionine were more susceptible to S. typhimurium. Mice fed diets limited in 75% methionine or threonine were more susceptible to L. monocytogenes. Mice frow two of the experimental dietary groups challenged with 6 times the 50% lethal dose of S. typhimurium cells on day 5 after immunization with 108 heat killed cells exhibited a lower resistance than the control group, however, by day 8 after immunization, all dietary groups developed adequate resistance. The increased susceptibility of mice to S. typhimurium was reversed by replenishing the diet after 2 weeks with the limiting amino acid, histidine or threonine. The replenishment of the diet was not effective to reverse the increased susceptibility of mice to L. monocytogenes. The increased susceptibility to L. monocytogenes is likely to be due to the impairment of mouse cell mediated immunity which is not reversible by dietary replenishment.     

Use of arginine to reduce the severity of retinoid-induced hypertriglyceridemia

Previous research with an animal model of retinoid-induced hypertriglyceridemia, rats fed a 13-cis retinoic acid-containing diet in which casein was the dietary protein, has demonstrated that replacement of dietary casein with soy protein isolate can reduce the severity of this condition. A depressant action of soy protein vs. that of casein on serum triglyceride concentration has also been demonstrated in rats fed purified diets without supplemental retinoid. Because this action of soy protein appears to be due to its having a higher arginine-to-lysine ratio than casein, a study was done to determine how feeding a casein-containing diet supplemented with sufficient arginine, to give a dietary arginine-to-lysine ratio equivalent to that of soy protein, would affect the development of retinoid-induced hypertriglyceridemia. Groups of five-week-old male Fischer 344 rats (n = 7/group) were fed a control diet containing casein or one of three 13-cis retinoic acid-containing diets in which dietary nitrogen was provided as casein, casein + arginine, or soy protein. Incorporation of dietary 13-cis retinoic acid resulted in hypertriglyceridemia, with serum triglyceride concentrations of 2.00 and 7.23 mmol/l, or 177 and 640 mg/dl, for groups fed the control and casein + 13-cis retinoic acid diets, respectively. For animals fed the 13-cis retinoic acid-containing diets, serum triglyceride levels for the casein + arginine and soy protein diet groups (4.75 and 2.92 mmol/l, or 421 and 258 mg/dl, respectively) were significantly lower than for the casein group (p < 0.05); however, the value was significantly lower for the group fed the soy protein diet than for the group fed the casein + arginine diet. Serum and dietary arginine-to-lysine ratios were highly correlated (r = 0.93, p < 0.0001). Thus, supplementing dietary casein with arginine reduced the severity of retinoid-induced hypertriglyceridemia, but not as effectively as replacing casein with soy protein.     

Effect of excess dietary methionine on weight gain and plasma amino acids in kittens

Four groups of five kittens each were individually fed for 6 wk either a purified control diet containing 0.5% methionine (Met) (based on 18% casein supplemented with arginine, cystine and threonine) or one of three experimental diets containing an additional 2, 3 or 4% L-Met. The diets with added Met caused a reduction in food intake (FI) on the second day and a negative body weight gain (BWG), in proportion to the level of added Met. After 10 d, kittens fed the 2 and 3% Met diets increased their FI and had positive BWG. During the last 10 d of the experiment, the control, 2% Met and 3% Met groups had BWG of 28, 15 and 0 g/d, respectively. Kittens given the 4% Met diet showed no adaptation and continued to lose weight. In these kittens plasma concentration of Met was 50-70 times and cystathionine about three times greater than in control kittens. Four male kittens were fed the same 4% Met diet for 6 wk and then switched to a diet containing 4% L-Met plus 4% glycine (Gly) for 12 d. Average daily FI was 21.4 +/- 1.3 g with 4% Met and 48.5 +/- 2.5 g after the addition of Gly, and BWG went from negative to positive. These results indicate that growing kittens are more sensitive than rats to excess Met and have a limited adaptive capacity. Kittens did not grow normally when the diet contained 2% or more dietary Met, which was equivalent to 0.6 g Met/(kg body wt.d).     

The sustained development of preneoplastic lesions depends on high protein intake.

