Effects of p-aminobenzoic acid, methionine, threonine and protein levels on susceptibility of mice to Plasmodium berghei

The effects of dietary p-aminobenzoic acid (PABA), protein, methionine and threonine on Plasmodium berghei infection in mice were investigated. Animals were fed diets containing 12 or 20% casein supplemented with PABA (0 or 2 mg/kg diet), methionine (0 or 15 mg/g casein) and threonine (0, 7.5, 27.5 or 47.5 mg/g casein). Percent mortality was lower in rats fed diets without PABA than in those fed diets containing PABA. All further experiments were conducted without supplemental PABA. While the mean day of death was greater in the groups fed 12% casein, percent mortality in these groups was nearly twofold higher than in the groups fed 20% casein. The presence or absence of 15 mg methionine per gram casein had no effect on percent mortality, mean day of death, or percent parasitemia, regardless of dietary casein level. Supplementation of threonine at any level to the 12% casein diet with supplemental methionine resulted in mortality rates similar to those from animals fed the 20% casein diets, but percent mortality was not altered by threonine in the absence of supplemental methionine. The mean day of death of animals fed 20% casein increased with increments of added threonine. Methionine had no influence on this phenomenon. It is concluded that the quantity of dietary protein and the quality of its amino acid composition can have profound effects on the susceptibility of mice to malaria.     

Threonine imbalance, deficiency and neurologic dysfunction in the kitten

The effect of feeding threonine-imbalanced and threonine-deficient purified diets (containing L-amino acids as the only source of dietary nitrogen on food intake, weight gain and blood plasma amino acid pattern has been examined in growing kittens. The imbalance was created by adding 17.5% of amino acid mixture lacking threonine to a low amino acid basal diet comtaining 17.5% amino acid mixture including 0.4% threonine. A depression in food intake and weight gain occurred while feeding the imbalanced diet which was corrected by adding an additional 0.2% threonine to the imbalanced diet. There was no adaptation with time in the form of increased food intake or weight gain while feeding the imbalanced diet. Plasma threonine was consistently and similarly depressed (10 to 35% of normal) while feeding the basal, imbalanced and corrected diets and increased to normal when the standard diet with 1.4% threonine was fed. Plasma threonine and total amino acid concentrations of kittens fed the imbalanced diet did not differ from those observed in kittens fed the basal diet. Signs of neurologic dysfunction and/or lameness developed in 14 of the 17 kittens fed threonine-imbalanced or deficient diets, which resolved as dietary threonine was increased.     

Soy protein isolate and its hydrolysate reduce body fat of dietary obese rats and genetically obese mice (yellow KK)

This study examined in dietary obese and genetically obese rodents the effects of soy protein isolate (SPI) and its hydrolysate (SPI-H) on the rate of body-fat disappearance. Male Sprague-Dawley rats (4-16 wk old) and yellow KK mice (6-10 wk old) were made obese by feeding high-fat diets containing 30% fat. They were then fed energy-restricted, low-fat (5.0%), and high-protein (35% casein, SPI, or SPI-H) diets for 4 wk at 60% of the level of the energy intake of rodents on laboratory chow. The body-fat contents of rats and mice fed a high-fat diet were 27.3 and 33.6 g/100 g body weight, respectively, at the end of the obese period. For rats, the apparent absorbability of dietary energy and fat was significantly lower in the SPI and SPI-H groups than in the casein group, but vice-versa for nitrogen balance. Body-fat content in mice fed SPI and SPI-H diets was significantly lower than in those fed the casein diet. In rats, plasma total cholesterol level was lower with the SPI-H diet, and plasma glucose level was lower with the SPI and SPI-H diets than with the casein diet. These results indicate that SPI and SPI-H are suitable protein sources in energy-restricted diets for the treatment of obesity.     

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.     

Histidine-imbalanced diets stimulate hepatic histidase gene expression in rats.