The effects of sequential alterations in the feeding of two levels of dietary protein (5% and 20% casein) on the postinitiation development of aflatoxin B1- (AFB1) induced gamma-glutamyl transpeptidase-positive (GGT+) preneoplastic foci were examined. Weanling male Fischer 344 rats fed AIN-76A diet (20% protein) were administered 10 intragastric doses of AFB1 (1 dose/day during the 14-day dosing period excluding weekends) at 250 micrograms/kg body wt (initiation). After AFB1 tissue clearance, rats were randomly assigned to dietary treatment groups. During the next 12 weeks (promotion), they developed AFB1-induced GGT+ preneoplastic lesions. The 12-week promotion period was subdivided into four three-week periods, during which rats were fed isocaloric diets containing 20% casein during all four periods (20:20:20:20), 5% casein during all four periods (5:5:5:5), or sequentially altered casein levels (20:5:20:5 and 5:20:5:20). Rats were killed at 3,6,9, and 12 weeks to examine the dependence of GGT+ foci development on protein intake. Animals fed 5% casein diets developed significantly fewer (p < 0.01) GGT+ foci than animals fed 20% casein diets despite greater total caloric intake. Similarly, in the intervention groups, preneoplastic development was enhanced when the 20% casein diet was fed and inhibited when the 5% casein diet was fed. These results indicate that the sustained development of AFB1-induced preneoplastic foci depends on a high protein intake. Alternatively, these results suggest that low protein intake inhibits lesion development.     

Methionine toxicity in the chick: nutritional and metabolic implications.

Several assays with young chicks fed crystalline amino acid diets were conducted to investigate the effects of supplemental glycine, serine, threonine, arginine, or adenine on the growth depression resulting from consumption of excess methionine. Glycine was partially effective in alleviating the growth depression caused by excess methionine. The addition of threonine together with glycine improved performance still further. Efficiency of food utilization for weight gain was greater in birds fed the methionine-imbalanced diet supplemented with glycine and threonine than in those fed the control diet. Supplemental glycine, threonine, or adenine, but not arginine, was effective in ameliorating the hypoglycemia resulting from consumption of excess methionine. The rate of oxidation of a tracer dose of threonine was increased markedly by feeding 1.25% excess methionine. This was reflected in a 20% depression in threonine utilization for weight gain as measured by slope ratio. The data suggest that both threonine and glycine are antagonized by consumption of excess methionine.     

The quality and quantity of dietary protein affect brain protein synthesis in rats.

The influence of the amino acid supply in diets with different quality and quantity of protein on the rate of protein synthesis in the brain was investigated. Amino acid concentrations in serum and brain altered in accordance with the amino acid levels of the diets, with the exception of some amino acids such as aspartic acid, glutamic acid, glycine and threonine. When rats were fed various levels of dietary casein (0, 5 and 20%), the aggregation of ribosomes increased and the fractional rate of protein synthesis tended to increase with the increase in dietary protein. When rats were fed a 20% casein diet, greater aggregation of brain ribosomes and protein synthesis rate were observed compared with those in rats fed 20% wheat gluten or gelatin diets. The RNA activity was related to the degree of the aggregation of brain ribosomes and the fractional rate of protein synthesis.     

Efficiency of methionine retention in ducks

The accretion of methionine and protein as a function of methionine intake was assessed in growing ducks between 22 and 42 d post-hatching. Four graded doses of dl-methionine at 0, 0 x 5, 1 x 0 or 1 x 5 g/kg diet were added to a methionine-limiting basal diet and fed to four replicate groups of four ducks each. The growth and efficiency of food use for growth increased linearly (P<0 x 05) as a function of methionine intake. The accretion of body protein increased (P<0 x 001) from 87 x 5 to 182 x 2 g, and that of methionine from 1616 to 3125 mg, over the 21 d period as dietary methionine increased. The accretion rate of methionine in the body (y, mg/d) as a function of methionine intake (x, mg/d) of ducks fed diets containing supplemental methionine at 0, 0 x 5, 1 x 0 or 1 x 5 g/kg diet from day 22 to day 42 post-hatching gave the regression equation: y=-148 x 86 (se 32 x 558)+0 x 312 (se 0 x 0384)X, r2=0 x 8253. For protein accretion rate in the body (y, mg/d) as a function of methionine intake (x, mg/d), the regression equation was: y= -9782 (se 2204)+19 x 505 (se 2 x 5994)x, r2=0 x 8009. There was a linear relationship between methionine (y, mg/d) and protein (x, mg/d) accretion in ducks that was described by the equation y=12 x 757 (se 7 x 4019)+0 x 01 525 (se 0 x 00 107)x, r2=0 x 9355. The results of these studies suggest a constant utilisation of methionine over the range 2 x 4-3 x 9 g digestible methionine/kg diet, with an efficiency of 31 %. Furthermore, the results suggest a quantitative relationship of 15 mg methionine for every gram of protein accretion.     

Effect of diets supplemented with amino acids on intestinal sucrase and leucine aminopeptidase activities in rats.

In order to investigate the relationship between dietary amino acids and protein, and activities of intestinal sucrase [EC 3.2.1.26] and leucine aminopeptidase [EC 3.4.11.1, LAPase] in rats, the effect of supplementation of amino acids into a protein-free diet and a low casein diet containing sucrose as the carbohydrate source on these enzyme activities was studied. The segmental weights of the small intestine and its mucosa of rats fed the protein-free diet supplemented with L-methionine or with L-methionine and L-threonine at 0.1 or 0.2% levels were significantly higher than those of rats fed the protein-free diet or one supplemented with L-glutamic acid, but there was no difference in the segmental activities of the sucrase and LAPase among rats fed these diets. On the other hand, the supplementation of methionine or methionine plus threonine to the 5% or 10% casein diet produced remarkable increases in the segmental weights of the small intestine and its mucosa as well as in the segmental activities of the sucrase and LAPase. There was no difference between the segmental sucrase activity of rats fed the 10% casein diet supplemented with 0.2% methionine ad libitum and that of rats fed this diet under restricted feeding conditions, although the segmental LAPase activity was affected by the amount of food consumed.     

Dietary pantothenic acid requirement of juvenile grass shrimp, Penaeus monodon

A feeding trial was conducted to estimate the minimal dietary pantothenic acid (PA) requirement for juvenile grass shrimp, Penaeus monodon. Purified diets with seven levels (0, 20, 40, 60, 120, 240, and 480 mg/kg) of supplemental PA were fed to P. monodon (mean weight 0.88 +/- 0.01 g) for 8 wk. The level of PA detected in the unsupplemented diet was 0.02 mg/kg. Each diet was fed to three replicate groups of shrimp. Feed efficiencies (FE) and protein efficiency ratios were highest in shrimp fed the diets supplemented with 120, 240, and 480 mg PA/kg diet, followed by the groups fed 60 mg/kg, then 40 mg/kg, and finally the unsupplemented control group (P < 0.05). Shrimp fed diets supplemented with PA had significantly higher survival percentages and lower hepatopancreatic lipid concentration than those fed the unsupplemented, control diets. Broken-line regression analyses of weight gain percentage and hepatopancreatic CoA and PA concentrations of the shrimp indicated that the adequate dietary PA concentration in growing P. monodon is 101-139 mg/kg.     

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.     

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.     

Sex- and age-specific immunomodulatory effects of dietary soya protein isolate and isoflavones in rats

The present study examined, using rats as a model, the effects of sex and age of exposure to dietary soya components on serum total and soya-specific antibody content. In Expt 1, Sprague-Dawley rats at 50 d of age were fed diets containing 20 % casein or 20 % alcohol-washed soya protein isolate (SPI) with or without supplemental isoflavones (ISF, 250 mg/kg diet) for 70, 190 or 310 d. The offspring were fed the same diets as their parents. In Expt 2, juvenile Sprague-Dawley rats at 30 d of age were fed diets containing 20 % casein with or without supplemental ISF (50 mg/kg diet) or increasing amounts of alcohol-washed SPI (5, 10 or 20 %) for 90 d. Exposure of rats to dietary SPI before the age of 28 d increased serum total IgA and IgM, and induced the production of SPI-specific IgA, IgG, IgM and IgE antibodies. Feeding of juvenile or adult rats with SPI elevated serum total IgA in females, while the opposite occurred in males, and markedly stimulated the production of SPI-specific IgM in females and IgG in males. Our data suggest that the effects of soya proteins and ISF on the production of serum total and SPI-specific antibodies appear to be sex dependent and also related to the age of exposure to soya in rats. However, the physiological significance of these immune responses remains to be determined.