A high protein concentration in the diet induces the gene expression of several amino acid degrading enzymes such as histidase (Hal) in rats. It is important to understand whether the amino acid pattern of the dietary protein affects the gene expression of these enzymes. The purpose of the present work was to study the effect of a histidine-imbalanced diet on the activity and mRNA concentration of rat hepatic histidase. Seven groups of six rats were fed one of the following diets: 1) 6% casein (basal), 2) 20% casein, 3) 35% casein, 4) an imbalance diet containing 6% casein plus a mixture of indispensable amino acids (IAA) equivalent to a 20% casein diet without histidine (I-20), 5) 6% casein plus a mixture of IAA equivalent to a 35% casein diet without histidine (I-35), 6) a corrected diet containing 6% casein plus IAA including histidine equivalent to a 20% casein diet, 7) a corrected diet containing 6% casein plus IAA including histidine equivalent to a 35% casein diet. Serum histidine concentration was inversely proportional to the protein content of the diet, and it was significantly higher in rats fed the corrected diets compared to their respective imbalanced diet groups. Hal activity increased as the protein content of the diet increased. Greater histidine imbalance resulted in lower food intake and higher Hal activity. Rats fed histidine-corrected diets had lower activity than their respective imbalanced groups. Differences in Hal activity were associated with differences in the concentration of Hal mRNA. These results indicate that rats fed a histidine-imbalanced diet exhibit reduced food intake and weight gain and increased Hal gene expression as a consequence of an increased amino acid catabolism.     

Effect of feeding lysine and threonine fortified bread during gestation and lactation on growth of the brain in rats.

First and second litter offspring brain development was studied following the feeding of bread, bread plus lysine, or bread plus lysine and threonine to rats during pregnancy and lactation. Results were compared with offspring brain growth following maternal consumption of a stock diet, 13% casein or 26% casein diet. Brain weight, protein and cellularity progressively increased as lysine and lysine plus threonine were added to the maternal bread diets. The bread plus lysine and threonine diet resulted in similar or higher total weight, protein and DNA values for all brain areas in comparison with results from the stock diet or 13% casein diet. The 26% casein diet resulted in greater brain weight, protein and DNA values in comparison to the bread, bread plus lysine, 13% casein, or stock diets. First litter pups whose mothers were fed the 26% casein diet, in comparison to the bread plus lysine and threonine diet, had higher or similar brain weight and DNA levels. For second litter pups, all values were similar for both diets with the exceptions of lower brain stem weight for 26% casein and lower cerebellum DNA for bread plus lysine and threonine.     

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.     

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.     

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.     

Growth,protein utilization,and secretion of pancreatic enzymes by rats in response to elevated levels of dietary protein

Rats were fed diets containing 15, 30, 45, or 60% casein for 30 days. The weight gains of rats fed diet containing 30% casein were higher (P<0.05) than that of rats fed 15, 45 or 60% casein diets. Food intake was similar in the 15% and 30% casein diets and lower in the 45% and 60% casein diets. Blood plasma urea level increased in parallel to the increase in dietary protein and was inversely related to PER. Water intake increased with the increase in dietary protein level. The relative weights of the pancreas and kidneys increased with the increase in protein level as did the activities of the digestive enzymes. However, while trypsin and chymotrypsin activities were increased both in the pancreas and in the intestine, lipase was increased only in the intestine and amylase only in the pancreas. The response to elevated dietary protein, as shown by the levels of digestive enzymes excreted in the feces, was very marked in the first week and was smaller and inconsistent later on. The digestibility of casein was related to trypsin and chymotrypsin activities in the pancreas and small intestinal contents.     

Effect of feeding lysine and threonine fortified bread during gestation and lactation on growth of the offspring in rats.

White bread, bread fortified with lysine, or with lysine and threonine was fed to female rats as the sole source of protein (13% of diet) during two closely spaced gestation and lactation periods. Comparisons were made with diets containing 13% or 26% casein protein, and a stock diet. Weight changes and food consumption of the mothers were determined, as were birth weights and weaning weights of the offspring. Lysine fortification increased food consumption and decreased weight loss of the mothers during lactation. When threonine was added with the lysine, food consumption during lactation was further increased and maternal weight loss prevented. Average weaning weights for first litters as a function of diet were: bread 17.0 +/- 1.9 g, bread + lysine 26.8 +/- 1.0 g, bread + lysine + threonine 39.8 +/- 3.1 g, 13% casein 42.8 +/- 3.8 g, 26% casein 62.5 +/- 4.7 g, and stock diet 41.5 +/- 7.9 g. First and second litter weaning weights were closely similar for each. Although average birth weights were not greatly affected by dietary treatment, total offspring weight at birth of mothers fed unfortified bread was reduced 40% in the second, as compared to the first litter. This reduction in total offspring weight did not occur in mothers fed amino acid fortified bread.     

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.     

Regulation of glucose-6-phosphate dehydrogenase and malic enzyme in liver and adipose tissue: effect of dietary trilinolein level in starved-refed and ad libitum-fed rats.

The responses of glucose-6-phosphate dehydrogenase (G6PD) (EC 1.1.1.49) and malic enzyme (ME) (EC 1.1.1.40) were studied in liver and adipose tissue of rats fed for 2 days a high glucose diet containing levels of synthetic trilinolein ranging from 0 to 25% (w/w) of the diet (trilinolein was substituted for glucose). One group of rats was starved for 2 days before the trilinolein-containing diets were fed (starved-refed); a second group of rats was fed a fat-free diet for 7 days before the trilinolein-containing diets were fed (ad libitum). Liver G6PD activity decreased exponentially and liver ME activity decreased linearly with increasing dietary trilinolein in starved-refed rats, but did not decrease significantly in ad libitum fed rats. Total liver lipid decreased exponentially with increasing trilinolein in starved-refed rats, but increased exponentially in ad libitum fed rats. Adipose tissue G6PD and ME activities decreased slightly with increasing trilinolein in starved-refed rats, but did not decrease in ad libitum fed rats. When the data were adjusted by analysis of covariance for differences in glucose intake, the liver responses in starved-refed rats were still significant but the adipose tissue responses were not, indicating that the responses of adipose tissue (but not of liver) may have resulted from decreased glucose intake rather than from increased trilinolein intake. The results suggest that dietary trilinolein inhibits the characteristic increase in liver G6PD, ME and total lipids upon starvation-refeeding. However, after the levels of these parameters have been increased by feeding a fat-free diet they cannot be decreased by dietary trilinolein in 2 days.     

Effect of dietary tryptophan levels on the urinary excretion of nicotinamide and its metabolites in rats fed a niacin-free diet or a constant total protein level

We previously found that the sum total urinary excretion of nicotinamide and its metabolites in growing rats did not increase as the level of dietary casein increased. So, we investigated how the urinary excretion of nicotinamide and its metabolites changed when rats were fed nicotinic acid-free diets containing 10, 20 or 40% casein, or when they were fed diets containing nicotinic acid and the same level of protein but different levels of tryptophan. The latter diets were 10% casein-30% gelatin, 20% casein-20% gelatin or 40% casein. With the nicotinic acid-free diets, the sum total urinary excretion of nicotinamide and its metabolites was almost the same in the groups fed the 20 and 40% casein diets; however, it was significantly lower in the group fed the 10% casein diet. With diets containing the same level of protein, the sum total urinary excretion of these compounds in the groups fed the 10% casein-30% gelatin, 20% casein-20% gelatin and 40% casein diets were about 5, 7 and 11 mumol/d, respectively, values that were significantly different from each other. Therefore, it was found that only when the total protein intake was constant did the sum total urinary excretion of nicotinamide and its metabolites increase with increasing intake of tryptophan.     

Physiological response of mature rats to replacement of dietary fat with a fat substitute

The effects of replacing dietary fat with a fat substitute on food intake, body composition and lipid metabolism were examined in rats. Female Sprague-Dawley rats (250 g) were fed diets containing between 2 and 63% of energy as fat for 64 d. Inclusion of a substitute resulted in diets of different fat content but similar texture. When 10% corn oil (21% kJ-fat diet) was replaced with the substitute supplemented with linoleic acid (2% kJ-fat diet), rats increased food intake so that there was no effect on energy intake, body weight, body composition or serum lipid profile. Rats fed a diet containing 10% corn oil and 30% Crisco vegetable shortening (63% kJ-fat diet) became obese and hyperinsulinemic. When half (51% kJ-fat diet) or all (30% kJ-fat diet) of the Crisco was replaced with the fat substitute, the rats increased food intake and were fatter than controls but less obese than rats fed the 63% kJ-fat diet. Hepatic lipid oxidation and ketone synthesis were proportional to the percentage of dietary energy as fat. Adipocyte de novo lipid synthesis was inhibited by 51% kJ-fat and 63% kJ-fat diets. Partial or total replacement of Crisco prevented the hyperinsulinemia observed in 63% kJ-fat rats, suggesting a protective effect against the development of insulin resistance with diet-induced obesity.     

Food intake, energy balance and serum leptin concentrations in rats fed low-protein diets

Studies examining the effects of low-protein diets on food intake and body weight have shown varied results. Many researchers have found low dietary protein to increase food intake, while others have found no effect or even a decrease. In 63 male Sprague-Dawley rats, we examined several low levels of dietary protein (2%, 5%, 8%, 10%, 15% vs. 20% casein) to determine the dose-response relationships between low dietary protein and food intake, body composition, energy balance and serum leptin concentrations. Food intake, over the range of low dietary protein, showed a quasi bell-shaped response curve with peak intake occurring in rats fed 8-10% casein. Peak feeding occurred at or just below the estimated protein requirement of the rats (10-12.5% casein). Compared to the 20% casein controls, food intake was severely reduced in rats fed 2% casein, while it was greater in the other low-protein groups. The amount of body fat steadily increased between the 15% casein group and the 8% casein group, and sharply declined between the 5% casein group and 2% casein group. The change in body fat reflected both the change in food intake and altered energy partitioning. Serum leptin concentrations were greater in rats fed the 5 and 8% casein diets than in control rats fed 20% casein. Serum leptin concentrations were positively associated with body fat content (r(2) = 0.763, P < 0.001). Increased serum leptin concentrations in the presence of increased food intake is suggestive of a state of leptin resistance. This animal model may provide important insights into diet-induced obesity.     

Dietary Alaska Pollack Protein Induces Acute and Sustainable Skeletal Muscle Hypertrophy in Rats

Our previous studies suggested that Alaska pollack protein (APP) intake increases skeletal muscle mass and that it may cause a slow-to-fast shift in muscle fiber type in rats fed a high-fat diet after 56 days of feeding. In this study, we explored whether dietary APP induces acute and sustainable skeletal muscle hypertrophy in rats fed a normal-fat diet. Male 5-week-old Sprague–Dawley rats were divided into four groups and fed a purified ingredient-based high-fat diet or a purified ingredient-based normal-fat diet with casein or APP, containing the same amount of crude protein. Dietary APP significantly increased gastrocnemius muscle mass (105~110%) after 2, 7 days of feeding, regardless of dietary fat content. Rats were separated into two groups and fed a normal-fat diet with casein or APP. Dietary APP significantly increased gastrocnemius muscle mass (110%) after 56 days of feeding. Dietary APP significantly increased the cross-sectional area of the gastrocnemius skeletal muscle and collagen-rich connective tissue after 7 days of feeding. It decreased the gene expression of Mstn /Myostatin, Trim63/MuRF1, and Fbxo32/atrogin-1, but not other gene expression, such as serum IGF-1 after 7 days of feeding. No differences were observed between casein and APP groups with respect to the percentage of Type I, Type IIA, and Type IIX or IIB fibers, as determined by myosin ATPase staining after 7 days of feeding. In the similar experiment, the puromycin-labeled peptides were not different between dietary casein and APP after 2 days of feeding. These results demonstrate that APP induces acute and sustainable skeletal muscle hypertrophy in rats, regardless of dietary fat content. Dietary APP, as a daily protein source, may be an approach for maintaining or increasing muscle mass.     

Effect of the protein and oil contents on calcium utilization in the diets of female Fischer rats that were fed by three different feeding methods

We studied the effects of varying the protein and oil contents in the diet and three different feeding methods on the utilization of dietary calcium (Ca) in female Fischer rats. The experimental diets were based on the AIN-76 diet. Rats were fed one of nine experimental diets containing different levels of protein and oil. The experimental diets contained 10, 20 or 40% of protein (milk casein) and 5, 10 or 20% of soybean oil. The three meal feeding methods were ad libitum feeding, pair feeding and adjustable feeding. The experimental diets that were given by prepared feeding were supplemented with a mineral and vitamin solution. The effects of the protein and oil contents in the diet were analyzed by two-way ANOVA. Among rats that were fed ad libitum, the oil content in the diet affected the degree of mineral and vitamin intake. Among the rats fed ad libitum, the oil content in the diet had a significant effect on the level of Ca intake, although it did not have a significant effect on the level of energy intake. For pair feeding, the feeding volume was limited in each group; therefore, there was a significant difference in energy intake and there was no significant difference in Ca intake among each diet group. For adjustable feeding, there was considerable mineral and vitamin intake, which effected the reduction of feeding volume depending on the oil volume in the diet. There were no significant differences in the energy, Ca and other mineral and vitamin intakes among the nine groups that were fed using adjustable feeding. In analyzing two-way ANOVA, in which the parameters were the contents of protein and oil in the experimental diets, there were differences in Ca utilization among rats that were fed using the three feeding methods. This result was shown to reflect on the difference as energy, mineral including Ca, and vitamin intake. However, the protein content in the diet had a significant effect on urinary Ca excretion in all three feeding methods. In addition, it was clear that the intake of protein and oil affected kidney calcification in all three feeding methods. The AIN Experimental Diet Committee reported that kidney calcification was found among rats that were fed the AIN-76 diet and that one of the causes of kidney calcification was the Ca/P ratio in this diet. The protein and oil contents in the diets had significant effects on the degree of kidney calcification among rats that were fed by pair feeding or adjustable feeding. The results of this study suggest that the protein and oil contents in the diet play an important role in kidney calcification.     

Effects of restricted diet on protein metabolism measured by [15N]glycine in high-fat-diet-induced obese rats

The effect of restricted diets on protein metabolism was studied in obese rats (obesity had been induced by ad libitum feeding of a diet containing 30% fat and 25% casein). The obese rats were fed on one of three restricted diets, each containing 5% fat, for 2 weeks (restricted feeding groups); a high-protein diet (HPD, 50% casein), a standard-protein diet (SPD, 25% casein), or a low-protein diet (LPD, 5% casein). The food intake was restricted to 5 g per day per rat. On the eleventh day, the rats were given [15N]glycine orally, and 4 days later, they were killed. The restricted feeding groups all showed similar weight losses (about 100 g), 2 weeks after the start of the restricted diet. The 15N distribution in whole body was measured and results were compared with those of control rats given 5%- or 30%-fat diet ad libitum. The whole-body distribution of 15N in the HPD group was similar to that in the rats fed ad libitum although the diet intake was restricted. The results suggested that the amount of protein in a restricted diet is important for maintenance of protein metabolism in obese rats.     

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.